Understanding battery runtime estimates
Release Updates
v1.1 (June 7, 2026)
- Added Ah, mAh, and Wh capacity inputs plus W, kW, A, and mA load inputs.
- Added required battery capacity sizing for users who know their load and target runtime.
- Added battery chemistry presets, AC/DC load handling, and series/parallel battery bank support.
- Expanded results with bank voltage, bank Ah, nameplate Wh, usable Wh, runtime, and required capacity.
Battery runtime is a simple energy balance: the battery stores energy, and your load consumes it. A battery rated at 48 V and 100 Ah holds about 4,800 Wh in ideal conditions, but real-world limits reduce that number. Depth of discharge (DoD) protects the battery by avoiding full depletion. Inverter efficiency captures the conversion loss from DC storage to AC output. An aging factor lets you derate older batteries that no longer hold their nameplate capacity. This estimator multiplies those factors to calculate usable watt-hours.
Once you know usable energy, runtime is simply energy divided by load. If the load is 500 W and the battery provides 3,500 Wh of usable energy, the estimate is about 7 hours. This is a planning approximation rather than a guarantee because real batteries behave differently at different discharge rates. Higher current draw can reduce effective capacity, especially for lead-acid chemistry. Temperature and inverter quality also matter. That is why it is common to include conservative buffers when planning critical infrastructure.
The calculator accepts Ah, mAh, or Wh for capacity and W, kW, A, or mA for load so you can match the units printed on your battery or device label. All math runs locally in your browser. It is useful for UPS sizing, portable power planning, lab setups, or any infrastructure scenario where you need a quick estimate without exposing sensitive power information. Use the results to compare scenarios and then confirm with manufacturer curves or site testing before making a final decision.
For AC backup loads, inverter efficiency accounts for DC-to-AC conversion losses. For DC loads connected directly to a battery or DC distribution bus, use the DC load option and treat the efficiency field as power path efficiency for wiring, converters, fuses, and battery management losses.
Batteries are specified in ampere-hours at a nominal voltage, which means total energy is a voltage-dependent value. If you increase system voltage, the same Ah rating delivers more watt-hours. This is why data centers often use higher-voltage battery strings for UPS systems. The calculator keeps the math explicit so you can see how voltage changes affect total runtime, especially when comparing 12 V, 24 V, or 48 V systems.
Battery banks combine cells or packs in series and parallel. Series connections increase total voltage while amp-hour capacity stays the same. Parallel strings increase amp-hour capacity while voltage stays the same. The calculator uses the single-battery voltage and capacity fields with the series and parallel counts to calculate total bank energy.
Keep in mind that aging is not linear. A three-year-old battery may deliver far less than its rated capacity if it has been exposed to heat or deep discharge cycles. When planning for critical loads, it is common to apply a larger aging factor or to schedule periodic load tests. The estimator provides a transparent starting point so you can document assumptions and adjust them as you gather real performance data.