What Size Solar Battery Do I Need?

What size solar battery do I need? For most homes, match the usable capacity to your evening electricity use, after the sun goes down. Add up what you use from early evening to bedtime, then pick a battery close to that figure. Bigger is rarely better, because you pay for storage you cannot fill.

The rule of thumb

Start with the hours when your panels make little or nothing. For most families that means about 5pm to midnight, because that is when lights, cooking and TV all run. Your battery has to cover that window.

Your smart meter or energy app shows how much you use in those hours. Read a typical week, not one day, since single days mislead. Then pick a battery with a usable capacity near that average.

Usage varies by season, so the winter figure matters most. In winter your panels make less, therefore the battery refills slowly. In summer it fills early, so a smaller battery still does the job.

A worked example

Take a household that uses 7 kWh between 5pm and midnight. These figures are assumptions for illustration, not a quote. Your own numbers will differ, because each home lives differently.

A battery with 7 kWh of usable capacity would cover that evening in full, if it starts the evening full. However, the system also needs to fill. A 4 kWp array can fill 7 kWh on a good summer day, but not on a dull winter one.

So the question becomes two sums, not one. How much do you use after dark, and how much surplus will the panels make? Thus the smaller of the two sets the useful size. Our guide to how many solar panels you need covers the second sum.

Sizing by household type

The table below gives example evening loads. They are assumptions to show the method, while real averages vary a lot.

Always replace these examples with your own meter data, because the best size is the one your own numbers support. Our look at the solar and battery package shows how installers bundle the two.

Usable versus nominal capacity

This is the most common trap, since quotes often mix the two figures. A battery has a nominal capacity, which is the total its cells can hold. It also has a usable capacity, which is the part the system lets you draw on.

The gap exists because a battery keeps a reserve to protect the cells. Therefore a battery sold as 10 kWh may give you less than 10 kWh in use. Some energy also turns to heat when it goes in and out.

Compare quotes on usable kWh only. If a quote lists one figure without saying which, ask, because the answer changes the sums. Our guide to the best solar batteries in the UK lists both figures where makers publish them.

A five-step way to size your battery

You can do this on paper in a few minutes. Follow the steps in order, and keep a note of each figure.

1. Read your evening use from your meter or app, from 5pm to bedtime.
2. Check your winter figure, since it is the hardest to meet.
3. Estimate your daytime surplus from the array, using your installer’s model.
4. Take the smaller of the evening use and the surplus.
5. Convert that to usable kWh, then add a small margin for losses.

The result is a target, not a final answer. An installer will refine it with your real data. Even so, arriving with a figure helps you spot a quote that is far too large or too small.

Mistakes that skew the size

Three errors crop up often. The first is sizing from the daily total instead of the evening. Daytime use is covered by the panels directly, so the battery never sees it.

The second is ignoring future changes. An EV, a heat pump or a home office all raise evening demand. Therefore it is worth asking whether you plan any of these within a few years. A system that can add a second battery later may suit better than one huge unit now.

The third is trusting averages. A household of four and a couple can both be “average” on paper. As a result, use your own meter data and not a national figure.

The cost of oversizing

A bigger battery costs more, yet it does not always save more. If it never fills, you pay for cells that sit idle. In contrast, a battery that is too small runs out early and leaves you buying peak-rate power.

Oversizing also adds weight and bulk, which matters for siting. Furthermore, a BSI specification, PAS 63100, covers where to place a battery. It prefers outdoors or a garage where practicable, and not in bedrooms or on escape routes.

Home batteries are VAT-free (0%) until 31 March 2027. Because the rate is due to revert to 5% from 1 April 2027, the timing of an order can matter. Even so, never buy a larger battery only to use up a tax relief.

When tariffs change the answer

A time-of-use tariff can shift the sums, because cheap hours change the maths. If you charge from the grid overnight, you no longer rely only on solar surplus. Intelligent Octopus Go listed a cheap rate of 7.6p per kWh in October 2026, for eligible EVs and chargers.

In that case, a larger battery can make sense, because grid charging fills it reliably in winter. However, you need a tariff that suits your battery and a smart meter. Our guide to how home battery storage works explains grid charging in more detail.

Export matters too. Under the Smart Export Guarantee, suppliers pay for power you send to the grid. However, a battery that soaks up your surplus can reduce those payments, so weigh both sides. Our page on smart energy monitoring helps you see the real split.

Sizing in four cities

Your local sun and housing change the picture. In Bristol, the EU’s PVGIS tool models about 4,092 kWh a year from a 4 kWp south-facing array. That is a strong result for the UK, so summer surplus can be generous.

Terraces make up 32.1% of Bristol households, so ask early about roof space and siting. See our page on solar battery installers in Bristol.

PVGIS models Leicester at about 3,923 kWh a year. Semi-detached homes are the largest group, at 36.4% of households. Therefore, a typical suburban semi is a common starting point for sizing. Our page on battery installers in Leicester has more detail.

PVGIS models Coventry at about 3,946 kWh a year, and 39.9% of its homes are terraced. Terraces can limit where a battery goes, so ask about siting early. Our page for solar battery installers in Coventry explains how we match you.

PVGIS models Derby at about 3,897 kWh a year, and 24.1% of its homes are detached. Detached homes often give an installer more choice on siting and size. Our page on battery storage in Derby covers local points.

These figures are for city-centre coordinates, not your roof. They show modelled output for a 4 kWp array at a 35 degree tilt, facing south, with 14% losses. Your own roof may do better or worse, so treat them as a guide.

Getting a sized quote

Give your installer your meter data, your evening use and your roof details. A good quote shows the usable capacity, the inverter rating and the sizing logic. Compare at least two.

Check the installer holds MCS certification for batteries as well as panels, because the standards differ. Solar Base Energy is a free matching service, not an installer. We match you with an MCS-certified installer after you send one short form. You can read more about our solar and battery bundles and our battery storage service before you start.

Meanwhile, our guide to the Tesla Powerwall 3 in the UK shows how one large unit compares.