Power Bank Charge Calculator
Calculate how many charges a power bank provides for your phone, tablet, or earbuds from mAh capacity, device battery size, and 85-90% efficiency loss.
A power bank’s true capacity and charging efficiency determine how many times it can charge your devices. Marketing specs show milliamp-hour (mAh) capacity, but real-world performance is always lower due to voltage conversion losses.
Key formulas: Usable Energy (Wh) = Rated Capacity (mAh) × 3.7V ÷ 1000 × Efficiency Device Charges = Usable Energy (Wh) ÷ Device Battery Energy (Wh) Device Battery Energy (Wh) = Device mAh × Device Voltage ÷ 1000 Charge Cycles = Power Bank mAh × Efficiency ÷ Device mAh
What each variable means:
- 3.7V — the nominal voltage of lithium cells inside power banks. Manufacturers rate capacity at this voltage.
- Device Output Voltage — USB-A outputs at 5V; USB-C fast charging at 9V, 12V, or 20V (PD protocol). The voltage step-up from 3.7V to 5V causes conversion losses.
- Efficiency — real-world power banks convert 70–85% of their stored energy to output. Average: ~75–80% for mid-range units.
Quick reference — charges per device type:
A 20,000 mAh power bank at 80% efficiency = 60 Wh usable:
- iPhone 15 (3,877 mAh, 3.7V = 14.3 Wh): 60 ÷ 14.3 = ~4.2 full charges
- Samsung Galaxy S24 (4,000 mAh = 14.8 Wh): 60 ÷ 14.8 = ~4.1 charges
- iPad Air (28.65 Wh): 60 ÷ 28.65 = ~2.1 charges
- MacBook Air 13" (52.6 Wh): 60 ÷ 52.6 = ~1.1 charges (requires high-wattage PD)
Worked example: Power bank: 10,000 mAh rating. Efficiency: 75%. Phone: 4,500 mAh at 3.7V. Power bank usable = 10,000 × 3.7 ÷ 1000 × 0.75 = 27.75 Wh Phone battery = 4,500 × 3.7 ÷ 1000 = 16.65 Wh Charges = 27.75 ÷ 16.65 = 1.67 charges (not the “2+” often implied by marketing)
Pro tip: Fast charging (higher voltage protocols like PD or QC) is less efficient than standard 5W charging. If you want maximum charges per power bank, use standard charging mode when time allows.