Round trip efficiency (RTE) is the ratio of energy discharged from a battery to the energy required to charge it. It is one of the most important factors in energy storage economics.
How RTE Works
Example: You put 10kWh into a battery, but only get 8kWh back out. Round trip efficiency = 8/10 = 80%.
The missing 2kWh is lost as heat during the charge and discharge process.
RTE by Battery Technology
- Lead-acid (flooded, AGM, GEL): 75-90% depending on charge rate and depth of discharge
- OPzV tubular GEL: 80-88% typical
- LiFePO4: 90-95% — significantly more efficient
Factors Affecting RTE
- Charge rate: Very slow or very fast charging reduces efficiency
- Temperature: Cold reduces chemical efficiency; optimal is 20-30C
- Depth of discharge: Deeper cycles have slightly lower RTE
- Battery age: Older batteries have lower RTE due to plate degradation
- System design: Inverter/charger efficiency, cable losses, and BMS consumption all affect overall system RTE
Why RTE Matters for Solar Economics
On a 10kWh battery cycling daily at 80% RTE vs 90% RTE:
Difference: 1kWh/day x 365 days = 365kWh/year lost to inefficiency
At $0.15/kWh: $55/year wasted. Over 10 years: $550 on a single small battery bank.
For commercial systems (100kWh+), the annual cost of inefficiency is $550-5,500/year.