Power Bank Charge Calculator

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 Battery Energy (Wh) = Device mAh × Device Voltage ÷ 1000 Device Charges = Usable Energy (Wh) ÷ Device Battery Energy (Wh)

You will also see the shortcut Charges = Power Bank mAh × Efficiency ÷ Device mAh, and it is fine for a phone, because a phone cell and a power bank cell are both nominally 3.7V so the voltages cancel. It falls apart the moment the device is a laptop. A 4-cell laptop pack sits at 14.8V, so its 5,000 mAh is four times the energy the same 5,000 mAh would be in a phone. Compare energy, not mAh, whenever the two voltages differ.

What each variable means:

• 3.7V is the nominal voltage of the lithium cells inside power banks. Manufacturers rate capacity at this voltage, which is why a “20,000 mAh” bank is 74 Wh and not 100 Wh.
• Device output voltage is a different thing again. USB-A outputs at 5V, and USB-C fast charging at 9V, 12V or 20V under the PD protocol. Stepping 3.7V up to any of those costs energy, and that cost is what the efficiency figure covers.
• Efficiency in the real world runs 70 to 85%, with 75 to 80% typical for a mid-range unit.

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.