Battery round-trip efficiency

The fraction of energy you actually get back out of the battery for every kWh you put in. A value of 0.92 means 92% — store 10 kWh, retrieve 9.2 kWh later. The missing 8% is lost as heat in the cells, the BMS, and the inverter conversion on both legs of the round trip.

How the simulator uses it

On every step the simulator applies the square root of the round-trip efficiency at both charge and discharge — so 0.92 round-trip → 0.96 one-way each direction. Battery throughput tracks the loss exactly: 10 kWh charged adds ~9.6 kWh to the battery; 10 kWh out takes ~10.4 kWh from it. Lower efficiency means more solar lost between panel and useful load.

Typical values

0.92 — modern lithium iron phosphate (LFP) home battery + hybrid inverter combo. This is the sensible default.
0.95 — newer high-end inverters with silicon-carbide power stages (some Tesla Powerwall 3 installs report this).
0.85 — older lead-acid, or AC-coupled systems with two separate inverters (PV inverter + battery inverter compounding losses).
0.80 or below — usually indicates a failing battery or a derated inverter, not a normal operating point.

Common pitfalls

Some manufacturers quote DC-DC efficiency (cell-only) which is ~96–98% but excludes the inverter losses. The number this field wants is the AC-to-AC round trip — what you measure between grid in and grid out.

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