Battery Runtime Calculator

Battery runtime formula

To calculate battery runtime, multiply battery voltage by amp-hours to get watt-hours, then adjust for depth of discharge, inverter efficiency, and battery age. Divide usable watt-hours by the load in watts.

Runtime hours = V × Ah × DoD × efficiency × age factor ÷ load watts

Example: a 12 V 100 Ah battery has 1,200 Wh before losses. At 80% DoD and 90% efficiency, usable energy is 864 Wh. A 100 W load would run for about 8.6 hours.

Understanding battery runtime estimates

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.

Formula

Bank voltage: single battery V × batteries in series

Bank capacity: single battery Ah × parallel strings

Usable energy: Wh = bank V × bank Ah × (DoD/100) × (efficiency/100) × (1 − aging/100)

Runtime (hours): Wh / loadW

Runtime (minutes): Runtime hours × 60

Example calculation

A 48 V, 100 Ah battery with 80 percent DoD and 90 percent inverter efficiency provides 48 × 100 × 0.8 × 0.9 = 3,456 Wh. If you apply a 10 percent aging factor, usable energy becomes 3,456 × 0.9 = 3,110 Wh.

With a 500 W load, runtime is 3,110 / 500 = 6.22 hours, or about 6 hours 13 minutes. This aligns with typical UPS planning rules that recommend leaving margin for battery temperature and discharge-rate effects.

Common battery runtime examples

These examples use 80% depth of discharge and 90% efficiency before battery aging. Real runtime may be lower in cold temperatures, with old batteries, or at high discharge rates.

FAQs

How is usable energy calculated?

Usable energy equals voltage times capacity, adjusted for DoD, efficiency, and aging.

Does this account for Peukert effects?

No. It is a first-order estimate and does not model discharge rate or temperature.

Why include an aging factor?

Battery capacity declines over time; derating improves planning accuracy.

Can I enter load in kW or amps?

Yes. Use the load unit selector for W, kW, A, or mA. Current-based loads are converted to watts using the battery voltage.

Is this private?

Yes. Everything runs locally in your browser.

How it works

This estimator calculates usable watt-hours from battery specifications, then divides by your load to report runtime. All math runs client-side for privacy.

Why real battery runtime may be lower

• High discharge rate: lead-acid batteries can deliver less usable capacity at high current draw.
• Temperature: cold batteries often provide less usable energy, and hot batteries may age faster.
• Battery age: older packs may no longer hold their rated amp-hours.
• Inverter losses: AC loads through an inverter use more battery energy than the AC wattage alone suggests.
• Cutoff voltage: many inverters, UPS units, and battery management systems stop discharge before the battery is completely empty.

Disclaimer

Runtime estimates are simplified and do not replace manufacturer discharge curves. Validate for critical systems.