Power bank labels show capacity in mAh, but that number is misleading. A 20,000mAh power bank does not charge a 4,000mAh phone five times. Voltage conversion and heat losses eat 15-25% of the rated capacity. This calculator accounts for real-world efficiency to give you an honest runtime estimate.
Typical Device Power Draw
| Device | Typical Draw | Full Charges from 20,000mAh |
|---|---|---|
| iPhone 15 (3,349mAh) | 5-10W | ~4 charges |
| Samsung Galaxy S24 (4,000mAh) | 5-15W | ~3.5 charges |
| iPad Air (28.6Wh) | 10-20W | ~2 charges |
| Nintendo Switch (16Wh) | 8-18W | ~3.5 charges |
| AirPods Pro case (0.52Wh) | 1-2W | ~100+ charges |
| GoPro Hero 12 (6.4Wh) | 3-5W | ~9 charges |
| Laptop (USB-C, 50Wh) | 30-65W | ~1 partial charge |
"Full charges" assumes 85% conversion efficiency from the power bank's 3.7V internal cells to the 5V USB output. Actual results vary by cable quality and ambient temperature.
How to Find Your Power Bank's Real Capacity
- Find the mAh rating on the label. This is the cell-level capacity at the internal voltage (usually 3.7V). A "20,000mAh" bank stores 74Wh (20,000 x 3.7 / 1000).
- Check for a Wh rating. Some manufacturers list the real energy in watt-hours. This is a more honest spec. Airlines require Wh for lithium battery limits (100Wh carry-on, 160Wh with airline approval).
- Apply the conversion loss. When the internal 3.7V is boosted to 5V USB, you lose 10-20% to heat. A 74Wh bank delivers roughly 59-67Wh at the USB port. For USB-C PD at 20V, losses are similar because the boost ratio is higher.
- Divide by your device battery. Your phone battery in Wh = mAh x voltage / 1000. Most phones run at 3.85-3.87V internally. A 4,000mAh phone battery is about 15.4Wh.
Frequently Asked Questions
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Power bank marketing overstates real capacity by 20-40% because they quote cell-level mAh without accounting for conversion losses. Always convert to watt-hours for honest comparisons, and expect about 60-70% of the headline mAh to reach your device in practice.
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Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.