WattCalc LAB

Home battery sizing without guesswork

Turn measured household demand into a practical battery-capacity estimate.

Reviewed 8 September 2026 · WattCalc Lab editorial team

Measure the load first

Smart-meter or plug-meter data is more useful than adding appliance nameplate ratings. Identify daily energy, evening energy and the minimum essential load during outages.

Separate energy from power

Capacity in kWh determines approximate runtime. Inverter output in kW and surge capability determine which appliances can run at the same time.

Keep a reserve

Usable capacity, conversion losses, battery ageing and cold weather all reduce delivered energy. A backup reserve can also preserve essential operation or EV driving range.

Try the numbers