mAh ↔ Wh — Battery Capacity
Convert capacity and energy with voltage, and size a series/parallel pack.
Capacity, voltage and energy
A battery's energy is its charge times its voltage:
That's why milliamp-hours alone can't compare two packs at different voltages — only watt-hours can. For a pack, series cells multiply voltage and parallel cells multiply capacity:
Use the nominal cell voltage (≈3.7 V Li-ion, 3.2 V LiFePO4, 1.2 V NiMH). Real usable energy is a bit lower because of the discharge cut-off and internal resistance.
Frequently asked questions
How do I convert mAh to Wh?
Wh = (mAh ÷ 1000) × voltage. For example a 2500 mAh cell at 3.7 V holds 2.5 × 3.7 = 9.25 Wh. To go back: mAh = (Wh ÷ voltage) × 1000.
What voltage should I use?
Use the battery's nominal voltage: about 3.7 V for a Li-ion/LiPo cell, 3.2 V for LiFePO4, 1.2 V for NiMH and 1.5 V for alkaline. The label's mAh rating is specified at that nominal voltage.
How do series and parallel cells change capacity?
Cells in series add voltage (pack V = cell V × S) and keep the same mAh. Cells in parallel add capacity (pack mAh = cell mAh × P) at the same voltage. Total energy in Wh multiplies by S × P either way.
Why use Wh instead of mAh to compare batteries?
mAh only means something at a fixed voltage, so you can't compare packs of different voltages by mAh. Watt-hours include the voltage, so Wh is the fair way to compare energy — and it's what airlines and shipping rules use.