Battery sizing means matching storage to how much spare daytime power you have and how much you use after dark. It is not a function of roof size, and it is not a function of panel count.
The clearest way to show that is with real jobs. Below are three of our published installations, chosen because they land at three different battery sizes for three different reasons.
Before you read the numbers, remember that a nominal figure is not what you get out. Read usable versus nominal battery capacity for why.
Key Points
- Two homes with the same array can need different batteries. Two of our published Lewes jobs have 14 panels each, one with 5 kWh and one with 10 kWh.
- The question is how much you use in the evening, not how much the roof makes at noon.
- A battery you never empty is capacity you are not using.
- A battery that empties by nine every evening is working, but a larger one might have carried you further.
- Generation well above household use points at export, not automatically at more storage.
- Generation close to or below household use is where storage does the most work.
- The battery capacities on our published work are 5 kWh, 5.76 kWh, 9.32 kWh, 10 kWh and 20 kWh. The 10 kWh Anker is the most common by a wide margin.
Common Misconceptions
"Size the battery from the panel count." No. Two published Lewes jobs have 14 panels each and different batteries, because the households differ.
"A bigger battery always stores more of my solar." Only if you use what it stores. Otherwise it sits full.
"A battery makes me independent of the grid." It does not, and no size does in a British winter.
"If my system makes far more than I use, I need a huge battery." Often the opposite. Low evening use means a smaller battery empties properly.
"10 kWh is the standard size." It is the most common size on our published work, not a default. Three of our published jobs are not 10 kWh.
Real-World Advice
| Project | Town | Panels | Array | Battery | Design generation against household use |
|---|---|---|---|---|---|
| 14-panel-solar-installation-polegate | Polegate | 14 | 6.58 kWp | 5 kWh Anker | Close to four times what the home uses |
| 12-panel-solar-installation-hastings | Hastings | 12 | 5.52 kWp | 10 kWh Anker | Almost the same over a year |
| 21-panel-solar-installation-ninfield | Ninfield | 21 | 9.66 kWp | 20 kWh Anker | Slightly less than the home uses |
Example one: a big roof and a small user
Our Polegate 14 panel job is 6.58 kWp with a 5 kWh Anker battery. See the Polegate case study. The installer's note calls it the biggest gap we have published between power made and power used. The design makes close to four times what the household was expected to get through.
That looks like a case for a huge battery. It is not. When a home uses very little, the evening it has to cover is small. A 5 kWh battery covers that evening, and the rest of the day's power leaves the house on an export tariff. A bigger battery would sit part full most nights.
Example two: generation and use in balance
Our Hastings 12 panel job is 5.52 kWp with a 10 kWh Anker battery. See the Hastings case study. The installer's note puts what it makes and what the house uses at almost the same level over a year.
This is where storage earns its place. The note explains why. There is less spare power to export and buy back later. So holding the middle of the day for the evening does more here than on a roof with a large surplus. The same reasoning gave the 13 panel Hastings job next door the same 10 kWh battery.
Example three: a high user with a large roof
Our Ninfield 21 panel job is 9.66 kWp with 20 kWh of Anker storage, the largest battery on anything we have published. See the Ninfield case study. The installer's note says the design showed it making slightly less than the household uses in a year.
That is the sentence that justifies the size. There is enough demand in that house to empty a battery that big. Put 20 kWh on the Polegate house above and it would rarely be used. Put 5 kWh here and it would be flat every evening.
UK-Specific Considerations
Winter is the limit. A UK home does not run through December on solar and storage alone, whatever size the battery is. Sizing is about covering evenings across the year, not about leaving the grid.
Cheap overnight tariffs change the arithmetic. If you can buy power cheaply at night, the battery is useful even in the months when the panels contribute little. That is a tariff decision as much as a hardware one.
What sits behind your meter matters too. Around Hailsham and Polegate we often see a car charger, a hot water tank or a heat pump on the same supply. That changes both how much you use and what the battery should power first, which is a setting we agree at the start.
Grid rules count the battery. Storage is included when the network operator works out your connection route. See UK Power Networks and G99 timescales.
When This Isn't Right For You
Sizing on published examples alone will not fit your house. These three are illustrations of the reasoning, not templates. Your own annual consumption and your own evening pattern are what the sizing should be built on.
A battery is also the wrong first purchase if the array is not right yet. Fix generation first, then size storage to it.
Before You Spend Money
Get your annual electricity use in kWh from a recent bill. This is the single most useful number.
Work out roughly how much of that you use after dark. That is what the battery has to cover.
Ask your installer to show what the design expects the panels to make, against your use, for a full year.
Ask what the usable capacity of the quoted battery is, not just the nominal figure.
Ask what happens if you add a car or a heat pump later, and whether the system can be expanded.
Ask whether a hybrid inverter now would let you add or extend storage later.
Frequently Asked Questions
Q: How do I know what size battery I need?
A: Start with your annual electricity use and how much of it happens in the evening. Then match usable capacity to that, not to the roof.
Q: Is a bigger battery always better?
A: No. A battery you do not empty is capacity you are not using.
Q: Can I add more storage later?
A: Often yes, depending on the inverter and the battery family. Ask before you buy, because it is easier to plan for than to retrofit.
Q: What size do most of your customers have?
A: Across our published work, 10 kWh is the most common. It is not a default, and several jobs are smaller or larger.
Q: Will a battery cover a power cut?
A: Only if the inverter has an emergency supply output and it is set up for it. Ask for it at the quote stage.
Summary
Battery size follows the household, not the roof. Our published Polegate job has a big array, a very low user and a 5 kWh battery. A small evening does not need a large store. Our published Hastings job has generation and consumption in balance and a 10 kWh battery, which is where storage does the most work. Our published Ninfield job has a high user and 20 kWh, because there is enough demand to empty it. Start with your own annual consumption, work out your evening, and size on usable capacity.