What do you want to work out? Power bank charges Device runtime Power bank capacity mAh Phone / gadget battery mAh Charging efficiency % Real-world is usually 60–70%. Battery capacity Wh Printed on laptops and power banks. mAh × volts ÷ 1000 = Wh. Average power use W Efficiency % Calculate Uses 3.7 V for power-bank cells and 3.85 V for phone batteries. Heat, cable quality, and fast-charging modes change real results.