For most homes in England with a 4kW to 4.4kW solar system, the right solar battery size is 5 to 10kWh of usable capacity. The quickest guide is your evening and overnight use: a family home using 3,500 kWh a year uses roughly 5.8 kWh a day after the panels stop producing, so a battery of about 5 to 6kWh fits. If you own an EV, have a heat pump or want to charge cheaply overnight, 10kWh or more starts to make sense.
- 5 to 10kWh usable capacity suits most homes with 4 to 6kW of solar
- 2,500 kWh a year is Ofgem's typical household electricity use from July 2026
- March to September is when a 4.4kW system in Birmingham makes enough spare solar to fill a 5.8kWh battery on an average day
- 89% round-trip efficiency on a current battery such as Tesla Powerwall 3, so about 11% of stored energy is lost
Getting the size right matters because battery capacity is expensive. Too small and you still export cheap solar in the afternoon and buy dear electricity at 7pm. Too big and you have paid for capacity that sits empty most evenings. Here is how to find your number.
The three numbers that decide battery size
- Your evening and overnight use. This is the electricity you use between roughly late afternoon and the next morning, when solar is not covering you. It sets the useful ceiling for a solar-only battery.
- Your daily solar surplus. The solar left over after your daytime use. A battery bigger than your usual surplus will rarely fill from the panels alone.
- Your tariff. If you can buy electricity cheaply overnight, a battery can fill from the grid as well as the roof, especially in winter. That makes a bigger battery worth more.
You can get the first number from your smart meter app or your supplier's online account, which usually shows half-hourly use. If you cannot, the table below gives a starting point.
Quick sizing guide by household
These bands are our rule of thumb, assuming about 60% of your electricity is used outside solar hours. Ofgem's typical household figure is 2,500 kWh a year.
| Household | Yearly use | Daily use | Evening and overnight use | Suggested usable battery size |
|---|---|---|---|---|
| 1 to 2 people, smaller home | 2,000 kWh | 5.5 kWh | 3.3 kWh | 3 to 5kWh |
| Typical home (Ofgem medium) | 2,500 kWh | 6.8 kWh | 4.1 kWh | about 5kWh |
| Family home | 3,500 kWh | 9.6 kWh | 5.8 kWh | 5 to 8kWh |
| Large family, home working | 4,500 kWh | 12.3 kWh | 7.4 kWh | 8 to 10kWh |
| Home with EV or heat pump | 6,000 kWh or more | 16.4 kWh | 9.9 kWh | 10kWh or more, with an off-peak tariff |
Worked example: a family home in Birmingham
A household of four in Birmingham uses 3,500 kWh a year and is thinking about a 4.4kW solar system with a battery.
- Daily use: 3,500 kWh / 365 = 9.6 kWh a day.
- Evening and overnight share: 9.6 x 60% = 5.8 kWh. The remaining 3.8 kWh is used in daylight.
- Summer surplus: PVGIS modelling gives 515 kWh in June for 4.4kW on a south-facing roof in Birmingham. 515 / 30 = 17.2 kWh a day. Take off 3.8 kWh of daytime use and 13.4 kWh is spare, more than enough to fill a 5.8kWh battery.
- Winter surplus: December gives 148 kWh, or 4.8 kWh a day. After 3.8 kWh of daytime use, only about 1 kWh is spare. The panels will not fill any battery in midwinter.
- Answer: about 5.8kWh of usable storage covers the evening need almost exactly and fills from the roof for seven months of the year. That is the size in our 4.4kW solar plus 5.8kWh battery package.
Remember the losses. A battery with an 89% round-trip efficiency returns about 5.2 kWh from 5.8 kWh stored, so on the fullest days you will still buy a little from the grid late in the evening. That is normal and not a reason to oversize.
Month by month: when does the battery fill?
Here is the same Birmingham example across the year. Surplus is the average daily solar left after 3.8 kWh of daytime use. Real days vary around these averages.
| Month | Average daily solar (4.4kW) | Average daily surplus | Fills 5.8kWh? | Fills 10kWh? |
|---|---|---|---|---|
| January | 5.2 kWh | 1.4 kWh | No | No |
| February | 8.1 kWh | 4.3 kWh | No | No |
| March | 12.2 kWh | 8.4 kWh | Yes | No |
| April | 15.9 kWh | 12.1 kWh | Yes | Yes |
| May | 16.8 kWh | 13.0 kWh | Yes | Yes |
| June | 17.2 kWh | 13.4 kWh | Yes | Yes |
| July | 16.9 kWh | 13.1 kWh | Yes | Yes |
| August | 15.0 kWh | 11.2 kWh | Yes | Yes |
| September | 13.3 kWh | 9.5 kWh | Yes | Nearly |
| October | 8.8 kWh | 5.0 kWh | Nearly | No |
| November | 6.2 kWh | 2.4 kWh | No | No |
| December | 4.8 kWh | 1.0 kWh | No | No |
On solar alone, a 10kWh battery in this home would fill on an average day for only five months, and the family only needs about 5.8 kWh in the evening anyway. The extra 4.2kWh would mostly sit idle. That changes with a cheap overnight tariff, which is where larger batteries earn their place.
When a bigger battery makes sense
You can charge cheaply overnight
Some tariffs sell electricity at a low rate for a few hours at night. Intelligent Octopus Go, for EV drivers, offers 8p per kWh between 11:30pm and 5:30am. A battery charged in that window can cover the whole of the next day's use in winter, when the panels are weak. Here the right size is closer to your total daily use outside the cheap hours, not just your evening use, so 10kWh or more is reasonable for many homes. Each kWh shifted saves the gap between your day rate and the night rate, less about 11% in losses. Our guide to the best energy tariffs for solar compares the options.
You have or plan an EV
A home battery is rarely the best way to charge a car: most EVs hold far more energy than any home battery and are better charged directly on a cheap night rate. But EV owners often qualify for those cheap tariffs, which makes a larger home battery worthwhile for the rest of the house. See our EV charger installation page if you are planning one.
You have a heat pump
Heat pumps use most of their electricity in winter, exactly when solar is weakest. A larger battery charged overnight can move a good share of that heating onto cheaper hours.
You have a larger solar array
A 6kW or bigger system makes much larger summer surpluses. Our 6.2kW solar plus 10kWh battery package pairs the two for that reason.
Capacity is not the only number: power and usable kWh
- Usable vs total capacity. Some batteries quote total capacity, but hold part of it back to protect the cells. Always compare usable kWh. Tesla, for example, quotes 13.5 kWh usable for Powerwall 3.
- Power (kW). This is how much the battery can supply at once. Powerwall 3 can be set to between 3.68kW and 11.04kW of output depending on the connection. A low power rating means a kettle, oven and electric shower together will pull some energy from the grid even with a full battery.
- Charge rate. Charge rate limits how fast a battery can fill in a short cheap-rate window. With six cheap hours, a 5kW charge rate can put in far more than a 13.5kWh battery holds, so it is rarely the bottleneck overnight.
- Warranty. Home batteries typically carry 10 years or more of manufacturer warranty, often with a minimum capacity guaranteed at the end. Our warranties page explains what we cover.
Grid rules: if the combined inverter capacity of your solar and battery is more than 3.68kW on a normal single-phase supply, your local network operator must approve it before installation under the G99 process. We make the application for you, but it can add a few weeks to the timeline.
What does each size cost?
Batteries are priced mostly on capacity. Which? reports a range from under £2,000 to £10,000, with a Tesla Powerwall at £6,200 for 13.5 kWh. Our own prices: adding a 5.8kWh battery to a 4.4kW system takes the package from £6,495 to £9,295, an extra £2,800, and a battery added to existing panels starts from £3,995. See our full guide to solar battery costs, or if you already have panels, how we add a battery to an existing system.
Honest caveats
- The 60% evening share is an average. Home workers use more in daylight and may need less storage; households out all day may need more.
- Your usage will change. A new EV, heat pump or induction hob can double your electricity use, so allow for the next five years, not the last one.
- Batteries lose some capacity with age, usually slowly. A battery that is exactly the right size today may be slightly small in ten years.
- A battery does not normally power your home in a power cut unless it has a backup or emergency power function and the wiring to support it. Ask about this at survey if it matters to you.
If you are not sure, send us a screenshot of your half-hourly usage when you ask for a free quote and we will size the battery from your real data rather than an average.
Sources
Typical household use: Ofgem, revised typical domestic consumption values, July 2026. Battery specifications: Tesla Powerwall 3 datasheet. Battery prices: Which?, solar panel battery storage. Off-peak tariff: Octopus Energy, Intelligent Octopus Go. Grid connection rules: MCS, notifying DNOs. Generation: our modelling with PVGIS, European Commission Joint Research Centre.
