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Understanding mAh vs Wh
Milliamp hours (mAh) measures how much charge a battery can hold at a specific voltage. Watt hours (Wh) measures actual energy storage, making it the better unit for comparing batteries of different voltages.
When to use each unit
Use mAh
Charge- Estimate number of phone charges
- Compare power banks at same voltage
- Match charger output to device needs
Use Wh
Energy- Compare power banks of different voltages
- Check airline carry-on limits (100 Wh)
- Size a solar panel for charging
Common battery voltages
- 3.7V — Single lithium-ion cell (phones, power banks internally)
- 5V — USB output voltage (what you actually get from a power bank)
- 7.4V — Camera batteries, some laptops (2 cells in series)
- 11.1V — Drone batteries, laptop packs (3 cells in series)
Why mAh ratings are misleading
Power banks advertise their capacity at the internal cell voltage (3.7V), but deliver power at USB voltage (5V). The voltage conversion loses about 15–25% of the rated capacity. A “20,000 mAh” power bank actually delivers about 14,800 mAh at 5V — equivalent to 74 Wh.
Real-world examples
At the internal 3.7V, this stores 74 Wh. At 5V USB output with ~85% conversion efficiency, usable capacity is about 62.9 Wh. This charges a typical 5,000 mAh phone about 3 times in practice — not the 4 times the mAh ratio suggests.
A Samsung 30Q 18650 cell at 3,000 mAh and 3.6V stores 10.8 Wh. A DIY powerwall with 140 cells (14S10P) stores 1,512 Wh — enough for a 60W camping setup for over 25 hours.
A smartphone battery at 5,000 mAh and 3.85V stores 19.25 Wh. A 74 Wh power bank can charge it 3.8 times theoretically — about 3 times in practice after conversion losses.
When you'll need this conversion
- Sizing a portable solar panel for a power bank — To charge a 74 Wh power bank in one day with 5 sun hours and 80% efficiency: 74 ÷ 0.80 ÷ 5 = 18.5W minimum panel. A 20W panel works in ideal conditions; 30W provides margin for clouds.
- Drone operations from a solar setup — A DJI Mavic battery at 5,000 mAh and 15.4V stores 77 Wh. Four batteries need 308 Wh. A 100W foldable solar panel producing for 5 hours generates 500 Wh — enough for remote site surveys.
- Emergency power calculations — Daily essentials: phone charge (19.25 Wh) + USB LED light (30 Wh) + USB fan (24 Wh) = 73.25 Wh/day. A 30,000 mAh power bank (111 Wh) provides ~1.5 days. Pairing with a 20W solar panel extends this indefinitely.
