Understanding your result
Nominal watt-hours come from voltage and amp-hours. Available energy removes the share you intend to leave unused. Delivered energy is what reaches the load after conversion and any separately entered idle draw.
For the grid electricity cost of running that load, the Appliance Electricity Cost Calculator also allows a separate standby schedule.
The formula
The inverter draws more power from the battery than it delivers when efficiency is below 100%. Any additional battery-side idle consumption is added to that demand. Dividing the available watt-hours by battery-side watts gives hours of constant-load operation.
Worked example
A 12 V, 100 Ah battery contains a nominal 1,200 Wh. Using 80% leaves 960 Wh available. A 100 W output load through a 90% efficient converter draws about 111.11 W from the battery, giving an estimated 8.64 hours with no additional idle draw.
How to use this calculator
- Enter the pack’s nominal voltage and amp-hour capacity.
- Choose a usable energy share appropriate to the equipment.
- Enter steady output watts, conversion efficiency, and any separate idle consumption.
Voltage and amp-hours belong to the same pack
Use the complete battery bank’s voltage and amp-hour rating. Do not multiply voltage from one configuration by capacity from another. If a power station already specifies watt-hours, divide that value by its nominal battery voltage to obtain an equivalent amp-hour input, while retaining the manufacturer’s usable-energy guidance.
Real loads and battery behavior
Starting surges, changing loads, temperature, age, discharge rate, and cutoff settings can reduce usable runtime. Lead-acid capacity is especially sensitive to discharge conditions. The model is an energy balance, not a chemistry simulation. Check both continuous and surge output ratings separately: a large stored-energy figure does not mean an inverter can start every appliance.
Assumptions & limitations
What this calculation assumes
- Load and effective conversion efficiency remain constant.
What to keep in mind
- Excludes Peukert effects, voltage sag, thermal limits, and dynamic battery management.
Common questions
Should idle draw be counted twice?
No. If the efficiency value already includes inverter self-consumption at this load, enter zero additional idle watts.
Is nominal energy the same as usable energy?
No. The usable share accounts for the portion you choose or are permitted to discharge before conversion losses are applied.