Round-trip efficiency (RTE)

Round-trip efficiency (RTE) is the share of electricity a battery energy storage system returns to the grid or load compared with what it took in to charge, expressed as a percentage. A 90% RTE means charging a BESS with 100 kWh yields about 90 kWh of usable discharge, with the rest lost to internal resistance, inverter conversion and thermal management. Modern lithium-ion BESS typically achieves 85-95% RTE at the AC-to-AC level, with DC-coupled configurations edging higher because they avoid one round of AC/DC conversion; older lead-acid and some long-duration chemistries fall well below that range. RTE degrades gradually over a system's life as cells age, and warranty terms increasingly guarantee a minimum RTE alongside capacity retention. For data center operators pairing BESS with on-site generation or grid arbitrage, RTE directly sets the economics: a lower RTE means more generation or grid draw is needed to deliver the same usable megawatt-hours, raising the effective levelized cost of storage.