Battery Runtime Calculator
Whether you're working with a full-size power station or a small USB power bank, runtime comes down to the same relationship between stored energy and power draw. Use the calculator below with any capacity in watt-hours — and if your battery is only labeled in mAh, convert it first using the box further down.
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Enter your power station’s capacity in Wh to see a runtime estimate.
This is an estimate. Real-world runtime can be lower than the theoretical figure due to inverter efficiency losses, fluctuating (not constant) device draw, ambient temperature, battery age/health, and other factors. Always check your power station’s official specifications before relying on it for critical use.
The universal battery runtime formula
Regardless of battery size or brand, runtime follows the same two-step calculation:
Usable Energy (Wh) = Battery Capacity (Wh) × Efficiency
Runtime (hours) = Usable Energy (Wh) ÷ Total Power Consumption (W)
The only variable that changes between a phone power bank and a home backup power station is the scale of the numbers — everything from a 74 Wh USB battery to a 3000 Wh home unit uses the exact same math.
Converting mAh to Wh
Small power banks and phone batteries are usually labeled in milliamp-hours (mAh) rather than watt-hours, because manufacturers assume a fixed internal voltage. To use one with the calculator above, convert it first:
Watt-hours (Wh) = (mAh × Voltage) ÷ 1000
Most USB power banks use a nominal 3.7V lithium cell internally (even though they output 5V/9V/12V via USB — that conversion happens inside the pack). So a 20,000 mAh power bank is approximately:
20,000 × 3.7 ÷ 1000 = 74 Wh
| Capacity (mAh) | Approx. capacity (Wh, at 3.7V) |
|---|---|
| 10,000 mAh | 37 Wh |
| 20,000 mAh | 74 Wh |
| 26,800 mAh | 99 Wh |
| 30,000 mAh | 111 Wh |
26,800 mAh is a common ceiling for carry-on air travel under most airline lithium-battery rules (roughly a 100 Wh limit) — always check your airline's current policy before flying with any battery pack.
Frequently asked questions
Why is my power bank's real-world capacity lower than advertised?
The mAh rating on the box measures the raw cell capacity, before any conversion losses. By the time that energy passes through the pack's internal circuitry and out through a USB port, you typically only recover 70–85% of it as usable charge for your device — which is exactly what the efficiency setting in this calculator accounts for.
How long will a power bank charge my phone?
Roughly: divide the power bank's Wh (converted from mAh as above) by your phone's battery capacity in Wh, then multiply by your chosen efficiency. A 74 Wh power bank charging a ~15 Wh phone battery at 85% efficiency could provide around 4 full charges (74 × 0.85 ÷ 15 ≈ 4.2).
Does fast charging affect runtime calculations?
Fast charging changes how quickly energy is delivered, not how much total energy is available — the same Wh of stored energy is consumed either way, just faster. Runtime math based on watt-hours stays accurate regardless of charging speed.
What voltage should I use if I don't know my battery's exact voltage?
For small lithium-ion/lithium-polymer packs (phones, power banks, laptops), 3.7V per cell is a safe general assumption. For power stations and larger systems, the manufacturer's specification sheet will state the Wh capacity directly — use that figure rather than estimating from mAh.