Home Battery Backup Sizing: Usable kWh, Power and Runtime

Battery backup needs two independent checks: enough usable energy in kWh for the desired outage duration, and enough inverter power in kW to run and start the selected appliances. A house size or battery brand alone cannot establish runtime. Use measured loads, a stated reserve and the exact equipment specification.

Build a measured essential-load list

List each circuit, its average power, operating hours and any startup surge. A refrigerator cycles, so rated maximum watts multiplied by 24 is not necessarily daily energy. Use accumulated kWh from a suitable meter where possible. Medical or other life-critical equipment needs a professionally assessed backup plan, not reliance on this estimate.

Calculate energy before selecting units

Daily energy is the sum of average watts multiplied by hours and divided by 1,000. An illustrative 15 W router running 24 hours uses 0.36 kWh; four 10 W lights used six hours consume 0.24 kWh. These entered loads total 0.60 kWh/day before adding appliances, communications and inverter standby energy.

State what usable capacity means

Nameplate energy, manufacturer usable DC energy and usable AC energy delivered to the backed-up loads are not interchangeable. Do not apply depth of discharge twice if a specification already states usable capacity. If a usable AC budget already includes inverter losses, do not apply an extra discharge derating to it.

Worked runtime examples

Assume an illustrative 13.5 kWh usable AC energy budget, with 20% withheld as reserve, leaving 10.8 kWh for the selected loads. Dividing by a 0.5 kW average load gives 21.6 hours; 1 kW gives 10.8 hours; 2 kW gives 5.4 hours. This is a calculation example, not a capacity or runtime guarantee for any named battery.

Check continuous output and surge separately

A battery with ample stored energy can still fail to start a compressor or pump. Confirm continuous inverter output, surge magnitude and duration, voltage, circuit limits and which loads can run simultaneously. Ask the installer to document motor-start compatibility rather than infer it from kWh.

Do not use round-trip efficiency twice

Round-trip efficiency spans charging and discharging. Runtime from an already charged battery uses the applicable deliverable energy and any one-way discharge loss not already included in that boundary. Battery payback from shifting solar uses a round-trip energy balance. Those are different questions.

Plan for uncertainty and recharge

Runtime changes with temperature, degradation, actual load cycles and reserve settings. Solar charging during an outage requires compatible islanding equipment and suitable conditions. Model a no-recharge conservative case separately from a sunny recharge case; annual solar production does not guarantee power during the outage.

Installer verification checklist

Request a circuit list, measured daily energy, simultaneous running power, surge assessment, usable-energy boundary, reserve setting, expected standby draw, battery warranty conditions and commissioning test. Check what happens when reserve is reached and whether solar can restart the system safely. Installation and electrical work require qualified professionals.