Estimate usable battery energy and backup runtime
A home battery is described by storage energy, commonly in kilowatt-hours, and by maximum charge and discharge power, commonly in kilowatts. Those figures are not interchangeable. Storage energy relates to how long loads might operate, while power ratings determine whether a battery system can serve those loads at all. This calculator estimates a simplified runtime from energy and a constant average backup load. It does not verify surge capability, inverter compatibility or the capacity of any individual electrical circuit.
Begin with the nominal battery capacity shown by the manufacturer, then enter the permitted depth of discharge and an assumed conversion efficiency. Some products publish only usable energy rather than gross capacity; if that usable number is entered as nominal capacity and depth-of-discharge reduction is applied again, the result will be unnecessarily low. Compare the calculation with the manufacturer's usable AC energy specification wherever available.
Battery storage and duration formulas
Usable AC energy = nominal bank × depth of discharge fraction × conversion efficiency fraction
Backup time (hours) = usable AC kWh ÷ average backed-up kW
Installed budget = batteries + inverter + transfer equipment + labor + permits + contingency
Backup time changes rapidly when loads are not steady. Refrigerators cycle, well pumps start intermittently and heating or cooling equipment can have substantial momentary power demands. The calculator's average-load input should represent the intended critical-load panel rather than the rating of every electrical appliance in the house. The installer must check both continuous and starting requirements.
Example: two nominal 13.5 kWh batteries
Two batteries with nominal capacities of 13.5 kWh each hold a combined 27 kWh. Using a 90% allowable depth of discharge and 90% AC conversion efficiency leaves 27 × 0.90 × 0.90 = 21.87 kWh of modeled AC energy. At a constant backed-up load of 1.5 kW, that supports approximately 21.87 ÷ 1.5 = 14.58 hours.
This is a steady-load arithmetic example, not a promise that the battery can run a specific appliance or that it will last that long in practice. Real backup capability also depends on battery temperature, state of charge when the outage starts, reserve settings, age, inverter output and electrical integration.
Separate storage equipment from electrical work
Battery system proposals should identify the batteries themselves, any hybrid or standalone inverter, protected-load distribution equipment, communications and control devices, installation labor and required permits. A battery package advertised as an all-in-one unit may already include an inverter, in which case charging for another inverter would double-count equipment. Likewise, systems connected to an existing solar inverter may require different integration work from a new AC-coupled installation.
The result is best used to compare clearly described configurations at the same assumed load. Do not infer that a system is suitable for whole-house backup from battery kilowatt-hours alone. Circuit isolation, backfeed protection, emergency disconnects, inspection requirements and suitable placement are matters for the installer and local authority.
Home battery questions
Why are usable kWh lower than the battery's advertised capacity?
The calculation can subtract an allowed reserve and conversion losses from a nominal capacity. Not every manufacturer presents battery specifications in the same way, so confirm whether the advertised figure is gross cell energy, usable DC energy or usable AC energy before adjusting it.
Can a battery that runs for ten hours power my air conditioner?
Not necessarily. Runtime is based on average energy demand, but an air conditioner also needs sufficient continuous and starting power. The battery, inverter, backup panel and equipment must be compatible, and the intended load must be assessed by a qualified installer.
Does the cost include future electricity savings?
No. The cost panel covers purchases and installation only. Utility rate plans, charging sources, export restrictions, cycle count and battery degradation affect economics and should be modeled separately with the actual tariff and operating strategy.