Battery Runtime Calculator
Estimate how long a battery can power an electronic device from battery voltage, capacity, load power or current, efficiency and usable battery capacity. Calculate battery energy in watt-hours and estimated operating time in hours and minutes.
Battery Runtime Estimator
Enter the battery specifications and your expected load. The calculator estimates usable energy and runtime.
Battery Runtime Formula
Battery runtime is fundamentally an energy calculation. Battery capacity can be represented in amp-hours or watt-hours, while the load can be represented by power or current.
Ah vs Wh: Understanding Battery Capacity
Amp-hours and watt-hours describe battery capacity in different ways. Watt-hours are especially useful when comparing batteries with different voltages or calculating runtime from a known load power.
Amp-Hours (Ah)
Amp-hours represent a quantity of electrical charge. A nominal 10 Ah battery can theoretically supply 1 amp for 10 hours, 2 amps for 5 hours, and so on under idealized conditions.
Watt-Hours (Wh)
Watt-hours represent stored electrical energy. They are calculated approximately as voltage multiplied by amp-hour capacity and are convenient for comparing batteries and loads.
Battery Runtime Efficiency
Real systems rarely deliver all nominal battery energy to the load. Power converters, wiring, regulators, inverters and other components introduce losses.
100% Efficiency
An idealized calculation where all available battery energy reaches the load. Useful as a theoretical reference.
90% Efficiency
A useful example for a system with some conversion losses. The actual efficiency depends on the specific regulator, converter or inverter.
Lower Efficiency
Systems with substantial conversion losses, high current, inefficient electronics or unfavorable operating conditions may deliver less usable energy.
Usable Battery Capacity
Nominal battery capacity does not necessarily mean that all stored energy should be used. The usable-capacity setting lets you estimate runtime based on a selected discharge limit.
100%
Uses the full nominal energy for an idealized estimate.
90%
Leaves approximately 10% of nominal capacity unused.
80%
Uses 80% of nominal battery energy in the estimate.
50%
Uses half of nominal battery energy before cutoff.
Worked Battery Runtime Examples
These examples demonstrate the difference between ideal and practical runtime calculations.
100 Wh Battery, 10 W Load
A 100 Wh battery powering a 10 W device provides:
With 90% efficiency and 90% usable capacity: 100 × 0.90 × 0.90 ÷ 10 = 8.1 hours.
12 V, 10 Ah Battery, 10 W Load
First calculate nominal battery energy:
At 90% efficiency and 90% usable capacity, estimated runtime is 120 × 0.90 × 0.90 ÷ 10 = 9.72 hours.
24 V, 100 Wh Battery, 20 W Load
Assuming 90% efficiency and 90% usable capacity:
The same energy-based approach works regardless of whether the battery is 12 V, 24 V or another voltage.
5 V USB Device at 2 A
The load power is approximately:
A battery feeding this device through a converter will have additional conversion losses, so the actual runtime can be lower than the ideal energy calculation.
Common Battery Runtime Examples
The table below uses 90% efficiency and 90% usable capacity, producing an overall usable-energy factor of 81%.
| Battery | Nominal Energy | Load | Estimated Runtime |
|---|---|---|---|
| 5 V · 2.5 Ah | 12.5 Wh | 2 W | 5.06 h |
| 12 V · 7 Ah | 84 Wh | 10 W | 6.80 h |
| 12 V · 10 Ah | 120 Wh | 10 W | 9.72 h |
| 12 V · 20 Ah | 240 Wh | 20 W | 9.72 h |
| 24 V · 10 Ah | 240 Wh | 30 W | 6.48 h |
| 100 Wh | 100 Wh | 20 W | 4.05 h |
| 500 Wh | 500 Wh | 50 W | 8.10 h |
Factors That Affect Real Battery Runtime
Load Variation
A device that changes power consumption over time should be evaluated using average or measured power, rather than only its maximum rating.
Discharge Rate
Battery capacity can depend on discharge current. High-current operation can result in less usable capacity than a low-current laboratory rating.
Temperature
Battery performance can change significantly with temperature. The effect depends on the battery chemistry and operating conditions.
Battery Age
Capacity and internal resistance can change as a battery ages, reducing the runtime compared with its original rating.
Converter Efficiency
DC-DC converters, USB regulators and inverters consume some energy themselves. Their efficiency varies with load and operating conditions.
Cutoff Voltage
Electronics often stop operating before a battery is completely discharged. The cutoff voltage therefore determines how much of the nominal energy is actually usable.
Battery Runtime Calculator Applications
Embedded Systems
Estimate operating time for microcontrollers, sensors, development boards and portable electronics.
Portable Electronics
Estimate how long a portable device can operate from a battery pack or power bank.
Solar & Backup Power
Estimate storage runtime for small backup systems, battery banks and off-grid electronics.
IoT Devices
Estimate battery life for connected sensors, monitoring equipment and low-power wireless devices.
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Battery Runtime Calculator FAQ
How do you calculate battery runtime?
A simple battery runtime estimate is runtime = battery capacity in Wh divided by load power in W. If battery capacity is given in amp-hours, runtime can also be estimated from Ah divided by load current in amperes when the battery voltage and load conditions are compatible.
What is the battery runtime formula?
For an energy-based estimate, runtime in hours is approximately battery energy in Wh divided by load power in W. When efficiency and usable capacity are included, runtime = battery Wh × usable fraction × efficiency ÷ load W.
Can I calculate runtime using Ah instead of Wh?
Yes. Battery energy can be estimated as Wh = V × Ah. Therefore, runtime can be estimated from voltage, amp-hour capacity and load power. For a direct current-based estimate under compatible conditions, runtime is approximately Ah divided by average load current.
Why does battery runtime differ from the simple Ah divided by A calculation?
Real battery runtime can be affected by discharge rate, battery chemistry, temperature, age, internal resistance, converter losses, cutoff voltage and the battery discharge curve. The simple calculation is therefore an estimate.
What is battery efficiency in the calculator?
Efficiency represents the fraction of stored battery energy that reaches the load. It can account for losses in DC-DC converters, inverters, regulators, wiring and other parts of the power path.
What is usable battery capacity?
Usable capacity is the fraction of nominal battery capacity that you intend to use. For example, a usable capacity of 80% means the calculation assumes only 80% of the nominal stored energy is available before the system stops or the battery reaches its selected discharge limit.
How long will a 100 Wh battery power a 10 W load?
Ideally, a 100 Wh battery powering a 10 W load would provide about 10 hours. With 90% conversion efficiency and 90% usable battery capacity, the estimate becomes about 8.1 hours.
Does battery voltage matter for runtime?
Yes. Battery voltage is needed when converting amp-hour capacity to watt-hours. A 12 V 10 Ah battery has approximately 120 Wh of nominal energy, while a 24 V 10 Ah battery has approximately 240 Wh, assuming the nominal ratings are comparable.