Home Battery Backup Runtime Calculator

How to use this calculator

1. Enter the battery capacity in kWh and the usable depth of discharge (how much you can safely use).
2. Enter the backup load in watts and the inverter efficiency.
3. Click Calculate runtime for the usable energy and how long it lasts.

How runtime is calculated

Usable energy is the capacity times the depth of discharge; runtime is that energy divided by the load drawn through the inverter:

The inverter efficiency means a 1000 W load actually pulls a bit more from the battery. High loads, cold temperatures and battery age all shorten real runtime.

Example runtimes (10 kWh, 90% usable)

To back up high-power heating or an EV charger, a single home battery rarely lasts long — prioritise essential circuits.

Frequently asked questions

How long will a home battery last in a power cut?

Divide the usable energy (capacity × depth of discharge) by your load. A 10 kWh battery at 90% usable running a 500 W load lasts roughly 16–18 hours including inverter losses.

What is depth of discharge?

The share of the battery you can safely use. Many lithium home batteries allow 90–100%; using less than the full capacity extends battery life.

Why is real runtime shorter than the simple calculation?

Inverter losses, high or surging loads, cold temperatures and battery ageing all reduce the energy actually delivered to your loads.