Short answer
Allow 1 to 1.5 kWh of usable capacity per 1,000 kWh of yearly consumption. For an average household with solar panels that is about 5 kWh. Doubling to 10 kWh only raises the saving from €242 to €293 a year, while the battery costs €2,000 more. With a heat pump or an electric car, 7.5 kWh is a better fit.
Rules of thumb for capacity
- Look at your consumption in the evening and at night. That is what the battery has to deliver. An average household uses 9.6 kWh a day, about 67% of it outside the hours between 9:00 and 17:00.
- Look at what you export in summer. If you export less on a sunny day than the battery can hold, you will never fill it.
- The smaller of the two sets the useful capacity. Anything above it is only used on a handful of days a year.
What does each size earn?
For the average household (3,500 kWh of consumption, 3,500 kWh of solar power) we calculated six sizes. Without a battery this household uses 31% of its solar power itself.
| Capacity | Guide price | Solar power used at home | Saving per year | Per extra kWh of capacity | Payback period | Result after 15 years |
|---|---|---|---|---|---|---|
| 2.5 kWh | €2,500 | 53% | €160 | €64 | more than 15 years | -€382 |
| 5 kWh | €3,500 | 64% | €242 | €33 | more than 15 years | -€151 |
| 7.5 kWh | €4,500 | 68% | €277 | €14 | more than 15 years | -€540 |
| 10 kWh | €5,500 | 70% | €293 | €6 | more than 15 years | -€1,241 |
| 15 kWh | €7,500 | 72% | €311 | €4 | more than 15 years | -€2,943 |
| 20 kWh | €9,500 | 74% | €321 | €2 | more than 15 years | -€4,769 |
The column 'per extra kWh of capacity' shows the diminishing returns. The first kilowatt-hours each earn €64 a year; from 10 to 15 kWh that is only €4 per kWh, while each extra kWh costs about €400. The best result here comes from 5 kWh, although at this guide price that too stays just below zero.
Why a bigger battery earns so little extra
A battery only earns from electricity it charges and releases within one day. Two things limit that:
- In summer it fills quickly, but you use too little in the evening and at night to empty it. The next morning it is still half full.
- In winter there is hardly any surplus solar power. The battery then stays empty for days, however large it is.
A home battery shifts electricity from the afternoon to the evening, not from summer to winter. No affordable capacity can do that.
With a heat pump and an electric car
A household using 8,500 kWh with 6,000 kWh of solar power gets more out of a bigger battery, because there is more to deliver in the evening.
| Capacity | Guide price | Solar power used at home | Saving per year | Per extra kWh of capacity | Payback period | Result after 15 years |
|---|---|---|---|---|---|---|
| 2.5 kWh | €2,500 | 45% | €165 | €66 | more than 15 years | -€331 |
| 5 kWh | €3,500 | 54% | €289 | €49 | 13.5 years | €358 |
| 7.5 kWh | €4,500 | 60% | €377 | €35 | 13.0 years | €632 |
| 10 kWh | €5,500 | 65% | €444 | €27 | 13.4 years | €623 |
| 15 kWh | €7,500 | 70% | €515 | €14 | more than 15 years | -€175 |
| 20 kWh | €9,500 | 73% | €548 | €7 | more than 15 years | -€1,558 |
Here 7.5 kWh gives the best result. Note that a heat pump mostly runs in winter, when the battery has little to do. The car's extra consumption in the evening helps the battery more.
Power matters too
Besides the capacity in kWh, the power in kW matters: how much the battery can deliver at once. Cooking or a tumble dryer needs 2 to 3 kW. A 5 kWh battery with 2.5 kW saves the average household €242 a year; with 0.8 kW, like a plug-in battery, that is €218. The difference is smaller than you might think, because the battery also delivers for hours on end outside the peaks.
How to choose in four steps
- Find the purchase and feed-in figures on your annual statement and your production in your inverter app.
- Enter them in the calculator under 'use your annual statement'.
- Look at the table 'Which battery size fits best?' and choose the size with the best result, not the highest saving.
- Then enter the price from your quote. Also read what a home battery may cost.
Sources
- HTW Berlin (Weniger, Tjaden, Quaschning), Sizing of Residential PV Battery Systems, Energy Procedia 46 (2014). Reference for self-consumption and self-sufficiency: about 30% without a battery and 55–60% with 1 kWh of storage per 1,000 kWh of consumption.
- Milieu Centraal, Home battery for solar power (in Dutch). Independent assessment of home batteries and the rule of thumb that households use about 30% of their solar power directly.
- CE Delft, voor Vereniging Eigen Huis, Study of home battery payback times (in Dutch). Self-consumption rises to 60–70% with a battery; payback times range from just over 6 to 22 years.