What is Watt-Hour Efficiency of a Battery?

Watt-Hour (Wh) efficiency is the ratio of the total Electrical Energy output delivered by the battery during discharging, to the total Electrical Energy input required to fully charge it back.

Since Energy = Power $\times$ Time (and Power is measured in Watts), this efficiency gives the true picture of energy loss in the form of heat during the chemical reaction.

Watt-Hour Efficiency (%) = (Watt-hours of Discharge / Watt-hours of Charge) $\times$ 100

Alternatively, since Power (Watts) = Voltage (V) $\times$ Current (A): Watt-Hour Efficiency = (Average Discharge Voltage $\times$ Ampere-hours of Discharge) / (Average Charge Voltage $\times$ Ampere-hours of Charge) $\times$ 100.

• Ampere-Hour (Ah) Efficiency: This only measures the quantity of electrical charge (current) stored and returned. It ignores voltage changes. For a normal lead-acid battery, Ah efficiency is usually very high, around 90% to 95%.
• Watt-Hour (Wh) Efficiency: This takes into account both Current AND Voltage drops. Because a battery requires a higher voltage to charge (e.g., 14V) but delivers a lower voltage when discharging (e.g., 12V), the Wh efficiency is always lower than Ah efficiency. For a lead-acid battery, Wh efficiency is usually around 75% to 80%.