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Calculation details
Equipment estimate only. Verify voltage, load profile, losses, duty cycle, manufacturer specifications, and electrical safety requirements before relying on the result.
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What does this calculator estimate?
Estimate how long a battery will power a device. Enter battery capacity (mAh, Ah, or Wh), voltage, and device load (mA, A, or W), optionally adjust system efficiency and usable capacity, and get runtime in hours — with unit conversions handled automatically, entirely in your browser.
- Runtime = battery energy (Wh) ÷ load (W), adjusted for efficiency and depth of discharge.
- Energy (Wh) = capacity (Ah) × voltage (V); 1,000 mAh = 1 Ah.
- Power (W) = volts × amps.
- Real runtime is usually less than theoretical due to heat, resistance, and discharge rate.
Understanding battery capacity units
mAh and Ah measure charge capacity; Wh measures energy. Because Wh = Ah × voltage, a higher-voltage battery with the same Ah stores more energy. Use the capacity unit dropdown to enter your battery the way its label reads — mAh for phones and small electronics, Ah for car and deep-cycle batteries, Wh for laptops and power stations.
Why efficiency and depth of discharge matter
No battery delivers its full rated energy. Heat, internal resistance, voltage regulation, and discharge rate all cause losses, and many chemistries should not be drained to zero (depth of discharge). The system efficiency and usable-capacity inputs scale the ideal runtime down to a more realistic estimate.
Common device power consumption
Smartphone: 1–4 W active, under 1 W standby. Laptop: 20–65 W. LED flashlight: 1–5 W. Raspberry Pi 4: about 3–7 W. Arduino: under 1 W typical. Use these as rough starting points, then measure or check your device's label for exact figures.
How this calculator works
Formula
Runtime (hours) = Battery energy (Wh) ÷ Device power (W), adjusted by system efficiency and usable capacity. Battery energy is capacity × voltage (for Ah/mAh) or directly the Wh rating; device power is watts directly or volts × amps (for A/mA).
Worked example
A 100 Ah 12 V battery stores 1,200 Wh; at 80% usable depth and 90% inverter efficiency that is 864 Wh delivered, running a 300 W load for 2.88 hours (173 minutes).
Assumptions to verify
- Steady-load estimate, not a discharge-curve simulation.
- Efficiency and depth-of-discharge reduce the energy actually available.
- Battery chemistry, age, temperature, inverter idle draw, discharge rate, and manufacturer cutoffs can materially reduce runtime.
Frequently asked questions
How do I calculate battery runtime?
Divide the battery's energy in watt-hours by the device's power in watts, then multiply by your system efficiency and usable capacity. Energy is capacity (Ah) × voltage (V), or the Wh rating directly. This calculator does the unit conversions for you.
What is the difference between mAh, Ah, and Wh?
mAh and Ah measure charge (1 Ah = 1,000 mAh); Wh measures energy. Wh = Ah × voltage, so a 3,000 mAh battery at 3.7 V holds about 11.1 Wh. Use Wh when comparing batteries with different voltages.
How do I convert amps to watts?
Watts = volts × amps. A device drawing 2 A at 12 V uses 24 W. If you know the load in amps or milliamps, this calculator converts it to watts using your battery voltage.
Why does my actual battery life differ from the calculation?
Real batteries lose energy to heat, internal resistance, voltage regulation, and discharge rate (Peukert effect), and they age and run less efficiently in cold. The efficiency and usable-capacity inputs give a more realistic estimate than the theoretical number.
Cite this tool
BoringToolsKit. “Battery Runtime Calculator.” boringtoolskit.com/battery-runtime-calculator/ (reviewed August 2026). Free to reference in articles, syllabi, and answer posts with a link.
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