What is RTE (round-trip efficiency)?

RTE is the round-trip efficiency: if we charge 100 kWh of energy into the storage system, how much of it can we get back. For modern industrial lithium-ion systems, RTE is typically 86–92%, measured on the AC side — meaning the losses of the bidirectional converter are included. The loss leaves the system as heat, which is why active cooling of the containers is not optional. RTE is a key element of the arbitrage calculation: the net spread is not simply the difference between the discharge price and the charging price, but discharge price − (charging price ÷ RTE). At 88% RTE, energy bought at 100 EUR/MWh actually costs 113.6 EUR/MWh — arbitrage is only profitable above that level. A lower efficiency therefore requires a larger daily price difference for the same margin. RTE deteriorates slowly over the service life, and partial-load operation gives a somewhat better value than charging at maximum power. When comparing offers, always clarify whether the stated RTE is DC-side or AC-side — the difference between the two can be as much as 3–5 percentage points, which is significant in the payback calculation.