Round-Trip Efficiency (RTE)
Round-Trip Efficiency (RTE) is the ratio, expressed as a percentage, of the net electricity discharged from an energy storage system to the gross electricity input required to charge it over a full cycle, capturing energy losses across the battery cells, cabling, bi-directional power conversion system (PCS), step-up transformer, and thermal management.
How it is measured and specified
Round-trip efficiency is measured over a complete symmetric reference cycle (returning the battery to its exact starting State of Charge) at a specified C-rate and ambient temperature, and must always state its measurement boundary:
- DC-to-DC Cell or Rack RTE (typically 92%–95% for LFP at 0.5C): Measured at the DC terminals of the battery module or rack, capturing only electrochemical overpotential and internal busbar resistance losses.
- AC-to-AC System RTE excluding Auxiliaries (typically 88%–91%): Measured at the low-voltage or medium-voltage AC terminals of the bi-directional Power Conversion System (PCS), incorporating two passes through the inverter IGBT/SiC bridges (rectification during charge and inversion during discharge, roughly 96%–98.5% each way).
- Net AC-to-AC RTE at Point of Common Coupling including Auxiliaries (typically 84%–88%): Subtracts the parasitic energy consumed by liquid-cooling chillers, HVAC fans, BMS electronics, fire-suppression panels, and step-up transformers over a 24-hour cycle.
How it matters when sourcing from China
When reviewing technical proposals from Chinese cell and BESS suppliers, procurement teams regularly encounter datasheets quoting 94% or 95% efficiency. Almost invariably, that figure represents DC-side cell efficiency at 0.25C or 0.5C in a 25°C lab—not the AC-to-AC efficiency metered at the facility's utility interconnection point. As noted in how battery and storage TCO is modelled, round-trip efficiency and auxiliary loads directly govern the charging cost line and net delivered kilowatt-hours in every multi-year storage cash flow:
- Complete AC-to-AC Container Efficiency (87% RTE): In the 3 MWh India harbor BESS TCO study, the model applies an 87% AC-to-AC system round-trip efficiency at 0.5C across a containerized LFP + bidirectional PCS plant. Every 1.00 kWh of gross cycle throughput yields 0.87 kWh of net AC output (1,740 kWh/day net from 2,000 kWh/day gross), directly shaping the Levelized Cost of Storage (LCOS).
- Rack/Cabinet DC & Coupled Efficiency (92% RTE): In the 3 MWh Middle East & Africa hospital ESS TCO study (referencing CATL EnerC and BYD MC Cube DC blocks with 93%–95% nameplate DC RTE), the financial model applies a bounded 92% round-trip efficiency assumption, requiring 1,013,587 kWh of annual grid charging energy to deliver 930,750 kWh of Year-1 discharged energy.
- Direct Impact on Arbitrage Margin: In the 100 kWh South African museum battery storage TCO study, a 92% RTE on a 90 kWh daily cycle (90% DoD) reduces net delivered AC energy to 82.8 kWh/day (24,840 kWh/year across 300 cycles) while requiring 27,000 kWh/year of charging input—demonstrating how an 8% round-trip energy loss consumes nearly the entire gross time-of-use price difference when off-peak and peak tariffs are close together.
What to confirm before procurement
- Require the Chinese supplier to state whether the quoted RTE is measured DC-to-DC at the battery rack or AC-to-AC at the PCS output terminals, and at what C-rate (0.25C, 0.5C, or 1C).
- Request a separate auxiliary power consumption schedule (in kWh/day at 25°C, 35°C, and 45°C ambient temperatures) for the container's liquid-cooling chiller or HVAC unit.
- Include both PCS conversion losses and chiller/HVAC parasitic loads when modeling annual charging electricity costs and LCOS.