Battery Runtime Calculator

Direct answer

For a steady direct-current load, estimated runtime is usable ampere-hours divided by load current. Victron’s battery-monitor manual gives the matching example: a 100 Ah battery discharged at a constant 5 A lasts 20 hours, while also warning that discharge rate affects capacity.

Variables and formula

• Battery capacity: rated ampere-hours.
• Usable charge share: capacity intentionally used before recharge.
• Steady load current: direct current drawn from the battery.

Runtime hours = Capacity Ah × Usable share / Load current A

Reproducible example

With a 100 Ah battery, 80% usable capacity and a 5 A load: 100 × 0.80 / 5 = 16 hours.

Interpretation

This is a planning estimate for a steady load. Use the C-rate result and the battery data sheet to judge whether the entered current matches the rating conditions.

Assumptions and limitations

• The load is steady and measured in direct-current amps.
• Temperature, battery age, inverter losses and Peukert effects are excluded.
• The result is not a safety limit or a substitute for a battery-management system.

Common mistakes

• Dividing watts by ampere-hours without accounting for voltage.
• Assuming all rated capacity is usable in every application.
• Treating a fluctuating load as constant.

Assumptions

• The load is steady and expressed directly in amps.
• Temperature, battery age, discharge rate and conversion losses can reduce actual runtime.

Sources and methodology

CalcMotive publishes the formula and assumptions so you can decide whether the estimate fits your use case. See our methodology standards.

• Battery Capacity and Peukert Exponent; Victron Energy; accessed Jul 27, 2026