Battery Runtime Calculator

Q: How do I calculate battery runtime?

Battery runtime = (Ah × Voltage × Efficiency) ÷ Load Watts. For example, a 100Ah 12V battery running a 200W load at 80% efficiency lasts about 4.8 hours.

Q: Does temperature affect battery runtime?

Yes. Cold temperatures reduce effective capacity. At 0°C, lead-acid batteries may deliver only 70–80% of rated capacity. Factor this into your efficiency setting.

Find out how long your battery will last under load. Works for 12V, 24V, 48V systems and any battery type.

Calculate Battery Runtime

Battery Capacity (Ah)

Battery Voltage (V)

Load in Watts (W)

Battery Efficiency (%)

Result

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Runtime (hours)

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Hours & Minutes

Formula

The battery runtime formula is:

Runtime (hours) = (Ah × Voltage × Efficiency) ÷ Load Watts

Where efficiency is expressed as a decimal (e.g. 85% = 0.85). The formula calculates the usable watt-hours stored in the battery, then divides by the power draw.

Example Calculation

You have a 100Ah, 12V lead-acid battery powering a 200W load at 85% efficiency.

Runtime = (100Ah × 12V × 0.85) ÷ 200W
Runtime = 1020 Wh ÷ 200W
Runtime = 5.1 hours (5h 6min)

This is a theoretical estimate. Real-world runtime may vary based on battery age, temperature, and discharge rate (Peukert effect).

Practical Notes

Lead-acid batteries should not be discharged below 50% to preserve lifespan (use 50% of rated Ah).
LiFePO4 batteries can safely discharge to 20% — use 80% of rated Ah for these.
Cold temperatures reduce battery capacity. At 0°C, expect 70–80% of rated capacity.
Inverter losses add to your load. A 200W device needs ~220W from the battery through a 90% efficient inverter.
Always add a 20% safety margin to avoid over-discharging.

Frequently Asked Questions

How do I calculate battery runtime?

Use the formula: Runtime = (Ah × Voltage × Efficiency) ÷ Load Watts. For a 100Ah 12V battery running a 200W load at 85% efficiency: (100 × 12 × 0.85) ÷ 200 = 5.1 hours.

What efficiency should I use for lead-acid batteries?

Lead-acid batteries typically operate at 80–85% efficiency. Lithium (LiFePO4) batteries are closer to 95–98%. AGM batteries fall around 85–90%.

Does temperature affect battery runtime?

Yes. Cold temperatures reduce effective capacity significantly. At 0°C, lead-acid batteries may deliver only 70–80% of their rated capacity. Account for this by lowering your efficiency setting or your Ah input in cold environments.