Start with evening and night consumption
The battery should cover consumption from the moment PV is no longer enough until morning. For an ordinary house without a heat pump that is roughly half to two thirds of daily consumption. With annual consumption of 4,500 kWh the daily average is around 12 kWh, so roughly 6 to 8 kWh in the evening and at night.
Then check that the roof can charge it
The battery needs something to charge from. Useful capacity is limited by the surplus PV produces above daytime consumption. With a smaller PV, a larger battery charges fully only on the best summer days and part of it stays unused for the rest of the year.
Summer and winter are two different houses
In summer there is plenty of surplus and the battery fills every day. In winter PV produces a fraction and a house with a heat pump uses much more. A battery designed for winter would be oversized in summer and vice versa. Only a year-round hourly simulation shows a sensible compromise. If you have a dynamic tariff, the battery can also charge from the grid in cheap hours in winter.
Backup takes part of the capacity
If you want the battery to keep the house running during an outage, it is wise to keep a reserve in it, for example 20 to 30 percent. That reduces the capacity available for everyday savings. That is why a slightly larger battery is chosen when backup is wanted.
Do not forget power in kW
Capacity in kWh says how long the battery lasts. Power in kW says how many appliances it can run at once. A house with an induction hob, heat pump and car charging needs higher power, otherwise it draws from the grid at peak times even with a full battery.
A rule of thumb and an exact calculation
For a first estimate use the capacity calculator. For an actual design, send us hourly consumption and production data. We show what a 5, 10 and 15 kWh battery would do and choose the one where an extra kWh still pays off.
Send us your consumption, we will design the battery
Phone and e-mail are enough to start. We reply within 24 hours on working days, ask for the data we need and send a proposal with the capacity, inverter compatibility and installation steps. Free of charge and without obligation. Prefer to talk? 775 654 443