Battery and charging
What are watt-hours (Wh) and how do you calculate battery runtime?
Watt-hours (Wh) measure how much energy a battery stores; watts (W) measure how much power a device draws at any moment. Dividing the battery's Wh by the device's W gives a rough idea of how many hours it can run, after subtracting conversion losses.
How to estimate runtime
Rough formula: hours = battery Wh × 0.85 ÷ device W. The 0.85 accounts for inverter and electronics losses; every unit has its own efficiency.
Example: a 500 Wh station running a 10 W router and a 10 W bulb (20 W total) would last about 21 hours (500 × 0.85 ÷ 20). A refrigerator, which cycles on and off, makes the estimate more variable.
From mAh to Wh
Power banks are sold in mAh, but that figure is at 3.6–3.7 V, the voltage of their cells. To convert: mAh × voltage ÷ 1,000. A 10,000 mAh power bank at 3.7 V stores about 37 Wh.
That is why two batteries with the same mAh can hold different amounts of energy if their voltage differs: Wh is the fair way to compare. For flights, IATA allows batteries up to 100 Wh in the cabin without special approval.
What to check before you buy
- Add up the watts of what you want to keep running and estimate hours from the Wh.
- Do not confuse Wh (how long it lasts) with output W (how many devices it can run at once).
- To compare power banks, convert their mAh to Wh.
Guides to help you choose
Frequently asked questions
Both. Wh tells you how long the energy lasts; output W tells you whether the battery can power your devices at the same time.