Battery Capacity & Runtime Calculator

How this is worked out

Watt-hours are the universal energy unit that lets you compare batteries of different voltages: Wh = Ah × V. Runtime is then that usable energy divided by the load: hours = Wh ÷ W. Two adjustments matter for a realistic estimate: depth of discharge (most battery chemistries shouldn't be fully drained — lead-acid is often limited to 50%, lithium commonly to 80–100%, to protect battery life), and system efficiency (an inverter or charge controller loses some energy as heat, typically 85–95% efficient). Both default to sensible middle-of-the-road values here, but check your own battery and inverter's specifications for an accurate figure.

Frequently asked questions

Why not just use the full rated capacity?

Because real-world runtime is always less than the simple Ah×V/W arithmetic suggests — discharging fully shortens most batteries' lifespan, and any inverter or converter between the battery and your load wastes some energy as heat.

Does this work for UPS sizing?

Yes — enter your UPS battery's Ah and voltage, and the load you need to keep powered, to estimate how many minutes of backup you'll get; for sizing a new UPS, work backwards from your required runtime and load to find the Wh capacity you need.

Related tools: Ohm's Law Calculator, Electricity Cost Calculator, Solar Panel Savings Calculator.

Source / Last verified: standard electrical engineering definitions (Wh = Ah × V; runtime = usable Wh ÷ load) — read/checked 1 October 2026. Depth-of-discharge and efficiency defaults (80% and 90%) are typical, commonly-cited reference figures for home battery/inverter setups, not a specification for any particular product — always check your own equipment.