Solar battery round-trip efficiency: the energy lost when you store and return electricity

Round-trip efficiency measures how much electricity you get back out of a battery compared with how much you put in. Charging, holding and discharging all lose a little energy, mostly as heat, along with small losses in the inverter and battery management electronics.

For a modern lithium battery the figure is often somewhere in the region of 85 to 95 per cent, depending on the technology, the temperature and how hard the battery is worked. If you store 10 kWh, you might draw roughly 9 kWh back. The rest is the cost of storing power for later.

Solar is most valuable when you use it directly, with no storage losses at all. A battery earns its keep by shifting daytime generation to the evening, but every unit that passes through the battery arrives slightly reduced. That loss sits between the solar you generate and the grid power you avoid.

This affects the sums. If your battery returns 90 per cent of what it stores, then ten per cent of the stored solar never reaches your appliances. The saving still usually beats buying that power from the grid in the evening, but the gap is smaller than a simple before-and-after comparison suggests.

Manufacturers usually quote a round-trip figure on the datasheet, sometimes measured under favourable lab conditions. Ask whether the quoted number includes the inverter, because an AC round-trip figure and a DC one are not the same thing and can differ by several points.

Efficiency is one factor among several, alongside usable capacity, warranty, cycle life and how well the battery holds up in a cold plant room. A small difference in round-trip efficiency rarely decides a project on its own, but it is a fair question to put to any installer or funder before you commit.