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Battery Runtime: How to Estimate Hours From Wh, Voltage and mAh
Battery labels can use mAh or Wh, while devices are often described by watts. This guide shows how to put those numbers on a common basis before estimating runtime.
Why battery labels can be confusing
A battery's capacity can be expressed in milliamp-hours, or mAh, while a device's power use is expressed in watts. These are not directly comparable units. To estimate runtime, you need energy and power on compatible terms. Watt-hours, or Wh, are often the most convenient bridge.
A battery with a larger mAh number is not automatically larger in energy capacity if its voltage is different. For example, 5,000 mAh at 3.7 volts and 5,000 mAh at 7.4 volts represent different amounts of stored energy. Voltage must be included when converting mAh to Wh.
Convert mAh to watt-hours
Suppose a battery is rated 5,000 mAh at 3.7 V. Convert 5,000 mAh to 5 Ah, then multiply by 3.7 V: 5 × 3.7 = 18.5 Wh. This is the nominal energy figure under the stated rating.
Use the voltage appropriate to the battery specification. Some products quote nominal voltage while others describe a pack with multiple cells. If the manufacturer provides a Wh figure directly, it is usually preferable to use that published figure rather than reconstructing it from incomplete specifications.
Wh = (mAh ÷ 1000) × Voltage
Estimate runtime from power
If a battery provides 18.5 Wh and a device continuously consumes 5 W, ideal runtime is 18.5 ÷ 5 = 3.7 hours. This is an ideal arithmetic result, not necessarily the real-world runtime.
Real devices lose energy through conversion, heat and other system inefficiencies. If the overall usable efficiency is estimated at 85%, a simple planning adjustment is 18.5 × 0.85 ÷ 5 = 3.145 hours, or about 3 hours 9 minutes. The efficiency assumption should be treated as an estimate unless measured or specified.
Ideal Runtime (hours) = Battery Energy (Wh) ÷ Device Power (W)
Average power is often the missing input
A device may not draw a constant number of watts. A laptop can consume more power while processing heavily and less while idle. A refrigerator cycles on and off. A phone's power use changes with screen brightness, radio activity and workload. Runtime estimates should therefore use an average power figure for the intended use.
If a device averages 3 W for one hour and 7 W for another, its energy use is 10 Wh over two hours, or an average of 5 W. Averaging the actual energy consumption over a representative period is more useful than assuming the maximum rated power is always consumed.
Worked comparison
The table shows two important relationships. Doubling battery energy doubles ideal runtime when device power stays constant. Doubling device power halves ideal runtime when battery energy stays constant.
These relationships are useful for checking specifications. If a proposed runtime does not match the basic energy-to-power relationship, inspect whether the quoted capacity, power draw or efficiency assumption uses a different basis.
| Battery | Energy | Average load | Ideal runtime |
|---|---|---|---|
| A | 18.5 Wh | 5 W | 3.7 h |
| B | 37 Wh | 5 W | 7.4 h |
| C | 37 Wh | 10 W | 3.7 h |
Why rated capacity is not always usable capacity
Battery systems can limit how much of the nominal capacity is available to protect the cells or maintain stable operation. Voltage also changes during discharge. A power bank may lose energy through its internal voltage conversion before the connected device receives it.
For this reason, the best runtime estimate uses measured or manufacturer-provided usable energy when available. A calculation based on nominal capacity is still useful for comparison, but it should be labeled as an estimate.
Why real battery life is shorter than the estimate
Common errors include treating mAh as Wh, ignoring voltage, dividing mAh directly by watts, and assuming maximum power is continuous. Another mistake is comparing batteries with different voltages using mAh alone.
Keep the units visible in every step. Convert mAh to Ah, multiply by voltage to get Wh, then divide by watts. If efficiency is included, state the assumed percentage and apply it to usable energy rather than randomly reducing the final time.
How to verify a runtime estimate
Multiply estimated runtime by average power to reconstruct the assumed energy use. If the result is 18.5 Wh for a 5 W device, the arithmetic is internally consistent. If an efficiency factor was used, check whether it was applied once rather than twice.
Then compare the estimate with an actual test under similar conditions. A measured runtime that differs substantially may indicate a different average power draw, a battery condition issue, a capacity definition difference or an efficiency assumption that was too optimistic.
Runtime testing is more informative than labels alone
If you can measure actual power draw, compare the measured average with the specification used in the estimate. A device that averages 3 W instead of a presumed 5 W will have a substantially longer runtime from the same battery. The opposite is also true.
Battery age and temperature can change usable capacity. For repeatable comparisons, test batteries under similar loads and conditions. Record the starting charge, average power, runtime and relevant device settings so the result can be reproduced.
A quick energy sanity test
If a 20 Wh battery powers a 4 W device, ideal runtime is 5 hours. If someone reports 20 hours at the same constant 4 W load, the arithmetic would imply 80 Wh of energy, so at least one input must be different. These reverse checks are useful when reading technical specifications.
Apply the same test when comparing power banks. A capacity expressed only in mAh is incomplete until its voltage basis is known.
When a device changes power use
A device that averages 2 W for most of a day but briefly draws 8 W can have a much lower energy use than a calculation based on 8 W continuously. If it consumes 2 W for 5 hours and 8 W for 30 minutes, the energy is 10 Wh + 4 Wh = 14 Wh over that period. This illustrates why average energy use matters more than a peak figure for runtime planning.
Frequently asked questions
Can I convert mAh to Wh without voltage?
No. Voltage is required because Wh = Ah × V. If the manufacturer gives Wh directly, use that value when available.
Does higher mAh always mean longer battery life?
Not necessarily. Voltage and device power also matter. Runtime depends on energy capacity relative to the device's average power demand.
Why is actual runtime shorter than the ideal calculation?
Conversion losses, battery limits, changing voltage, heat and variable device power can all reduce usable runtime.
Conclusion
Battery runtime calculations work best when everything is expressed as energy and power. Convert mAh to Wh using voltage, divide energy by average power, and state any efficiency assumption. Treat the result as a planning estimate unless real measured data supports it.
Figures in this article are illustrative. Results depend on your own rates, fees, taxes and circumstances, and are for educational and planning purposes rather than financial advice.