From battery capacity to hours
A battery runtime calculator estimates how long a device can run when its battery capacity is known in watt-hours and its average electrical load is known in watts. Watt-hours describe stored energy; watts describe the rate at which the device uses that energy. Dividing usable watt-hours by watts yields hours. The efficiency input expresses the share of nominal capacity that is available after losses and practical limits. This is more informative than dividing a milliamp-hour figure by a watt figure, because those units cannot be combined without the battery voltage. If the label gives only mAh, convert that rating to Wh using its stated nominal voltage first.
A worked 100 Wh example
Enter a battery capacity of 100 Wh, a continuous device load of 20 W, and an efficiency of 80%. Usable energy is 100 × 0.80 = 80 Wh. The estimated runtime is then 80 ÷ 20 = 4 hours, or about one sixth of a 24-hour day. If the same equipment averages 10 W rather than 20 W, the estimate doubles to 8 hours. Conversely, a 40 W load halves the four-hour estimate. This proportional relationship helps check whether a result is plausible before relying on it. Keep the 80% assumption visible when comparing the estimate with a product specification.
Choose realistic input values
The load should be an average over the period you want to predict. A laptop may draw less power while reading a document than while rendering video; a router may vary with traffic. An inverter can also consume energy even when the connected appliance is idle. Measure the actual setup with a wattmeter when practical, or calculate several cases with different load and efficiency values. The calculator does not know the battery chemistry, discharge curve, cutoff voltage, inverter idle draw, or protective reserve. Use a wider margin for equipment that must stay running, such as monitoring and emergency systems.
Limits and common mistakes
Do not treat the result as a guaranteed shutdown time. Cold weather, an older battery, a high discharge rate, and cell imbalance can reduce accessible energy. Charging while operating changes the energy balance completely. A power bank may advertise the cells’ capacity at one voltage while its USB output is delivered at another, so comparing mAh labels alone can be misleading. The efficiency percentage here is a single overall assumption rather than a laboratory measurement. If the available energy is uncertain, try a low and high scenario and use the shorter result for planning. The calculation intentionally rejects zero or negative capacity, load, and efficiency.