Round-trip efficiency (battery)

Written and checked by the Energy Merchants desk · Reviewed 3 October 2026

Round-trip efficiency measures how much of the energy you put into a battery you get back out. It counts losses from charging, storing and discharging, including heat in the cells and conversion in the inverter.

Home lithium batteries typically quote somewhere around 85% to 95%. The datasheet gives the exact figure and the conditions it was tested under, which matter, because efficiency drops at high power and in extreme temperatures.

A worked example

A household charges its battery with 10 kWh of surplus solar. At 90% round-trip efficiency it can draw 9 kWh back out later. If the evening grid price is higher than the feed-in tariff, the 9 kWh is still worth more than the 10 kWh would have earned if exported. The lost 1 kWh is the cost of shifting energy in time.

Efficiency compounds with the system layout. An AC coupled setup converts energy more times than a DC coupled one, so its whole-system figure is usually a little lower, even if the battery itself is excellent.

In practice

Installers use efficiency to compare products honestly for a customer. Two batteries with the same usable kWh can pay back differently if one loses several points more. It also feeds payback maths: see simple payback period.

It does not affect the STC count. Under the Cheaper Home Batteries Program, certificates come from usable capacity multiplied by the factor, with the tier percentages applied. Efficiency, power rating and brand do not change the number.

Common confusion

Round-trip efficiency is not the same as depth of discharge. One is a loss rate, the other a usable fraction. A battery can be strong on one and weak on the other.

It is also not inverter efficiency alone, which is only the conversion step. Check whether a quoted figure is for the battery, the inverter or the whole system.

See battery STCs and the Cheaper Home Batteries Program installer guide. More terms are in the glossary.