Game Design, Programming and running a one-man games business…

Economics of solar batteries (big and small)

I’m lucky enough to be the proud owner of a 9.5kwh battery in my cellar. Its a GivEnergy battery (A UK company, although the actual battery is predictably made in China). Its AC-coupled, which means it sits on the ‘house’ side of the fusebox/consumer unit. We got it installed about 5 months ago and its been super awesome. Not flawless by any means, but its just so cool as a gadget for someone like me.

There are many reasons to get a home battery. Its a cool gadget, its also an incredibly strong way to reduce your energy bills (we basically run our whole house 24/7 on off-peak electricity at 75% off), and its also a great thing to partner-up with solar panels to ensure you use all that free power and don’t go exporting it to the evil energy company for a pitiful rate. I have to admit, although I was fully aware that we exported a lot of power on those days we were out, or we were not running much stuff, I had totally underestimated the impact. I’m currently running a big desktop PC/Monitor, router, wifi boosters and a bunch of other stuff, all from solar, on a cloudy day in march in the UK, AND filling the battery slightly…

Obviously the real reason to get a home battery is because you get to obsess over charts and stats:

I’m going to talk a bit more about the economics of the battery though, rather than the coolness of it, or its usefulness in balancing the grid and helping enable more renewable energy.

The economic bonus of my battery is it lets us buy off-peak electricity that costs (currently) £0.12 per kwh as opposed to peak power which costs £0.43 per kwh. In other words, each unit of power we use saves us £0.35. We use roughly 600kwh per month, but I reckon half of that is for the car, and can be done off-peak through charger scheduling regardless of the battery. So that leaves 300kwh a month or a saving of £105 a month, or £1,260 per year at current electricity rates.

There is an additional economic benefit though, which is that we are self-consuming all of our export. To look at how we manage this, I just need to look at the total solar->battery flow for the last month, which is about 40kwh. Thats energy that would previously have been wasted (exported to the grid, but not metered, so the amount doesnt matter). Thats another 40 * 0.43 so £17.20 a month, or a total of £206 a year.

So the income from our battery, at current prices, is 206 + 1260 = £1,466 a year. The installed cost for the battery was £6,720. So the payback period is 4.5 years. Thats really not bad, given the battery will definitely outlast that. This is also with a pretty rubbish old array of terrible 12 year old solar panels. So it seems like the economics of home battery storage are pretty good.


Now lets take a look at grid-level storage, and the size that I was considering. For those new to my blog, I run an energy company that is building a 1.2mwp solar farm here in the UK. Originally we were considering battery storage, but dropped the idea. We were originally looking to add a 500kwh, later increased to 583kwh battery, in a shipping-container enclosure that was AC-coupled to the farm. We are export-limited to 900kw despite peaking at 1.2kw, so this would be a good way to do peak shaving. I blogged about how that works in detail here.

So why are we not doing it? Basically its the economics. They just do not work, for that scale, at the current time, for a variety of reasons. The main one is that the costs have gone up, but also the cost of larger storage systems have gone down. It looks like economies of scale are massively at work here.

From what I understand, a 583kwh battery unit like the one pictured above is basically deployed as a kit. You get a lot of equipment, including pallets full of batteries, and a shipping container enclosure for all the gear, and then its assembled on site to create a finished, grid-connected 583kwh battery. The installation of this is about £28k out of a total installed cost of roughly £370k.

Lets pick apart the finances of this!

Firstly, a 583kwh battery is the same as 9.7 Tesla model 3 standard range batteries. A brand new Tesla model 3 standard range in the UK costs £43,000. 9.7 of those cars costs £417,000. That is MORE than the grid battery…but Jesus Christ its not MUCH more, and the 9.7 cars come with stuff like wheels, doors, touchscreens and…other car stuff. There is NO WAY that the cost to Tesla for making/buying the batteries in a model 3 are anything close to as high as people are being charged for a grid connected battery.

Whats going on?

Actually, the economics seem to be the case that grid-connected utility storage is being manufactured in a grossly inefficient way, and hugely overcharged for, especially by companies not called Tesla. Lets take a look at the real core of the problem by looking at a battery in a Tesla:

The actual batteries are the little cylindrical cells. They are being slowly changed to be a different configuration, but the same design principles apply: pack the batteries in with as efficient a volume-utilization as possible. Note that its actually much more safety critical in a car to prevent battery fires than in a shipping container sat in a soggy field, so if anything, this is the ultra-conservative and inefficient Tesla battery pack. Also…its a car, so weight REALLY matters (whereas with stationary storage we absolutely don’t care about weight), so thats another constraint. What we are looking at here then, SHOULD be the really inefficient way to do things.

But how are things done for grid scale utility batteries? Well… let me show you :D. This is a lithium ion battery for grid storage:

Thats an 8.1kwh battery. Not enough? It doesn’t matter because we can put them in groups in a rack!

Thats still not enough though, so we can then put the racks in a box!

And so it goes on, with what seems to be a Russian-doll approach to stacking boxes inside boxes inside boxes, until the amount of casings, handles, surrounds, spacers, racks, shelves and other non-battery components becomes ridiculous. Worse still, it seems that a lot of these are still being *assembled on site*. This is, as I am sure you must be aware, ludicrously inefficient.

This is the Tesla Megapack:

Its still racks-in-a-box, for now, but its not built on-site, or in a ‘room’ where you can walk down an aisle. its a big solid block of batteries (plus supporting equipment) that is manufactured like this at their factory. The whole thing is shipped as a single already-connected block, that you then hook up to your grid connections. No pallets, no wasted space. No lengthy install.

This is just a lot of tech-geek inside baseball until you realize the economics of it. The best way to find out the *real* price for these things is to look at recent installs. There was a recent one in the UK: with 196 megapacks for £75million. That comes out at a per-MWh of installed battery cost of £222k. Thats 42% cheaper than the quote I got for a 583kwh pack.

So the answer is simple right? Just ditch the battery provider you got a quote from and use Tesla? No. The megapack STARTS at 3MW. Thats a £666k installed cost, on top of the solar farm. It *might* make sense if the output of the grid connection was higher, and the farm itself was larger, but nobody would couple a 3MWH battery with a 900kw limited 1.2kwp farm. It would just be idle too much of the time to justify the expense.

We are still in the early days of grid-connected storage. Its accelerating like absolute crazy right now, so we are getting there…but there is a lot of change happening. As an investor in green-tech, I cannot see any other grid storage company surviving after the next 2 years other than Tesla. (The megapack is not surprisingly sold out for the next 2 years). The grid-storage business is VERY price sensitive. You cannot just shrug your shoulders with a 42% cost difference…

So how does that leave me? Well it leaves me building a solar farm with, for now, no battery storage (although with planning permission to add it later if the numbers change). I would love to revisit it when BESS prices drop, and definitely will. I think we are going to see a real push to make those systems cheaper, and thats just based on design optimization and economies of scale, not even considering better battery chemistry or any tech breakthroughs.

For home storage though, its a laughably good deal. If you can afford a home battery, get one.

Solar farm battery storage business case calculations

I’m currently in the throes of building a 1.2MW solar farm somewhere in England. Its an adventure, to put it mildly :D. So far the planning process and bureaucracy has taken just under 2 years. In that time a lot has changed.

Originally the plan was for a 1.2mwp solar farm, and a battery of either 256kwh or 512kwh. The current situation is that I’m evaluating a 583kwh battery (they can basically pack a bunch more cells in there now), but the economics have got a bit trickier and it may not actually be viable. This does not mean the project itself is not viable, the farm likely still is profitable (I hope!).

First things first, I’ll explain why any of this would make any sense. You cannot just rent a field, fill it with solar panels (assuming the farmer lets you, and planning permission is granted), and make a pile of money. Its not that simple. (And trust me: none of that is simple). The main problem you face is the grid connection. Basically solar farms go in fields, so we are already limited to relatively rural locations. This means that the land is large enough, and also its not going to upset too many people. Our site is surrounded by fields and hills and generally…nothing, so there were zero objections. This is all good news..

…but it also means you are on the edges of the power grid. If you are REALLY lucky, you find a site that has a field to rent, thats rural, but also just-so-happens to be a site where a major grid connection cable runs right through it. This is super ideal, and no, there no sites left like this any more in the UK because all the canny early developers have already snapped them up :(

So…we end up with a perfect field, planning permission, and a happy landowner, but we are basically connected to the rest of the grid by what amounts to a thin USB cable (not really). This is the point at which you ask the grid to upgrade that line and send you the bill, they do, then you laugh at how they accidentally added a few zeroes to the bill, and then you start to cry. Basically upgrading a power line is a nightmare. They cant just add heavier cable and be done. That cable may then sag too much in summer, or be too heavy, so you need new pylons, and then the ground under the pylons is too weak so you need foundations and omg expenseahontas.

In practice, that means that the local DNO (power company) has told us ‘you can generate 900kw at any moment from here but no more’, and thats AFTER we pay for a major grid upgrade. This sounds nuts, but when you think about it, if a power cable was only ever built to supply 2 farm houses, its not ever expecting a power flow of more than about 40kw total maximum. Then suddenly we arrive on the scene with 3,000 solar panels and want to push 20x as much power through those cables…

What does this have to do with batteries?

You might have noticed the difference between our 1,200kw solar farm and the 900kw limit on export. Well spotted. Its quite glaring isn’t it? Actually, this is exactly what most people do, and if you have solar panels at home, you do it too. Its called inverter under sizing.

The inverter is the box that converts DC to AC, and is the device that determines the maximum power that can come from your solar panels. In many cases, its rated BELOW the maximum output of your panels. This must seem insane…because surely that means at peak sunshine some of your power is just wasted right? YES. This is exactly what happens, and its normally because the bigger inverters are very expensive, and the PEAK output of your solar panels only triggers quite rarely, maybe for just 1 or 2 weeks in summer around midday. This means that the extra spend on the ‘bigger’ inverter might not be worth it.

Example: You have a choice of a 2kw inverter or 2.5kw inverter. The difference is $1,000. You have 2.5kwp of solar panels. Which do you choose? Well its actually hellishly complex. You need to model the output of your panels for every hour for every day in a typical year, and then ‘clip off’ the top portion where output would have been above 2kw. You then need to value that power at whatever you get paid to sell it, or what it would have cost you to buy it. You do that for the lifetime of the inverter, then work out if its a good deal.

I’m currently basically doing a similar thing, except the limitation for me is export limits, not inverter sizing. Our site will have 10 inverters anyway, and the bigger you get, the more granular you can be with how many you have. (For those curious, yes, thats like 300 panels per inverter, but each inverter supports a bunch of strings).

What does this have to do with batteries?

Well batteries let you cheat the system! They can do it in home installs, with what they call DC-coupled batteries, and you can do it at the grid/utility scale with batteries the size of shipping containers. (I’m evaluating a 20ft long one). Before I get into this, I need to list the different ways the battery might make sense in our solar farm. The battery would generate revenue basically 3 ways:

Frequency support services:

This is where the national grid basically pays you to act as a sort of ‘cache’ or ‘buffer’ for the grid. If they suddenly have slightly too much power, then dump it on you, then grab it back a bit later (maybe even a few seconds or minutes). They do this to keep the electricity grid at 50hz, to prevent every transformer in the country going bananas and maybe even failing. This is a constant battle, fought every second of every day. Here is a chart of the last 36 hours of it:

You can make money from these services, just by having a battery connected to the grid, and being registered to rent out your battery to act as a local buffer.

Time-based arbitrage stuff

You might think the price of electricity is fixed for your current contract term. Ha. Yes, at the consumer level it is, but behind the scenes, the wholesale price of electricity fluctuates like crazy, not only each month, or each day, but each 15 minutes. And it fluctuates a LOT. You can decide to ‘keep’ some of that solar power you generated at midday and sell it later. In fact in theory this can be hugely profitable, based on the current volatility. Again, here is a price chart the last 36 hours:

In that chart, the price gyrated between negative £70 (They pay YOU to use power) to positive £200 for a MWH. In theory, if you could buffer the whole farms output, and only sell it at the peaks, you would make decent money


This is a system where you take that extra power that you are not allowed to export, you buffer it in the battery, and trickle it out later. This transforms the usual bell curve of solar output into a different shape:

We fill the battery with the ‘red’ energy, then trickle it out later to extend the peak output duration of the site. This energy would otherwise be totally lost, and we would not earn a penny for it!

Ok, so these are three ways storage can make money alongside your solar farm. What the problem? The problem is two-fold. Firstly, there has been a HUGE increase in the amount of grid-connected storage in the UK, which means lots of batteries bidding for the same grid services regarding frequency and price arbitrage. That means that naturally, the profits from these mechanisms have dropped. There are a lot of VERY big companies doing this now, and tons more on the way. Plus, there are other technologies that you compete against. The main one would be hydro (basically a gravity-water battery), but also there are experimental things like gravitricity, and companies working with flywheels and other tech. Its VERY volatile.

Secondly, the price of the battery has gone fucking nuts. In 2 years, the price has gone up 50%. Thats crazy. This may partly be due to the EV revolution, which is scooping up all those lithium ion batteries I was hoping to pack my shipping container with. It might also be related to components shortages for power electronics (lots of chips in a stationary battery), and general inflation for stuff like shipping and labour to install a concrete base and so on…

So right now, with the first two potential battery income streams kinda poor, we have to think whether the basic and most obvious one (peak shaving) is actually worth it. It looks right now that a 583kwh battery cost is about a quarter the total cost of the entire project.

That MIGHT mean its just not worth doing. Sure, being curtailed at peak output for a few weeks each summer may suck, but the big question is how much? I don’t have access to the data yet, and do not have software that accurately measures it, nor is it likely worth the hassle of buying any. I am digging further.

The maths I need is pretty simple, IF you have the modeled data: Given the output each day, for 25 years, what amount of KWH is curtailed on the site given a 900kw power limitation. I then need to guess the market price for this and compare it to the battery install and maintenance cost for 25 years. (I know, its optimistic to expect the electronics to last 25 years, but hey).

Hopefully it DOES make a convincing case, because if not, I basically have to scrap the idea of a battery, or at least wait and see if prices fall. I can still install the solar farm in the meantime.

My experience of having a 9.5kwh home battery in the UK for about a month

Recently, we got a 9.5kwh givenergy battery fitted in our cellar. I was very excited about it, and keen to dive into the stats, and wrote a blog post about it here.

I’ve now had it for about a month and thought it was worth typing up the inevitable impressions having got used to the thing! So here goes…

First, some context. This is in a 2.5 bedroom (attic doesn’t count really) detached house, thats very old (pre napoleonic), but has been insulated to the best of our ability. 2 people working from home, in the southwest UK. Also be aware that this was during November/December, and a pretty cold December. As I type this, there is snow everywhere…

I had some initial confusion when the battery was first set up. Day 1, they calibrate it, by basically filling it with grid energy and then discharging it, which feels horrible when you see the first thing your battery does is suck up some prime-time expensive power! Luckily this is a one off thing :D. Once that first day is out of the way, you can then choose your settings and.. to be honest then completely forget about it! My settings, because I have cheap power (75% off!) from 12.30am – 4,30am, is for the battery to fill to 96% during that time and then be in ‘eco mode’ for the rest of each day.

GivEnergy’s eco mode is basically a maximise self-consumption, minimize grid import system. So if you have solar panels, and are producing more power than you are using (fat chance for here in December), the excess gets diverted into the battery. Any power load during the day gets sourced from the battery, so you see the battery state of charge slowly trickle down through the day as its used to power the house:

On the far left is the battery filling up (purple below the line) and my car charging. Combined, the battery charge and car charger hit 9,000w! You will see a few spikes during the day which are basically kettles and coffee machines, and cooking. It looks like breakfast was a big spike load on the grid! and then later mini spikes (below 3,000w) are handled entirely by the battery, slowly draining down to about 8% by midnight. That sustained power draw from 4pm-6pm is a gaming PC and huge monitor playing battlefield V :D.

The thing is… once you have watched these charts a few times, you kind of get the hang of it, and then never really need to look at them, or go near the battery ever again. Its just a magic box somewhere in your house that cuts your energy bill by 75%. The only tweak I have made is that now its even colder, and we are cooking more and for longer, I’ve adjusted it to fill to 96% instead of my original 90%, because we need a bit more energy each day (and if I can possibly avoid any prime-time energy consumption…I will!).

So this is all very well, but what have I learned that might be relevant for people who are considering installing a battery?

Firstly, you really need to get the size of the battery right. I kind of lucked-out a bit, and ended up with the perfect size, but nearly didn’t. At one point, we were going to get an 8.2kwh battery, then a 9.5kwh, then maybe 2x 9.5kwh ones, and even had a board installed on the cellar wall to support a 2nd one, but we ended up with a single 9.5kwh which feels right. Obviously, when you think about it, all you need to do is check your energy bill for how many kwh you use on average each day… and thats the size of the battery you want!

Its a bit more complex if you have solar, because if you have a decent solar array, there may be days where you are generating more than your daily usage, and want to store some in case its cloudy/raining the next day, to maximise your usage. Remember, the goal is to NEVER export any energy to the grid, because they pay you a pittance. So there are circumstances where you might need to oversize things…

For example, if your daily usage is 10kwh, but your solar array in June/July regularly produces 20kwh, then you will be using 10kwh, and sticking 10kwh in the battery for tomorrow. If you dips in solar power are fairly sparse, you will be often faced with surplus solar power and a full battery. IF you have an electric car too, and are bothered enough to trickle-charge it with the excess, then you can of course do this. There are setups and systems that can automate this BTW, that involve cables running to the EV charger from your battery/inverter, but I found it to be prohibitively complex, especially with our EV charger about 100ft from the fuse box.

I reckon for the vast majority of people, whether you have solar or not, you probably should stick to a simple format of just buying a battery that can hold 100-150% of your average daily usage. Its not like you can precisely pick a size anyway, as our options were basically 8.2kwh or 9.5kwh or some multiple.

Something that IS worth paying attention to is the inverter. You need an inverter coupled with your battery, or batteries. Its the thing that converts the stored power (DC) back to AC so the house appliances can use it. We have a first generation GivEnergy inverter, that runs at 3kw, and the ones run at 5kw. If at all possible get the higher output one. Get the highest output inverter you can. Why is this?

Its important to understand the difference between kw (kilowatts, thousands of watts) and kwh (kilowatt hours). The first is a measurement of power as in, the amount of oomph that is running down a cable to a thing, and the second is a measurement of stored energy, ie: the amount of oomph multiplied by how many hours you can provide it, before you run out. Or think of kw as your salary and kwh as your savings :D,

In my case, we have a 9.5kwh battery, fed by a 3kwh inverter. That means that even if the battery is FULL, if I plug in some theoretical device that wants to draw power at 9,000 watts… the battery can only squeeze out 3,000. The rest will get imported from the grid. Why does this matter? It matters because British homes have kettles! and also sometimes electric heaters! In an ideal world your day to day current draw will never exceed the power of your inverter. Every time it does, you will draw the excess from the grid.

With something like charging an EV, you need to just admit defeat. Most EV chargers at home are about 7kw, and you are not going to power your home AND an EV charger with a simple domestic battery and inverter. You need to schedule any EV charging for off peak anyway. The real culprits for going over 3kw are stuff like a kettle, a power-shower, or multiple induction hobs going at once. You might think 3,000 watts is a lot, but boil the kettle and fry some bacon while someone is in the shower and you zip right over that, no problem.

So… I’d suggest a 5kw or better inverter, and probably a 9.5kwh battery, or if you use slightly more energy than me, maybe a 13.5kwh Tesla powerwall. If you have a 4 bedroom house and power-hungry kids, and can afford it, maybe you have a good case for getting 2 9.5kwh givenergy’s or 2 powerwalls, especially if you also have solar.

So there ya go. I’m a total home-battery geek, and look at my stats every day, but if you arent that into it, but just want cheap electricity, then you can just go for it, set it up once, and then never look at it again! Ours is in the cellar and I only even see it if I go down to the cellar to get something out of the freezer :D. Batteries come with apps that will soon ping you if there is an error, so you can comfortably just ignore them.

I worked out on the basis of our first month that payback time for us was 5.7 years. That will fall a LOT in march when our fixed price tariff comes to an end, and fall AGAIN in summer when we get the advantage of saving up our surplus solar power during the day. (At the moment the only financial benefit for us is to buy cheap power overnight and use it during the day). I think in the long run the payback time for us will be maybe 3-3.5 years. Thats crazy good.

PLUS! We did it as a retrofit to existing solar. Right now the govt charges zero VAT on new solar, and batteries can be included, so if you can get solar+battery right now, its an even better deal. I highly recommend it!

I finally have a home battery!

At long last, its finally installed and actually working, and after a very long and very tedious process that I will not bore you with (much) my house now has a 9.5kwh home battery connected up and working in the cellar. Its actually powering everything in my house right now, and is super cool.

This has been a long process because just when I started to ask a company to install one, UK electricity prices went absolutely insane, and everyone and their dog suddenly wanted a home battery installation. Getting a Tesla powerwall would have meant possibly an even longer wait, and to be honest, they were quite pricey compared with what I eventually chose (a Givenergy 9.5kwh one), but even when I found an installer, and agreed to have a battery installed, there were endless delays. Initially I was going to have a 9.5kwh (latest model) battery, then it became obvious they were hugely delayed, so opted for a single 8.4kwh one, then at the very last minute it turned out a 9.5kwh one was available, and as this was the latest tech, that allowed 100% depth of discharge and unlimited cycles (basically you can fully fill/empty the battery whenever you like without affecting the warranty), so we went with that.

Getting people to come around and install the battery was one thing. Getting it actually finished was another. The installation has been a real pain, but TBH that seems to be mostly down to huge demand and chaos among all the companies doing this sort of thing right now. We had the battery installed, but dormant for about 4 weeks until a vital missing component showed up (a wifi dongle that allowed it to talk to my home network, and thus back home to the battery management system run by the battery company).

Now its all installed, it all feels like it sort of went ok though, because my ideal scenario was to put it in our cellar, which is damp, and dark and empty, and thus the perfect location. Why take up room in the house with a great big metal box when you are almost never going to have to physically go see it? I had worried that the installers would be negative about anything ‘non-standard’, but were happy to run the power cable from our kitchen fuse box outside the house, along a wall, then down through a hole into the cellar and the battery. I’m very happy with where it is:

When it was installed I asked them to set up a double size wooden board on the wall and allow space for a second similar size battery if I wanted to add another one at some stage. In practice, I doubt this will happen. The picture shows the battery (at the bottom) and above it is the inverter. Basically an inverter converts DC to AC or vice versa. This battery is ‘AC-coupled’ which means its on the ‘house’ side of the setup, and is wired in to the fusebox pretty much like anything else. It needs the inverter to convert that AC power so it can be stored in, or sucked out of the battery itself. On the plus side, if I got a 2nd battery, I don’t need another inverter, as you can just daisy-chain em. The red switch is an emergency cut-off.

Back upstairs in fusebox land, you end up with an extra widget called an EM115 that takes up one fuse slot. Its a fancy management thing that seems to be required to check everything works:

Connected to this widget is something called a CT clamp. This is a thing that wraps around a cable and tries to detect the power load going through it, and the direction of it. I already have one, used by my car charger in the garage, to ensure the charger never overloads the house grid connection. This new one is connected to the EM115 and the battery so the battery can tell whats happening with my grid connection:

This information is important because the battery uses it to do cool stuff. For example, you can say to the battery ‘Try to never export power to the grid!’, and then on a day where the solar power is high, and your consumption low, the CT clamp tells the battery that we seem to have a negative power flow (exporting to the grid) of X watts, and it therefore tells the battery to soak up X watts of power to balance it out and ensure any ‘surplus’ solar is kept in my house and not given for free to the evil energy supplier!

Whether or not this is what you want your battery to do is dependent on your circumstances. We got our solar panels 12 years ago on an early ‘feed-in-tariff‘. This means that we get credited a nice amount of money for each kwh of power our solar generates, regardless what happens to it, and we get a ‘deemed export’ of 50% of that. This means if our panels generate 10kwh, we get paid 10xFIT plus an extra 5xExportTariff. The actual export tariff is super low, so it wouldn’t be a good deal anyway, but as our tariff doesn’t measure the actual real export, we are kinda gaming the system a bit there by trying not to do any :D

For people with newer solar panels, they may actually have an export meter, and genuinely be paid per kwh they export. So why would you not do this? Because the very very best rates you get to export are insulting. I’ve seen a high of £0.07/kwh which is an insult considering that companies SELL you the same power for £0.24/kwh… Thus even if you do have an export meter, it makes more sense to keep that power in your home and use it later rather than sell it, then buy it back at a huge loss…

For people with normal jobs this is even more acute. In the UK summer your home solar will generate tons of power while you are out at work, which it sells for £0.07 for you to buy back in the evening to run your home at a huge loss. Sod that!

But enough about the economics! lets look at the real reason anybody gets a home battery – fun charts to look at:

This is one of the many views you get from the givenergy website. It also has app for your phone, obviously. This is my first day with the battery, and I screwed a lot of stuff up which I will explain. As part of its first-run ‘calibration’, the battery gets filled to 100% from the grid. Thus it was at about 80% full around midnight. I had then stupidly told it to fill to 100% between 00.30 and 4.00AM which is when my electricity is 75% off. It did this (see the top purple line going up to 100%) by importing that power from the grid. Thats the red section below the base line on the left, and the purple is it filling the battery

The red is a bit higher than the purple (well…below…) because I also needed *some* power to run the house even overnight.

I had screwed up twice because I’d told the battery to only discharge from 4.00am onwards, so between about 1.30-4am you can see those red downward spikes showing me importing power to deal with our energy demand (green upwards). Then from about 4am onwards, that disappears and we are running entirely from the battery since then (including now). Thus the battery level is slowly running down (top purple line).

The UX of the app is not amazing, hence my screwups, but I think I’m getting the hang of it now. You can see on the RHS of the chart some downwards purple and some upwards yellow. Yellow is the output of our solar panels, and for a period there it exceeded our energy usage, so the excess was dumped back into the battery. Working as intended!

You also get real time data like this:

This sort of thing can drive someone insane, because very often the numbers do not match up exactly, and you start to wonder if everything is broken. It isn’t, but you have to get your head around two concepts:

Firstly, the accuracy is not perfect. We do not have an export meter, so we are relying on some pretty rough measurements of power flows being detected by a loosely attached metal clamp on a cable. Thus you have to allow for a bit of a fuzz factor.

Secondly, the battery response is not instant, but has a slight lag. This is REALLY important, and can cause confusion. For example, say you are consuming 200w of power, and the battery is happily supplying 200w from power you saved up earlier. All is good. Then suddenly you switch a 2,000w kettle on. That power HAS to come immediately, and the battery is not set to do it, so for a few seconds, the extra 2,000w comes from the grid. The clamp detects this, tells the battery, and the battery ramps up to output 2,200w instead. Kettle finishes boiling, but the battery is still at 2,200w. The excess automatically flows to the grid. The clamp then detects that, the battery is told, and ramps down to 200w again.

That all sounds ok, but what it means is we briefly imported 2,000w from the grid, and then later briefly exported 2,000w as well. In the grand scheme of energy flows, its a minor deviation, but that is why there is not a perfectly flat line between 4am and now, with zero grid import or export. The import/export is VERY low compared to normal, but it still does happen.

If you have a smart meter in-home-display you will already be fairly familiar with this. In theory your TV will use maybe 200w for example, but you only need a helicopter to explode in an action movie, with a big loudspeaker-driving bang, followed by a loudspeaker-minimal silence, to see that the per-second power draw of almost everything is very, very variable.

I guess this is early home battery technology. Maybe eventually this sort of thing will be somehow perfect with sub-second response times. Maybe thats really hard on some components and electrically tricky. I don’t know, I’m just some guy with a battery in his cellar.


You *can* wire a battery to be a backup situation if your home loses power due to a cut. We did not do this. Initially I had been told this could be done. I was then told that it can only be done for certain circuits in the house. I was THEN told it means adding yet another fusebox, at which point I bailed. Our kitchen already looks like a nuclear power control room.

One of the undiscussed issues with home battery as backup power is that the actual output RATE from a battery is quite low, in this case 3KW. 3KW is a lot, until you have a router, 3 wireless boosters, a TV, streaming stick, home theater unit and a subwoofer plugged in… and then turn the kettle on. Basically you run out of headroom, and things will start cutting out. Home batteries are great, and can provide a lot of power, but the big old electricity grid can deliver 100 amps at 230v and thats a lot of juice. If you want to be able to power everything, including kettle + electric cooker and also charge a car and do everything else all at once, you need some serious big-ass inverter, and frankly, powercuts here are rare enough that I decided to just not care.

I will do (inevitably) another post in a week or so, when I have long term data, and the energy bills to prove it. I’m still in the exciting early days of trying to find an excuse to go get something out of the freezer (also in cellar) as an excuse to gawp at my battery…

For those curious, the total installed cost was £5600 plus vat @20% £6720.

Solar Farm update: We finally have planning permission!

I should probably call this article ‘How I managed to get planning permission granted for a solar farm after initially getting refused’. But I’m sure the algorithm will find it anyway…

Its been a LONG time since the last update. Since then, we put in an application for planning permission and… got refused. This was pretty devastating, and I was fairly convinced that was the end. I’d close down the company, write off all the money already spent and spend my days grumpily complaining to people how the system was broken. Instead… we now have permission! and here is the epic story.

When we initially made our planning application, it included about 30 separate documents detailing stuff like what trees we would plant (to compensate for the horror of building a solar farm. We wont cut down any…), which roads our trucks would go down, what the soil was like, what bird species were common nearby, what panels we would use, what angle they would be at, where the substation would be, what the cables would be like, where the cables would go, what the mounting frames were like, whether or not any great crested newts had been seen in the vicinity of the site in the last decade…

The ridiculously long timeline for the council making a decision came to the end… and then they asked for an extension. We sighed and said yes (to be clear: you have zero real choice). Then when that expired they asked for another extension. We had to say yes again. Then eventually they turned it down.

They do not even email you to tell you. You have to keep checking the site. Yes I am serious.

The reason it was turned down was because its about half a mile away from a long hill (80 miles long in fact), called Offas Dyke. This is a historically interesting hill. Its technically a historical monument. There was absolute dread that there were places on this hill, that if you looked west, you might see our solar farm, and then presumably commit suicide out of horror. Thus we got turned down.

If you think this is insane, I haven’t even told you the best bit.

All the places on the hill that have an offensive view of our site… are private property. The public cannot even go there. I guess that the theory is that people trespassing on private property might interrupt their late night burglary attempts, look west and then recoil in horror at the sight? Needless to say there were no objections to our proposal from any residents.

To make it even more ridiculous, when you stand on the site of our farm and look at the dyke, there is a fucking house on it. A private house. How the hell did that get planning permission?


We decided that this decision was clearly insane, so we resubmitted, with some changes to the layout to mitigate the horrific devastation to the eyeballs that a 1.2mwp solar farm might cause. This resulted in a response from the council, from a hired consultancy company who they asked to appraise some of our paperwork, stating that our visual impact assessment was not good enough, because, among other comments, some of the photos were of low DPI, and one of the diagrams didn’t have a North-symbol on it. Yes really. And yes, amazingly, that company can be hired as consultants to do visual impact assessments which would have been much better. Oh yes, totally legit. Nothing to see here…

This painful smile is what planning permission looks like.

So how did we get it through this time?

I made a tactical error the first time, in that I thought if we include all the information, and make a reasonable proposal, it would be granted. This was naive. That time, the decision was made by a single ‘officer’ at the council (this is called ‘delegated powers’). Basically someone reads all 30 documents, then decides to grant or refuse, and they refused. This time around, that same planning officer was not available, so we got someone else, but the second time… I actually lobbied for support.

I emailed the local Councillors for the area, I emailed the mayor of the nearest town, I emailed every environmental pressure group I could find in the county. I emailed the local MP (who ignored me…too busy tweeting about renewable energy believe it or not), I emailed the Green party, The Labour Party and local conservative Councillors. I emailed and facebooked, and tweeted at everyone I could find who was remotely in any way associated with green energy, environmental politics in the local area, or local politics of any kind. I read every news article in every local paper that ever mentioned solar farms looking for names of people that might show their support.

Politically, the most supportive were probably the Green party, but in terms of practical help, it was mostly the conservative party. By a huge margin the least help came from the Liberal Democrats, who basically said ‘good luck, but we don’t get involved’. Thanks. Duly noted.


Eventually this all came down to a one hour planning meeting 3 hours drive from my house… so I went up there and arranged to speak. I was allowed to speak for 3 minutes, and not allowed to speak otherwise. This was a ‘planning committee meeting’ instead of a single officer’s ‘delegated powers’ decision. THIS IS WHAT YOU WANT. A single person will not want to be blamed for anything bad, so they default to refusing everything. In fact, the planning officer’s recommendation was REFUSE, due to archaeological and history concerns (again). Despite this, it was clear that every single Councillor on the committee was very very strongly in favor, and said so, sometimes pretty passionately, talking about climate emergencies and how ridiculous the claims of the history and archaeology groups were. It went to a vote and we won, unanimously! with a few easy conditions (every one of which is pointless, as they are all already addressed in the 30 documents in our proposal). I think it was overwhelming enough that my 3 minute talk didn’t ‘win’ it, but I do think it helped, as I preempted some concerns and addressed them.

Thats me at the back to the left. Looks like I’m wearing sunglasses… I am not.

It took about 3 days to get the website changed so it officially said granted. Again, obviously no email to tell me… *sigh*. The next morning after the meeting, we went along to the site for the first time to meet the landowner, who is a sheep farmer. He told me he has been trying to get renewable energy on part of his farmland since 1997. TWENTY FIVE YEARS. But yeah… we are totally going to be netzero soon right?

So what next?

Now we need to get a date from the DNO (power network company) for when they can finalize their grid connection and we can provide power. I am hoping for Q1/Q2 2023 but I bet it Q3/Q4. You basically have zero say in this, despite phenomenal cost. I’ve emailed them and am awaiting a reply. The site is muddy and hilly enough to preclude a winter build anyway, so we are looking at April next year to build it as a best case scenario. It would be amazing, but unlikely, to catch much of the 2023 summer output.

I still have 3,024 solar panels in a warehouse waiting to be fitted. I wont order battery or inverters until we get a connection date. Hilariously the panels have gone up in price since I bought them, so its not a bad thing I ordered early, especially given ongoing supply chain woes. There will likely not be updates on this blog for a few months, until I start ordering things and we have proper dates. I still suspect that the hardest part is over, because planning in the UK is so evil, it makes absolutely everything else look easy…