About the Round-Trip Efficiency
Round-trip efficiency (RTE) is the fraction of energy you get back out of a battery system relative to what you put in over a full charge-discharge cycle. It compounds the charging converter, the cells, the discharging converter, and auxiliary (HVAC/BMS) losses into one AC-to-AC number.
Formula
RTE (%) = Energy discharged (AC) ÷ Energy charged (AC) × 100
- Energy discharged
- AC energy delivered on discharge (kWh)
- Energy charged
- AC energy drawn to charge (kWh)
RTE ≈ η_charge × η_cell × η_discharge × η_aux
- η_charge / η_discharge
- PCS/converter efficiency each way
- η_cell
- Battery DC coulombic/voltage efficiency
- η_aux
- Auxiliary loads: thermal management, BMS
How to calculate it
- 1Take the PCS/converter efficiency for charging and discharging (each ≈96–98%).
- 2Multiply by the battery DC efficiency (≈95–98% for lithium).
- 3Apply an auxiliary-load factor for HVAC/BMS parasitic consumption.
- 4The product is the AC-to-AC round-trip efficiency.
Worked example
A lithium BESS with 97% charge, 97% discharge, 97% DC cell and ~98% auxiliary factor lands at 0.97 × 0.97 × 0.97 × 0.98 ≈ 89% AC round-trip efficiency.
What you can calculate
- RTE %
- Aux losses
- IEC 62933
- Chemistry compare
Standards & references
Frequently asked questions
What is a good round-trip efficiency for a battery?
Modern lithium (LFP) BESS achieve an AC round-trip efficiency of roughly 85–92%. Lead-acid is lower, around 70–80%. Flow batteries sit lower still. The figure quoted matters: a DC-side number is always higher than the AC-to-AC number that includes the PCS and auxiliaries.
Why is round-trip efficiency lower than the cell efficiency?
The cells themselves are very efficient, but every stage adds loss — the power-conversion system on the way in and out, and continuous auxiliary loads like thermal management and the BMS. RTE captures the whole system AC-to-AC, so it is always below the bare cell efficiency.
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