Round-Trip Efficiency

Round-trip efficiency is the percentage of energy that can be retrieved from a battery compared to the energy that was put in. It accounts for the energy losses that occur during the charging and discharging process.

For example, if you store 10 kWh of solar energy in a battery with 90% round-trip efficiency, you can retrieve 9 kWh — the other 1 kWh is lost as heat during the conversion process. Modern lithium-ion home batteries achieve round-trip efficiencies of 85–95%, meaning relatively little energy is wasted. Higher efficiency means more of your solar energy is usable and less money is lost to conversion losses. When comparing battery options, even a 5% difference in efficiency adds up over thousands of cycles and years of operation. AC-coupled battery systems typically have slightly lower round-trip efficiency than DC-coupled systems because AC coupling involves an additional conversion step (DC → AC → DC → AC). The Tesla Powerwall has a round-trip efficiency of approximately 90%, while Enphase IQ batteries achieve around 89%.