Guide: battery sizing
5kWh vs 10kWh home battery: which size actually fits?
The larger battery is not automatically the better buy. The useful size is the one your household can regularly charge and discharge without leaving expensive capacity idle.
Last reviewed: October 2026
Start with usable capacity, not the name
A battery sold as 5kWh or 10kWh may have a different usable capacity from the headline figure. Ask for usable capacity in writing and compare that number between systems.
Then compare it with the amount of electricity you could genuinely shift. If your household only uses a few kWh through the expensive evening window, doubling battery capacity does not double the saving.
When a 5kWh battery can be enough
- Your evening electricity use is modest.
- You mainly want to shift a cheap overnight tariff into a few expensive hours.
- Your solar system produces a limited daily surplus.
- You want to keep installed cost down and cycle the battery more fully.
- The battery's continuous power output still covers the loads you care about.
When 10kWh can make more sense
- Your household regularly uses well above 5kWh during the expensive part of the day.
- You have enough solar surplus to fill a larger battery for much of the year.
- You use an EV, electric heating or other loads that make your daily consumption genuinely high.
- A wide time-of-use tariff spread gives the extra stored kWh something useful to do.
- The extra installed cost still produces a sensible payback rather than simply a bigger specification.
Capacity and power are different
Capacity is how much energy the battery can hold. Power output is how quickly it can deliver that energy. A large battery with a modest output can still leave you importing from the grid when several appliances run together.
Ask for the continuous output in kW as well as usable capacity in kWh. If backup power matters, ask what output is available during an outage and whether additional hardware is required.
Test the installed cost as well as the size
Use the home battery cost and payback calculator if you are charging from a cheap tariff without solar. Change the capacity and installed cost together to see whether the larger system actually improves the result.
If you have panels, first estimate how much electricity you export with the solar calculator. A larger battery only earns from solar if there is enough surplus to fill it.