How long a battery lasts with different appliances

Use the calculator below to get quick estimates of how long batteries (different types and capacities) last with different appliances using inverters.

Note: The battery run-time values provided by the calculator are estimates. Actual values may differ because of the factors that affect battery run-time below.

Note:

This calculator assumes a fully charged battery. Inverter efficiency is set at 80%.
Battery depth of discharge used: deep cycle AGM or Flooded batteries is 50% while for Lithium ion or Lithium phosphate: 90%.
Higher current draws significantly reduce the time for which the appliance can be powered by the battery.

Formula for calculating how long a battery lasts

The estimated duration (hours) for which an appliance will run when connected to a battery is obtained using the formula: (c x d x i) ÷ p where:

c is Battery capacity (Ah)
d is depth of discharge (%)

i is inverter efficiency(%)
p is power consumption of the appliance (W)

What determines run-time of an appliance on battery?

The number of hours a fully charged battery lasts (battery backup time) when connected to an appliance primarily depends on:

battery capacity (Ah),
the power consumption of the appliance
power losses in the inverter. Some of the energy from the battery is lost as the inverter converts DC to AC electricity and is not available to power the appliance. Appliances connected to lower efficiency inverters run for a shorter time. In general, aim for inverters with an efficiency of 90%+

No load power: In addition, inverters will consume some power in standby mode (when no load is connected). This is shown as the no load power in the inverter’s technical specifications. This may be as low as 4 W (30mA) at 12V for the highly efficient ones.
Age of batteries: The life of a battery measured in cycles also expressed in number of years is finite. A battery’s capacity reduces with age. So for instance, a battery new battery with a capacity of 100Ah may lose, 10%, 20% or more with age. Actual battery capacity will be affected by how long the batteries have been in use.
Damage from high discharge rates: Frequent high current discharge beyond those advised by the manufacturer also damage the battery and reduce its its capacity.

Deep discharges beyond the depth of discharge: Routine discharges of a battery beyond the manufacturer’s recommended depth of discharge, typically (50%) for deep cycle AGM or 90% for Lithium also damages the battery and lowers its capacity.

Worked example

Using the calculator above, how long will a 100Ah AGM battery power an appliance rated 100W?

The estimated duration (hours) for which the appliance will run is 15 minutes (0.4 hours).