Power Bank & Battery Capacity Calculator
Estimate power bank Wh capacity, usable energy after efficiency losses, and how many full device charges it delivers from mAh, voltage, and efficiency.
Power Bank Calc — interactive calculator, computed instantly in your browser.
Power Bank Capacity Realized: mAh, Wh and Actual Charges
Labeled Capacity vs Energy You Actually Get
Power banks are sold on mAh — milliamp-hours — but batteries move energy, and energy needs volts too. A 20,000 mAh bank at the cell's 3.7 V stores 74 Wh, not some phone-sized multiple of your device's mAh. This calculator converts capacity into watt-hours, applies conversion efficiency, and tells you exactly how many full charges to expect.
1. From mAh to Watt-Hours
- Bank: 20,000 mAh × 3.7 V ÷ 1000 = 74 Wh of stored energy
- Phone: 4,500 mAh × 3.85 V ÷ 1000 = 17.3 Wh per full charge
- The naive mAh ratio (20,000 ÷ 4,500 = 4.4×) ignores voltage and always overpromises — energy conservation does not care how the label reads
- Charges = usable Wh ÷ device Wh — here 74 × 85% ÷ 17.3 ≈ 3.6 charges, not 4.4
2. Where the Missing Energy Goes
The cell runs at 3.7 V but USB delivers 5 V (or 9 V for fast charging), so a boost converter steps voltage up — and every conversion leaks as heat. Real-world round-trip efficiency lands at 80–90%; add cable resistance and a warm ambient environment and 85% is a fair default. Cheap no-name banks inflate capacity claims and use low-grade cells that sag under load, delivering even less.
3. Practical Rules and Travel Limits
- Airlines cap power banks at 100 Wh (carry-on only) — a 20,000 mAh / 74 Wh bank flies; a 30,000 mAh one usually needs airline approval above 100 Wh
- Charging a phone at 5 V/2 A is faster than 5 V/1 A but both draw the same total energy — speed changes time, not capacity
- Cells lose ~20% of capacity per year; a bank that once gave four charges will eventually give three
- Store banks at ~50% charge in cool places — full charge plus heat accelerates chemical aging
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Frequently Asked Questions (Power Bank Calc)
Divide the bank's usable watt-hours by your phone's battery watt-hours. A 20,000 mAh / 3.7 V bank stores 74 Wh; after ~85% conversion efficiency you get 63 Wh, and a 4,500 mAh / 3.85 V phone needs 17.3 Wh per charge — about 3.6 full charges in practice.
Because mAh measures charge, not energy, and energy depends on voltage. The bank's cells sit at 3.7 V while your phone battery averages 3.85 V, and the converter in between wastes 10–20% as heat — dividing the two mAh figures directly skips all three corrections.
Most airlines allow power banks up to 100 Wh in carry-on luggage with no approval, between 100 and 160 Wh with airline permission, and ban anything above 160 Wh. Lithium banks are never allowed in checked bags because of fire risk in the cargo hold.
The label reflects raw cell capacity at 3.7 V with zero losses. Boost-converter inefficiency, cable resistance, phone use during charging, and battery aging each shave off a slice — 15–25% total loss versus the naive mAh ratio is completely normal behavior, not a defective unit.