Use energy capacity, average load, and realistic losses together. A large mAh number does not establish compatibility or guaranteed runtime.
Start with watt-hours, then check watts
Estimated runtime is usable energy in watt-hours (Wh) divided by average device power in watts (W). Capacity describes stored energy; maximum output describes how quickly the device can deliver it. A high-capacity battery may still lack the voltage or charging protocol your laptop needs.
Use the manufacturer’s Wh rating where available. If only milliamp-hours and the corresponding rating voltage are supplied, calculate:
Wh = mAh × rating voltage ÷ 1,000
For example, 20,000 mAh rated at 3.7 V represents 74 Wh. Multiplying by an arbitrary 5 V USB output instead would incorrectly produce 100 Wh. Use the voltage that belongs to the stated capacity, not whichever output voltage you happen to see.
Account for losses without treating an assumption as a guarantee
Suppose, only for a worked example, that 80% of a 74 Wh rating reaches the load. Usable energy would be 74 × 0.8 = 59.2 Wh. Actual results depend on conversion, battery age, temperature, cables, output mode, and operating conditions.
| Assumed constant load | Calculation | Estimated runtime |
|---|---|---|
| 5 W | 59.2 ÷ 5 | 11.8 hours |
| 10 W | 59.2 ÷ 10 | 5.9 hours |
| 20 W | 59.2 ÷ 20 | 3.0 hours |
These are arithmetic examples, not measured product results. Phones change power use with screen brightness, radio activity, and charging stage. Do not turn this table into a guaranteed phone recharge count.
Add simultaneous loads and verify port sharing
Two 5 W lights and another device averaging 10 W create a 20 W combined load. Under the same example assumptions, runtime is about three hours—but only if the battery supports that combination.
A headline maximum such as 65 W may apply to one port. Connecting another device can redistribute available power or briefly renegotiate charging. Read the simultaneous-output specifications and verify device voltage, current, charging protocol, and cable capability. Correct energy arithmetic cannot compensate for incompatible output.
Test the exact equipment you will use
In a safe everyday setting, connect the actual devices and cables. Record starting charge, elapsed time, and device settings. A brief test can reveal a failed cable or unstable charging; it cannot precisely guarantee full-discharge runtime because percentage indicators are approximate.
Do not test or continue using a swollen, damaged, or abnormally hot battery. Follow its charging, operating, and storage instructions. For medically necessary powered equipment, obtain an individual backup and evacuation plan from the supplier and care team rather than relying on this general calculation.
Common questions
Does 20,000 mAh mean four charges of a 5,000 mAh phone? Not reliably. Voltage differences, conversion losses, charging losses, and phone use matter.
Does fast charging extend runtime? It changes delivery speed, not the amount of stored energy.
Does the formula apply to a power station? The principle does, but inverter losses, standby consumption, startup loads, and minimum-load behavior also matter.
Do this today
- Record the Wh rating and output specifications.
- List loads and required hours.
- Calculate an assumption-based energy budget.
- Test the intended cables and port combination.
Technical basis: energy (Wh) divided by power (W) gives hours. The 80% figure is an illustrative assumption, not a product efficiency claim.