A common mistake when selecting a home battery is relying only on monthly electricity consumption. That figure supports general analysis but does not show which appliances operate together, how long an outage lasts, or the power required at a particular moment.

Start with the operating scenario

First define the system objective. Backing up critical appliances, maximizing solar self-consumption, and pursuing full autonomy are different tasks with different sizing requirements.

For backup, list the equipment that genuinely needs to operate:

  • lighting and connectivity;
  • refrigeration;
  • heating controls and circulation pumps;
  • a well pump;
  • workstations;
  • medical or security equipment.

Record power and required runtime for each item. Power in kilowatts multiplied by time in hours gives estimated energy in kilowatt-hours.

Capacity is not the same as power

A battery may hold enough energy yet be unable to support simultaneous startup of all loads. Continuous power and short peaks must therefore be calculated separately. Compressors, pumps, and electric motors can draw several times their rated power at startup.

The inverter and battery must support this mode together. Check inverter peak power, battery current limits, BMS settings, and restrictions for the selected configuration.

Account for real losses

Calculated load energy should not equal nominal battery capacity. Conversion introduces losses, while available energy depends on permitted depth of discharge and temperature.

A practical calculation should allow for:

  • inverter and cable losses;
  • operating reserve so the battery is not constantly used at its limit;
  • reduced available energy at unfavourable temperatures;
  • possible load growth after commissioning.

Test realistic profiles

Two homes with equal daily consumption may need different batteries. One may have evenly distributed demand, while another concentrates energy use in several high-power appliances. Test several scenarios, including overnight operation, a working day, a winter outage, and a period with no solar generation.

The final design should include more than a kWh figure. Backup circuits, inverter power, protection, installation location, and charging logic together determine whether the system will be dependable and convenient.