You Should Get a Battery.

But only with a solar system.

Our home battery has stored and supplied almost exactly a quarter of our annual electricity use to the 1st October 2025. We've also exported surplus solar, and run some grid-trade during March to September too. Our system is a 5kWp solar system with 13.5kWh battery and we have an electric car (almost exclusively charged at home) & an air source heat pump. Our home is a 4-bed 1960s house (reasonable insulation) with 3 teenagers that enjoy long hot showers.

Our usage pattern isn't consistent though so this is hard to extrapolate from as we had building work going on Oct to May and more recently I've started to run the battery to charge overnight from Go instead of Agile.

However some basic maths shows that our direct solar use has saved us around £500 (1,960 x 25p), export earned us around £110 (747 x 15p) and the battery has saved us around £130 (1,332 x [25p-15p] ).

There's a difference between the solar-to-battery and battery-to-home in the two graphs as the latter includes the impact of grid-charging the battery on Go. That's given us around £260 ( [2883-1332] x [25p-8.1p] ) benefit; we've grid-charged 1,551kWh that's cost us the Go rate of 8.1p but saved us 25p if we've used that during the daytime.

Although I'm grid-charging I've not set it to discharge back to the grid as we generally end up using most of the battery stored energy in the evening; cooking, dishwasher, games machines, hot water (via the heat pump), washing machine (teenagers, remember...), etc.

The battery has therefore given us an annual benefit of just under £390 so pay back at around 10 years. Come May 2026 (a year after finishing the building work, and a longer period grid-charging) it'll be good to see if the annual figures improve by much.

And my calculation ignores any flexibility I've taken part in and benefitted from.

Battery prices have dropped significantly and still falling so this return on investment is going to get better and better. At Solar Storage Live (23-25 Sept 2025) this year I counted over 50 battery manufacturers showing a strong market.

Other than looking at the case design, here's my thoughts on what you need to look for when deciding which battery to get with your solar system.

• Size matters. Go for 10kWh plus. This is a bit subjective as it does also depend on the size of the solar install and your home consumption. We're consuming less than 10kWh daily in the summer and over 50kWh in the winter (caveat - builders and reduced insulation though) so our battery utilisation is pretty good. If you over-size the battery then it's just a waste of investment. My rule of thumb is aim for about the daily consumption and around double the kWp of the solar system.

• Whole home back-up during a power cut is now more than feasible; gone are the days of an emergency circuit. But check your house load to get this right. The inverter will have a kW limit (e.g. 5kW) so don't assume you really can run the whole house off the battery. When the grid is operational your home may draw from the battery and the grid at the same time because the battery can only discharge up to a certain amount (in kW).

• Don't get in a tangle over kW, kWp and kWh. The easiest way to understand this is the water analogy; kW is like the flow of water and kWh is like the volume of water in an hour. 1 kW flow rate over an hour provides 1kWh of electricity. Your home consumption (kW) will be varying all the time as per the fourth graph above; I find it useful having really granular data like this so check what each system provides. kWp means the peak flow of electricity that your solar panels can generate in maximum sunlight - you want to get the best kWp solar panels (at least 0.4kWp, or more commonly referred to as 400Watts peak, per panel now). Your inverter kW isn't always specified to the same kWp value - ours is actually clipped at 3.5kW. This is ok as there's really only a couple of weeks in July & August that we might reach the maximum generation (and of course, that mostly gets exported) but the rest of the year the high kWp potential means you'll still get plenty of generation. And you want your battery, as above, probably over 10kWh.

• If you can, get one of the latest systems that have the solar inverter built into the battery as that's less tech to install so makes for a cheaper, quicker, easier and more space efficient install. Do install it all outside - most of the systems at the show could be installed outside, but check.

• It's got to be online; which realistically they all are now. That means firmware updates automatically delivered, being able to check the health, temperature and the all important SoC (State of Charge) as well as all the metrics as I've described above.

• But most important of all, being online means being accessible to smart tariffs and flexibility - look out for the ever growing compatibility lists. This is where there will be a lot of growth in participation so really quiz the manufacturers as the compatibility lists are fairly short at the moment. Don't assume the manufacturer is right on their claims - check with us.

Something that's previously only been available from a very small number of unknown brands online is systems called "Balcony Storage" and there were three or four manufacturers demonstrating these this year for the first time.

It's a bit of an odd one hanging a solar panel on your balcony and having a battery sat there. The supplied energy is going to be a bit weak as you're hanging it vertically, you've got no choice over direction it's facing and you may well get shading issues from other apartment blocks etc. So the sales pitch is a bit of "well it's better than nothing" kind of thing. Coupled with these is a small battery pack that can sit on the balcony and plugged in (assuming you have an accessible socket on the balcony. A UK plug & socket is design for electricity to flow in one direction but now you've got electricity flowing the other way - feels a safety issue in the making but I've not looked into how safe that is. I also query putting a few kW back over the socket ring main when it was never really designed for that. A proper battery system is often wired right next to the smart meter separately from the main consumer unit. So use these balcony things with caution. For £s per kWh they're also over-priced too.

Sodium is coming. Eleven Energy are the first I've seen and this was the second event I've seen them exhibit at. Supported by Midsummer, they should be successful. The obvious benefit is getting away from the dubious sourcing of lithium, cobalt and other materials. Sodium is simply just salt so as well as the solving the source of materials we should see costs drop even further. But it's drawback is space - per kWh they're physically larger in size. Not an issue if you've got the outdoor space for a battery.

For the eagle eyed, GivEnergy had their new Home Energy Management System (HEMS) device on display for the first time. Several of the manufacturers talked about the integration of other energy tech in the home - car chargers, heat pumps, thermostats, hot water, etc. Integration can be either cloud-to-cloud (Intelligent Octopus obviously does this) or with in-home hardware devices such as GivEnergy's device. Time will tell which way is going to work out best, but I think it'll most likely depend on two use cases as I think these clash a little:

• To manage (balance) the grid it's all about sending flexibility signals and so increase/decrease grid energy at any moment.

• To manage your home it might be a desire to stay off-grid as much as possible so redirecting solar energy dynamically between the car, home battery, heating, hot water and anything else.

So does one participate in the grid or go a bit more selfish. A middle-path is microgrids (see other announcements on that) but that can wait for another article. Keele University is one of my favourite references on microgrids.

The cost of wind energy curtailment in 2025 tipped past £1Bn today (1st Oct 2025) yet studies, such as from Imperial, show the UK could save between £500m and £3.5Bn annually by investing in storage. Instead of paying to turn energy off we should be storing it.

Octopus Energy is a founding member of the Energy Storage Association which is a new industry organisation created to drive the storage market.

"The Energy Storage Association (UK) aims to drive innovation, investment, and policy that unlock the full potential of storage solutions, ensuring a cleaner, more equitable energy future. It will represent all forms of energy storage in our homes, buildings and on our energy grids, including but not limited to electrical batteries, thermal storage, pumped hydro storage and hydrogen-based storage from green hydrogen and green ammonia".

As per my visit to the Solar Storage event, with so many manufacturers available now there's a risk of a minefield of issues around standards compliance, safety, security, etc. Organisations such as MCS, NICEIC, NAPIT, ENA etc provide all the necessary standards and regulations for safe electrical installation (make sure you use a registered installer as well as certified kit). Less well covered is connectivity, control, integration, cyber security which is where I'm particularly interested in working on with the ESA.

This isn't to cause any doubt or uncertainty on any of the manufacturers; if you've seen a brand frequently referenced (such as our IO-Flux compatibility) you can rely on it. The more unknown brands (including some in my photo board) may be less so.