Round-trip efficiency

Round-trip efficiency describes how much energy a battery storage system can return compared with the electricity put into it during charging. If 10 kWh is charged and 9 kWh is later available from the same measured path, the round-trip efficiency for that path is 90%. The missing energy is the charging, conversion, storage and discharge loss inside the battery system. For a home battery, the percentage matters when comparing solar self-consumption, grid charging on a time-of-use tariff or expected usable energy after a charge. A manufacturer figure should be read with its test boundary and conditions, such as whether it is AC-to-battery-to-AC, the charge and discharge power, temperature and whether the battery is at the beginning of life.