Free tool
What size battery do you need?
Tell us where your panels face and how much you use after dark. We’ll work out the battery size your kit can actually fill, show how full it gets each month, and list the batteries in stock near that size.
- suits an 890 Wp kit facing south in Birmingham
- 1.2 kWh
- months it fills on an average day
- 6 of 12
- full on an average winter day
- 48%
The battery size that suits your kit
1.2
kWh usable capacity
That’s what your panels have spare on an average day in the six sunniest months. A bigger battery would sit part-empty from autumn to spring.
6 of 6
summer months it fills
48%
full on a winter day
+292
kWh a year used at home, £75
How full a 1.2 kWh battery gets from solar
| Month | Filled from solar |
|---|---|
| Jan | 25% of 1.2 kWh |
| Feb | 51% of 1.2 kWh |
| Mar | 90% of 1.2 kWh |
| Apr | 100% of 1.2 kWh |
| May | 100% of 1.2 kWh |
| Jun | 100% of 1.2 kWh |
| Jul | 100% of 1.2 kWh |
| Aug | 100% of 1.2 kWh |
| Sep | 100% of 1.2 kWh |
| Oct | 57% of 1.2 kWh |
| Nov | 37% of 1.2 kWh |
| Dec | 27% of 1.2 kWh |
| Usable | Extra solar used | Months it fills |
|---|---|---|
| 0.5 kWh | 153 kWh | 9 of 12 |
| 1.0 kWh | 257 kWh | 7 of 12 |
| 1.2 kWh (suggested) | 292 kWh | 6 of 12 |
| 2.0 kWh | 348 kWh | 0 of 12 |
| 3.0 kWh | 348 kWh | 0 of 12 |
| 5.0 kWh | 348 kWh | 0 of 12 |
In stock near 1.2 kWh
- Indevolt BK1600 Plug-in Home BatteryBuy ↗
1.64 kWh£399In stockElectrician install
- Thunder Vault 1.92 kWh Plug-In Home BatteryBuy ↗
1.92 kWh£1,049.99In stockElectrician install
- Thunder Bolt Plus 460W Kit with BatteryBuy ↗
1.92 kWh + 460 Wp panels£1,549.99In stockElectrician install
- Thunder Bolt Plus 920W Kit with BatteryBuy ↗
1.92 kWh + 920 Wp panels£1,749.99In stockElectrician install
Batteries aren’t covered by the GB plug-in rules: each needs an electrician to install. Will it pay back?
Solar from PVGIS for West Midlands, on an average day in each month. On dull days the battery fills less and on bright days sooner.
Typical homes
Battery sizes for typical homes in Birmingham
Usable capacity for an 890 Wp kit and 3 kWh used after dark, by the way the panels face and who’s home in the day. The size is the most the kit fills on an average day in six months of the year. The more you use in daylight, the less is left to store.
| Panels | Most spare in a dayhome some of the day | Out most of the day | Home some of the day | Home all day |
|---|---|---|---|---|
| SouthGarden stand, 30° | 1.7 kWhJun | 1.6 kWh | 1.2 kWh | 0.8 kWh |
| South-eastGarden stand, 30° | 1.7 kWhJun | 1.5 kWh | 1.0 kWh | 0.7 kWh |
| South-westGarden stand, 30° | 1.7 kWhJun | 1.5 kWh | 1.0 kWh | 0.7 kWh |
| EastGarden stand, 30° | 1.6 kWhJun | 1.1 kWh | 0.7 kWh | 0.4 kWh |
| WestGarden stand, 30° | 1.5 kWhJun | 1.1 kWh | 0.7 kWh | 0.4 kWh |
| NorthGarden stand, 30° | 1.0 kWhJun | 0.3 kWh | Not worth it | Not worth it |
| SouthBalcony or wall, 90° | 0.9 kWhMar | 0.9 kWh | 0.6 kWh | 0.4 kWh |
| EastBalcony or wall, 90° | 0.7 kWhJun | 0.6 kWh | 0.4 kWh | Not worth it |
| WestBalcony or wall, 90° | 0.7 kWhJun | 0.6 kWh | 0.4 kWh | Not worth it |
Solar from PVGIS 5.3 for Birmingham, with the 800 W output cap. Daytime use is a steady 100 W, 170 W or 240 W, as in the calculator. 3 kWh after dark is our assumption; put your own into the tool. “Not worth it” means under 0.3 kWh.
Month by month
How full a 1.2 kWh battery gets
The same 890 Wp kit facing south at 30° in Birmingham, with 3 kWh used after dark. Summer fills it; winter doesn’t come close.
| Month | Filled from solar |
|---|---|
| Jan | 25% of 1.2 kWh |
| Feb | 51% of 1.2 kWh |
| Mar | 90% of 1.2 kWh |
| Apr | 100% of 1.2 kWh |
| May | 100% of 1.2 kWh |
| Jun | 100% of 1.2 kWh |
| Jul | 100% of 1.2 kWh |
| Aug | 100% of 1.2 kWh |
| Sep | 100% of 1.2 kWh |
| Oct | 57% of 1.2 kWh |
| Nov | 37% of 1.2 kWh |
| Dec | 27% of 1.2 kWh |
| Month | Spare solar a day | Stored a day | How full |
|---|---|---|---|
| Jan | 0.30 kWh | 0.30 kWh | 25% |
| Feb | 0.61 kWh | 0.61 kWh | 51% |
| Mar | 1.08 kWh | 1.08 kWh | 90% |
| Apr | 1.61 kWh | 1.20 kWh | Full |
| May | 1.65 kWh | 1.20 kWh | Full |
| Jun | 1.74 kWh | 1.20 kWh | Full |
| Jul | 1.64 kWh | 1.20 kWh | Full |
| Aug | 1.38 kWh | 1.20 kWh | Full |
| Sep | 1.21 kWh | 1.20 kWh | Full |
| Oct | 0.68 kWh | 0.68 kWh | 57% |
| Nov | 0.45 kWh | 0.45 kWh | 37% |
| Dec | 0.33 kWh | 0.33 kWh | 27% |
An average day in each month. On dull days the battery stores less and on bright days it fills sooner, so even in summer it won’t fill every day. That’s about 292 kWh a year more of your solar used at home.
In stock now
Batteries in stock, by size
Every battery we track that’s in stock and has a published usable capacity, smallest first, with how it would fill from the south-facing kit above. The suggested size there is 1.2 kWh.
| Battery | Usable | Price today | Months it fills | Solar used a year | Buy |
|---|---|---|---|---|---|
 Indevolt BK1600 Plug-in Home BatteryIndevoltIn stockElectrician install | 1.64 kWh | £399 | 4 of 12 | +344 kWh | Buy ↗Indevolt |
 Thunder Vault 1.92 kWh Plug-In Home BatteryThunder EnergyIn stockElectrician install | 1.92 kWh | £1,049.99 | 0 of 12 | +348 kWh | Buy ↗Thunder Energy |
 Thunder Bolt Plus 460W Kit with BatteryThunder EnergyIn stockElectrician install | 1.92 kWhkit with 460 Wp panels | £1,549.99 | – | – | Buy ↗Thunder Energy |
 Thunder Bolt Plus 920W Kit with BatteryThunder EnergyIn stockElectrician install | 1.92 kWhkit with 920 Wp panels | £1,749.99 | – | – | Buy ↗Thunder Energy |
 Zendure SolarFlow 2400 AC+ZendureIn stockElectrician install | 2.40 kWh | £899 | 0 of 12 | +348 kWh | Buy ↗Zendure UK |
 Zendure SolarFlow 2400 ProZendureIn stockElectrician install | 2.40 kWh | £1,099 | 0 of 12 | +348 kWh | Buy ↗Zendure UK |
Kits with a battery come with their own panels, so we don’t fill them from the example kit. All are marked Electrician install: no battery is covered by the GB plug-in rules (why). Prices are the best in-stock offer. For payback at these prices, use the battery payback tool.
How we size it
Two limits on a useful battery
- The spare solar. Your kit’s output, less what the home uses as it’s made. We take the most the battery would fill on an average day in at least six months of the year. Bigger than that and the extra capacity sits empty most of the year.
- Your evening. A battery only saves money when it empties into your evening use. We cap the size at what you use after dark, allowing for the 10% lost in charging and discharging.
The smaller of the two is the suggested size. Use a figure for usable capacity when you compare batteries.
Finding your evening use
How much do you use after dark?
- Smart meter app. Most suppliers show half-hourly use. Add up from sunset to when you get up, outside any cheap-rate hours.
- In-home display. Note the reading at sunset and again in the morning for a few days.
- No data? Start at 3 kWh, our assumption for a typical home, and see how much the answer moves.
Battery size and plug-in solar: questions
What size battery do I need for plug-in solar?
Smaller than you might think. For an 890 Wp kit facing south in Birmingham, with someone home some of the day and 3 kWh used after dark, about 1.2 kWh of usable capacity. That's what the kit has spare on an average day in the six sunniest months. A bigger battery would sit part-empty for most of the year.
How often will a plug-in solar battery fill up?
In our Birmingham example, a 1.2 kWh battery fills on an average day in 6 months of the year, roughly April to September. From October to March it gets to about 48% on an average day, and 27% in December.
Does the direction of the panels change the battery size?
Yes. For a home with someone in some of the day, an 890 Wp kit in Birmingham suits about 1.2 kWh on a south-facing garden stand, 0.7 kWh facing east and 0.6 kWh upright on a south-facing balcony. Facing north there's little spare solar outside high summer (about 1.0 kWh a day at best, in June), so a battery isn't worth it.
Why not buy a bigger battery to be safe?
Because a plug-in kit can only fill so much. Each kWh of capacity beyond what your panels have spare on a typical day is used on fewer days of the year, so it costs more for each kWh it stores. If you also charge on a cheap overnight rate, a bigger battery can earn its keep in winter: try the battery payback tool with your tariff.
What does usable capacity mean?
The energy you can actually take out, which can be less than the battery's total (nominal) size. We use the maker's usable figure where they publish one. About 10% is lost between charging and use, which we allow for.
Can I plug the battery into a socket?
No. The GB plug-in solar rules don't cover batteries. The DESNZ product specification leaves out plug-in battery systems, so a battery needs an electrician to install it and notify your network operator.
Sources
Free tools
Plan the rest of your setup
- CalculatorOutput, savings and payback for your home
- Kit finderThree answers, a shortlist of kits in stock
- Shade plannerDraw your horizon, see what shade costs
- Best panel angleUpright, stand or flat: output by angle
- Will it fit?Which panels and kits fit your space
- Clipping calculatorWhat oversized panels lose to the 800 W limit