Round-trip efficiency
The share of energy put into a storage system that can be recovered on discharge, measured at a defined boundary (cell, DC, or AC grid connection).
Round-trip efficiency (RTE) is the ratio of energy delivered during discharge to energy absorbed during charge over a full cycle. The measurement boundary matters: a lithium-ion cell may reach 95–98% DC-to-DC, while an AC-to-AC figure at the grid connection, including inverter, transformer, cooling and standby loads, is typically 85–90%.
RTE ≈ η_charge x η_discharge
Example: 1 MWh charged at 88% RTE returns 0.88 MWh
Typical AC round-trip values:
- Lithium-ion BESS: 85–90%
- Pumped hydro: 70–85%
- Vanadium flow battery: 65–80%
- Diabatic CAES: 40–55%; adiabatic designs target about 60–70%
- Hydrogen (electrolysis plus fuel cell or turbine): roughly 30–45%
RTE directly affects arbitrage economics: with 85% RTE, a battery must sell power at least 1/0.85, about 1.18 times its purchase price just to break even on energy, before degradation. It also feeds into LCOS. Self-discharge, which is small for lithium-ion (1–3% per month) but large for thermal stores, is usually reported separately.
Related terms
All terms →- LCOS (Levelized Cost of Storage)
The average cost per MWh discharged over a storage asset’s lifetime, including investment, operation, charging energy and efficiency losses.
- BESS (Battery Energy Storage System)
A complete grid- or site-connected battery installation, including cells, battery management, power conversion, thermal management and controls.
- Pumped hydro storage
Energy storage that pumps water to an upper reservoir when electricity is cheap or abundant and releases it through turbines when power is needed; the largest form of grid storage worldwide.
- Flow battery
An electrochemical battery storing energy in liquid electrolytes held in external tanks, so power (stack size) and energy (tank volume) scale independently.
- Compressed air energy storage (CAES)
A storage technology that compresses air into caverns or vessels with surplus electricity and expands it through a turbine to regenerate power.
- Long-duration energy storage (LDES)
Storage able to discharge at rated power for roughly 8 hours or more, covering multi-day lulls and seasonal imbalances that 2–4 hour batteries cannot.