Battery Usable-Energy Calculator — Convert Rated Capacity Into Realistic Energy
Convert battery voltage and amp-hours into nominal energy, then reduce it for depth of discharge and conversion efficiency to estimate usable delivered energy.
Battery capacity inputs
Nominal versus usable battery energy
Battery energy is often quoted as voltage multiplied by amp-hours. That produces nominal watt-hours, but the full number is not necessarily available to the load.
Depth of discharge
A selected usable discharge percentage limits how much of the nominal stored energy is treated as available.
Conversion efficiency
In a real backup system, some stored energy is lost through power electronics, wiring and other conversion stages.
Reserve
An additional reserve reduces the design energy so the planner does not depend on using the entire theoretical usable range.
Runtime check
Dividing delivered energy by load power gives a first-pass runtime estimate under a constant-load model.
Limitations
Actual battery runtime can be lower at high discharge rates or under aged and cold conditions.
Frequently asked questions
Can Ah be converted directly to kWh?
Only after voltage is known. The basic conversion is volts × amp-hours ÷ 1,000.
Can depth of discharge be 100%?
The calculator permits it mathematically, but actual battery operating limits should come from the battery specification.
Can one battery produce enough energy for a long outage?
Possibly, depending on voltage, capacity, usable range and load.