Most UK homes with solar land on 5 to 10 kWh of usable battery storage. What decides your number is not your roof. It is how much electricity you use in a day, how much of that you can move out of expensive hours, and which single job you are buying the battery to do. Size it for one job and the answer is obvious. Size it for three and you overbuy.
To turn that into a figure for your house, put your daily usage into the solar battery size calculator, which answers what size solar battery you need from your own numbers. This page is the thinking behind that result: what actually drives the size, how the battery connects to your system, what it costs, and which specifications change the answer.
What actually decides the size you need
Five things move the number, and they do not move it equally. In rough order of how much they matter:
- Your daily electricity use. This sets the ceiling. A home getting through 8 kWh a day cannot usefully store more than about 10 kWh, because the extra never gets emptied. Ofgem’s 2026 typical figures are 1,600 kWh a year for a low user, 2,500 for medium and 3,800 for high, which is 4.4, 6.8 and 10.4 kWh a day.
- How much of the day you are at home. This decides how much solar you would otherwise export. A household out from nine to five wastes most of its midday generation and gets the most from storage. A household home all day already consumes it and gains less.
- Your tariff, and whether you are arbitraging. If you charge from a cheap overnight rate as well as from solar, the battery earns twice a day rather than once, and a bigger unit pays. If you are on a flat single rate, only the solar surplus fills it and there is no point buying capacity you cannot fill.
- Your array size, if you have one. A 5 kWp roof throws off 3 to 5 kWh of midday surplus on a clear summer day. That surplus is what a solar-only battery lives on, so it caps the useful capacity in the same way daily use does.
- What the marginal kWh is actually worth. The last kWh of capacity you buy is the one that fills least often. Capacity you fill 300 days a year pays back. Capacity you fill in July only, does not.
That last point is why sizing for the winter peak is a mistake people make in both directions. Size for the load you shift most days of the year, then check the number against a real quote.
Capacity: cover the day, keep 20% back
Capacity has to cover your daily usage with headroom, because taking a lithium iron phosphate pack below roughly 20% state of charge shortens its life. If you get through 10 kWh a day, you want at least 12 kWh of nameplate capacity to actually use 10.
My own house runs between 6 and 16 kWh a day depending on cooking, laundry and hot water, which is a wider spread than most sizing advice admits to. I sized for the middle of that range, not the top.
Size for the whole day rather than just the evening if you intend to charge on a flexible tariff in winter. From November to February the solar contribution is small, and a battery big enough to carry the house from a cheap overnight window is doing a completely different job from the one it does in June.
Peak power: the number that decides whether it can run the house
Capacity tells you how long. Power tells you whether it can run the appliance at all. Three figures have to line up: your peak simultaneous draw, the battery’s continuous discharge rating, and your inverter’s AC output.
Mine works out like this. Oven at 2 kW, washing machine at 1 kW, and base load from fridge, router and chargers at roughly 500 W. That is 3.5 kW. A 5 kW inverter covers it with room, and the battery needs at least 3.5 kW continuous or the shortfall comes off the grid regardless of how full the battery is.
A 5 kWh battery limited to 0.5C delivers 2.5 kW. It will run lights, fridge and a television. It will not run an oven on its own, and it will not go near an electric shower. Check the continuous discharge figure before the capacity figure if backup matters to you.
AC-coupled or DC-coupled: which one can you actually fit?
This is the decision that determines whether you can add a battery to the solar you already own, or whether you have to replace working equipment to do it. It comes down to one question: where does the electricity get converted from DC to AC, and how many times?
DC-coupled puts the battery on the DC side of a hybrid inverter. Solar charges it directly, with no conversion in between, and one conversion on the way out to your sockets. It is the efficient path and the simpler installation, one box instead of two. The catch is that it needs a hybrid inverter, so if your existing string inverter is working fine, going DC-coupled means throwing away a working component.
AC-coupled gives the battery its own inverter and connects it to your household AC wiring, the same side as your sockets. Your existing solar inverter is untouched. The cost is conversions: DC from the panels to AC, AC back to DC to charge the battery, DC to AC again to use it. Each pass loses a few percent. The Tesla Powerwall works this way, and it is why you can add a battery without disturbing a Feed-in Tariff installation.
| DC-coupled (hybrid) | AC-coupled | |
|---|---|---|
| Conversions solar to battery to home | One | Three |
| Typical round-trip efficiency | 95-97% | 85-90% |
| Your existing inverter | Replaced by a hybrid | Kept as it is |
| Boxes on the wall | One | Two |
| Best for | New systems, or an inverter due for replacement anyway | Adding storage to solar that already works |
The practical rule: if your inverter is healthy and you just want storage, AC-coupled is the right answer even though it is the less efficient one, because replacing a working inverter costs more than the efficiency gap will ever return. If you are building from scratch, or your inverter is old enough to be on the replacement list, go DC-coupled.
One thing neither route gives you automatically is backup during a power cut. Islanding, where the system disconnects from the mains and runs the house on its own, needs the right inverter and the right wiring at install. Most hybrid inverters support it through an emergency power supply output, and all-in-one AC units like the Powerwall handle it natively, but it has to be specified and wired deliberately. It is not a side effect of owning a battery.
What a battery is actually worth, and why
The whole case is one number: the gap between what a kWh costs to buy and what it earns to sell. At the July to September 2026 price cap you buy at 26.11p/kWh. Selling the same kWh back earns 12p on Octopus Outgoing, the highest flat rate I could confirm first-hand on 24 July 2026, and as little as 3 to 7p on the weaker Smart Export Guarantee tariffs. Every kWh the battery keeps at home instead of exporting is worth the difference.
That is also the reason the case gets stronger as your array gets bigger. A 5 kWp roof spills 3 to 5 kWh on a clear summer day. Without storage, that leaves at the low number and comes back at the high one every evening. The larger the array, the more expensive it is to leave the battery out.
The chart above is my own meter data from June 2023. The battery went in on the 5th and grid purchases fell to near zero for the rest of the month, with the solar capacity unchanged.
The part I did not expect was where the benefit came from. A big share of it was covering daytime peaks rather than the overnight discharge. When the panels are producing a steady 1.2 kW and I switch on a 2 kW oven, the battery covers the 800 W shortfall instead of the grid. That peak-shaving happens dozens of times a day and never shows up in a simple midday-surplus calculation.
How I went from 10 kWh to 15 kWh
I fitted 10 kWh of LFP, two Fogstar 5.12 kWh units, in June 2023. By autumn 2024 the metered data was unambiguous: on high-use days I was down to 20% state of charge by 10pm and buying from the grid until midnight. I added a third module to the existing rack, taking usable capacity to about 15 kWh. One afternoon, no change to the inverter.
The lesson is about the buying order rather than the number. Modular DC batteries let you start conservatively and expand when your own data tells you to, which is a much better position than guessing high on day one. The Fogstar rack setup is covered separately, but the principle holds for any stackable 48V LFP system.
One caveat from living with it: my batteries are in an unheated garage, and below about 5 degrees the management system slows charging to protect the cells. That is normal LFP behaviour rather than a fault, but it means you get less usable capacity in January and February. Factor it in if your battery is going somewhere cold.
Storing the surplus
On a sunny day the panels routinely make more than the house is using. Without a battery that surplus leaves. With one, it charges through the middle of the day and comes back out in the evening, and your inverter handles the switching without you touching anything.
Batteries on a zero-export system
Some homes are configured not to push anything back to the grid, because there is no export agreement or the network operator has not approved one. Without a battery, the inverter throttles its output down to match household demand, so the panels spend the middle of the day deliberately underproducing. A battery fixes that by acting as a load: it absorbs the surplus and lets the inverter run flat out. On a zero-export system, storage is not an optimisation, it is what makes the array work.
What a home battery costs in the UK
I am not going to give you a cross-brand price-per-kWh league table, because I cannot build one honestly. When I tried to verify the UK market on 24 July 2026, exactly one manufacturer published a first-party price I could check directly. What I can give you are the two ends of the range, and both ends stand up.
| DIY component route | Installed system route | |
|---|---|---|
| What you are buying | The LFP pack on its own | Battery, inverter, labour, network operator registration, any consumer-unit work |
| Price I can verify | Fogstar Energy 16.1 kWh at £121 per usable kWh, about £1,950. The 32 kWh pack works out at £109 per kWh, about £3,490, but was out of stock until September. Checked on fogstar.co.uk, 24 July 2026. | Tesla Powerwall 3, 13.5 kWh, roughly £700 to £780 per kWh installed. This is an installer-quoted range, not a manufacturer price: Tesla’s own UK page would not load for me. |
| Still to add | A hybrid inverter at roughly £770 to £1,050, labour at £500 to £1,500, and a rack or enclosure | Nothing. That is the total. |
| Who it suits | Anyone comfortable specifying their own inverter, or already running a hybrid with spare battery ports | Anyone who wants one company accountable for the whole system |
The gap between those two columns is roughly six to one per kWh, and it is the real story in UK battery pricing. Some of it buys you genuine things: an integrated inverter, a single warranty, backup that works out of the box, and nobody to argue with when a component fails. Some of it is just the price of not doing it yourself.
What I could not price, and why I am telling you. Pylontech appears only through resellers, at around £189 per kWh, which is a reseller figure rather than a manufacturer one. Puredrive quotes on request, and the £3,800 to £4,800 figure circulating for it is an installed-system estimate that keeps getting repeated as though it were a component price. Fox ESS shows up only on aggregator sites. Solis and Growatt have no findable UK price at all, and Deye has no listing in pounds. If you see a tidy table comparing all of these on price per kWh, most of it is inferred.
Which capacity band you are in
Capacity bands are more useful than a model list, because the model that fits changes every few months and the band does not.
| Usable capacity | What it covers | Typical situation |
|---|---|---|
| 3.5-5 kWh | An evening and overnight for a small household | Flat or small terrace, low daily use, or a first module you intend to expand |
| 8-10 kWh | A full day for a typical three-bed | The most common band, and where most people should start |
| 13-16 kWh | High consumption across the whole day | Heat pump, EV charging, electric heating, or serious overnight tariff arbitrage |
A note on GivEnergy, because it appears in a lot of older buying advice including some of ours. GivEnergy went into administration on 9 April 2026, ceased trading, and made its staff redundant. Hardware warranties and app and software support are not being honoured: the administrator reported over £17m owed to unsecured creditors against roughly £114,711 of realisable stock, so there is no realistic prospect of a warranty claim being met. Units already installed keep working on local control. If you own one, your routes are your original installer or the administrator at info@cb-br.co.uk. Do not buy remaining stock on price.
Do not compare a unit price with an installed price
This is the single most common way people misread battery pricing. An installed price covers the battery, the inverter if it is an AC-coupled unit, the electrician, the network operator registration and any board work. A unit price is the box on its own.
A £1,950 Fogstar pack is not a £1,950 system. Add a hybrid inverter and installation and you are realistically at £3,200 to £4,500 for the same storage, and that is the number to put next to an installer’s quote.
VAT is the other trap. The 0% rate on battery storage runs to 31 March 2027, but it is relief on installation: supply-and-fit work in a home. A pack you order online and fit yourself is standard-rated, so do not model a DIY build as though 20% comes off. The grants and incentives guide covers what the relief does and does not reach.
Chemistry: why everything sold here is LFP
Lithium iron phosphate, written LFP or LiFePO4, is the chemistry in effectively every home solar battery sold in the UK in 2026, and the reasons are practical rather than fashionable. It runs 6,000 or more cycles, it is more tolerant of the partial charging that solar actually produces, it has a lower thermal-runaway risk than the higher-density lithium chemistries, and it has the best cost per cycle. Lower thermal-runaway risk is not zero fire risk, and it should still be sited and ventilated properly.
The chemistries you can safely ignore
Lead acid, both flooded and sealed, still turns up in off-grid builds and is cheaper per kWh up front, but at 300 to 1,000 cycles the cost per cycle is far worse than LFP and it is effectively obsolete for grid-connected home storage. Nickel-cadmium handles temperature extremes but costs more and is environmentally poor. Nickel-metal hydride is denser than nickel-cadmium but is not a mainstream home option. None of the three is worth specifying for a UK house in 2026.
The specifications that change the answer
Battery datasheets are dense, and most of the numbers on them do not change a buying decision. These three do.
C-rate: how fast it can deliver, not how much it holds
Capacity in kWh tells you how much energy is in the pack. The C-rate tells you how quickly it can come out. 1C means the whole capacity in one hour. 0.5C means two hours, and a 5 kWh battery at 0.5C therefore delivers a maximum of 2.5 kW.
That is the difference between a battery that runs your house and one that runs your lights. Many UK batteries are limited to 0.5C to protect cell life, which is a legitimate engineering choice, but it means a single 5 kWh module cannot cover a 3.5 kW household peak on its own. Two modules in parallel can, because parallel modules add their power ratings together as well as their capacities.
At the top end, the Tesla Powerwall 3 is rated at 13.5 kWh with 11.04 kW of output, which is roughly 0.8C and is why it copes with heat pumps and EV chargers that would flatten a typical residential pack. Treat the 13.5 kWh figure with mild caution: the published sources are inconsistent about whether it is nominal or usable. The Enphase IQ Battery 5P sits at the other end of the range at 5.0 kWh usable and 3.84 kW, which is a genuinely high power-to-capacity ratio for a small unit.
Low voltage or high voltage
Low voltage means around 48V or 51.2V, and it is the standard for UK homes. Modules parallel up, the kit is widely stocked, installers know it, and it is straightforward to expand. My own system runs at 51.2V and adding to it has been uneventful.
High voltage systems connect in series instead, moving the same energy at lower current, which means less heat loss and thinner cabling. That matters for large houses, three-phase supplies and commercial installations. For a typical three or four bedroom single-phase home, low voltage is the practical choice and there is no efficiency argument strong enough to change it.
What a battery management system does
Batteries store and release DC. The inverter handles the conversion, but it is the battery management system inside the pack that decides what the inverter is allowed to do. Its job is to keep every cell inside its safe operating range, and it does that in four ways: balancing the charge across cells that drift apart, slowing or stopping charging at temperature extremes, tracking state of charge and state of health so the inverter knows what it is working with, and cutting the pack off entirely if it detects a fault such as overcharging.
Two consequences matter to you as an owner. The first is that a good management system adds years to the pack, which is most of why LFP warranties run to ten years. The second is that its protective behaviour looks like a fault when it is not: charging slowing to a crawl on a January morning in a cold garage is the management system doing exactly its job.
Charging from the grid, not just from solar
A battery does not care where its electricity came from. On a time-of-use tariff you can fill it overnight at a cheap rate and run the house from it through the expensive part of the day, which is a second income stream on top of storing solar and is what makes larger capacities pay.
Be careful with the rates you plan around. Octopus Go and Intelligent Octopus Go are different products with different windows, and it is worth naming which one you mean. Standard Go runs 00:30 to 05:30 at 9.5p/kWh and Intelligent Go runs 23:30 to 05:30 at 8p/kWh, and both of those overnight rates are set nationally: I read the same figures off Octopus’s own pages in two distant postcodes on 24 July 2026. What does vary by region is the day rate and the standing charge, so check those for your own address. Intelligent Go also needs a compatible electric car or charger to qualify, which rules it out for a lot of battery owners. The gap between an off-peak rate and the 26.11p/kWh cap rate is the thing you are buying, and it is worth confirming before you size around it. Automating winter charging covers how to actually run it.
Where it can physically go
The IP rating decides the location, and it is easy to check before you fall in love with a model. IP20 units, which includes most rack-mounted packs such as Fogstar and Pylontech server racks, are indoor only and need a dry, reasonably stable space. IP55 to IP65 units tolerate rain and dust and can go in a garage, carport or on a sheltered wall. IP66 units are built for full exposure.
Whatever the rating, LFP works best between 5 and 35 degrees. Below 5 the charge current drops to protect the cells, so an unheated garage costs you charging speed exactly when daylight is shortest. Some premium units include cell heating for this reason.
Which path fits you
Tariff arbitrage only Fogstar rack review
What I actually run Powerwall vs GivEnergy
Premium all-in-ones (read the GivEnergy note below first) Battery size calculator
Size from your usage DIY home battery UK
What you can and cannot DIY
Battery decision tree
- Do you already have solar? No, so start with a battery without solar, where the whole case is overnight tariff arbitrage. Yes, so continue.
- Is your inverter already a hybrid with free battery ports? Yes, so add a compatible DC module. No, so choose between an AC-coupled retrofit and replacing the inverter with a hybrid.
- What is your daily use? Under about 6 kWh a day puts you in the 5 kWh band. Around 8 to 12 kWh a day puts you in the 8 to 10 kWh band. A heat pump or EV puts you in the 13 to 16 kWh band. Check it against the battery size calculator.
- Do you want one company accountable for the whole system? Yes, so compare the closed all-in-one systems carefully on total installed cost. No, so a modular LFP pack behind a hybrid inverter is usually better value.
- Are you planning to do the work yourself? Read the legal and safety boundary before you buy anything: DIY home battery UK and connecting batteries.
How sizing plays out on three real roofs
- 4-bed south-facing: 16.1 kWh behind a 5 kW hybrid. The large case, where the capacity earns out on overnight arbitrage as much as on solar.
- 3-bed east and west: 5.12 kWh behind a 3.6 kW hybrid, bridging the midday dip between the morning and evening generation peaks.
- 3-bed north-facing: storage is not optional here. Without it the economics of the array do not work at all.
Frequently asked questions
What batteries do I need for solar panels?
An LFP battery of 5 to 10 kWh is the right starting point for most UK homes. LFP runs 6,000 or more cycles and handles the partial, variable charging that solar produces. Connect it through a hybrid inverter if you are building a new system, or through an AC-coupled battery inverter if you are retrofitting to solar that already works. Size for your full daily consumption rather than your evening use, and keep 20% headroom.
How many batteries for a solar system?
One or two modules gets most homes to 5 to 10 kWh of usable storage. The count depends on the module size, and modules parallel up so you can add later. A three-bed averaging 8 kWh a day wants roughly 10 kWh, which is two 5.12 kWh modules, one 10 kWh unit, or a rack you expand. I started with two and added a third eighteen months later, once the data showed I was hitting 20% state of charge before midnight.
How much do solar batteries cost in the UK?
The only UK price I can verify first-hand is Fogstar Energy at £109 to £121 per usable kWh for the pack alone (fogstar.co.uk, 24 July 2026), which is about £1,950 for 16.1 kWh. Add roughly £770 to £1,050 for a hybrid inverter and £500 to £1,500 for installation. At the other end, an installed all-in-one such as the Tesla Powerwall 3 is quoted at roughly £700 to £780 per kWh installed. Most other brands cannot be priced first-party at all, which is why this page has no cross-brand comparison table.
Do solar panels need a special battery?
No proprietary battery is required, but it has to be compatible with your inverter. LFP packs talk to hybrid inverters over CAN bus or RS485, and most current LFP batteries work with the common hybrid inverter brands, though you should confirm the specific pairing on the inverter manufacturer’s compatibility list rather than assuming. What you cannot do is hang a DC battery off a plain string inverter: that needs either a hybrid inverter or a separate AC-coupled battery inverter.
Can I add a battery to my existing solar system later?
Yes, and you do not have to replace your inverter to do it. An AC-coupled unit connects to your household AC wiring and leaves your solar inverter alone. If your inverter is already a hybrid with spare battery ports, you simply add modules, which is what I did. The AC-coupled and DC-coupled comparison above explains which route your system allows.
How long do solar batteries last?
LFP packs are typically rated for 6,000 cycles to 80% of original capacity, which is 16 to 17 years at one full cycle a day. Real household cycles are usually partial, which stretches that further. Warranties are normally ten years, though a warranty is only worth the company behind it: see the GivEnergy note above. Lead acid manages 300 to 1,000 cycles and older NMC lithium packs 3,000 to 4,000. My own LFP units show no measurable capacity fade after two and a half years.
What to do next
If you take one thing from this page, make it the sizing order. Decide the single job the battery is for, work out the daily load that job implies, add 20%, and check the continuous power rating covers your peak. Everything else on a datasheet is detail by comparison.
The practical next step is to run your own numbers through the battery size calculator, which will tell you what size solar battery you need for your usage, then take that figure into a quote rather than the other way round. If you are heading for the DIY route, connecting batteries covers how to reach the capacity and power rating you settled on.
