Round-Trip Efficiency Explained: How Much Energy Storage Gives Back
Updated October 5, 2026
Round-trip efficiency (RTE) is the percentage of electricity put into a storage system that you can later get back out. If you store 100 MWh and recover 85 MWh, the round-trip efficiency is 85%. The missing 15% is lost as heat and other conversion losses.
The formula
Round-trip efficiency = energy discharged ÷ energy used to charge × 100%
It's important to know where it is measured. "DC" efficiency at the battery terminals is higher than "AC" efficiency measured at the grid connection, which also includes inverter and transformer losses and auxiliary loads such as cooling. Project specs should say which one they quote.
Typical round-trip efficiency by technology
| Technology | Typical RTE (approx.) |
|---|---|
| Lithium-ion batteries (LFP/NMC) | 85–90% (AC, system level often low-to-mid 80s) |
| Pumped hydro storage | 70–80% |
| Flow batteries (e.g., vanadium) | 65–75% |
| Compressed air (CAES) | 40–70% depending on design |
| Thermal storage (power-to-heat-to-power) | Wide range, often 40–60% |
| Iron-air batteries | Well below lithium-ion |
| Hydrogen (electrolysis → storage → fuel cell/turbine) | ~30–40% |
Figures vary by system design, age, and operating conditions. U.S. EIA analysis of operating data has found average monthly round-trip efficiency of roughly 82% for utility-scale batteries and 79% for pumped hydro.
Why it matters, and when it doesn't
Higher efficiency means less wasted energy and better economics for storage that cycles every day. For a battery doing daily price arbitrage, a few points of efficiency directly hit revenue.
Efficiency isn't everything, though. For storage used only a few times a year, such as multi-day or seasonal storage, a lower cost per kWh of capacity can matter far more than efficiency. That's why technologies like iron-air or hydrogen can make sense despite lower RTE. When the charging electricity is surplus renewable power that would otherwise be curtailed, losses cost much less.
Related metrics
- Self-discharge: energy lost while sitting idle. Low for lithium-ion, near zero for pumped hydro apart from evaporation.
- Depth of discharge: how much of the rated capacity can be used.
- Degradation: efficiency and capacity typically decline over the system's life.
- Auxiliary load: HVAC and controls consume energy even when the system is idle.
Bottom line
Round-trip efficiency tells you how much energy storage gives back. For daily-cycling storage, higher is better and lithium-ion leads. For long-duration and seasonal storage, low capital cost can outweigh lower efficiency, so judge each technology by the job it's built for.
Frequently asked questions
What is a good round-trip efficiency for a battery?
Modern lithium-ion systems typically achieve about 85–90% at the battery level, and somewhat lower when measured at the grid connection including auxiliary loads.
Why is hydrogen storage efficiency so low?
Each conversion step (electrolysis, compression or storage, and conversion back to electricity) loses energy, leaving roughly 30–40% overall.
Is round-trip efficiency the same as battery efficiency?
They're related, but RTE specifically measures the full charge-then-discharge cycle, and it depends on whether it is measured at DC or AC.