If you put 100 units of electricity into a battery and take 85 back out, its round-trip efficiency is 85%. Simple on paper. On a real grid-scale system, that one number hides several underlying efficiencies, a temperature dependence, and a slow decline as the battery ages. A single percentage point up or down is worth real money over a year of dispatch - and the number on the EPC contract is rarely the number the asset actually delivers.
DC round-trip vs AC round-trip
The cleanest measure is DC round-trip efficiency: energy out of the battery terminals divided by energy into them, at the same SoC endpoints. Modern LFP cells run DC RTE around 96–98% at low C-rate and optimal temperature.
AC round-trip efficiency is the number that matters economically. It takes DC RTE and adds the losses from the power conversion system (PCS) and any transformers between the battery and the grid meter. Well-engineered modern PCS equipment runs around 97–98% one-way at partial load. Multiply these chains together and you arrive at a theoretical AC RTE in the low 90s.
Full-system AC RTE - the number the meter reports, including station auxiliaries - is typically 85–88% for grid-scale LFP systems in operation. ACCURE's 2025 Energy Storage Report, which analysed over 100 BESS sites representing 18 GWh globally, found best-in-class systems above 88% with roughly one-third of the fleet clearing that bar.
Parasitic loads are material
HVAC is the largest single parasitic line on a BESS, and it is almost never included in the datasheet RTE. A container yard in a hot climate running compressor-based cooling can spend 2–4% of throughput on HVAC alone. In a cold climate, heating is smaller but non-zero. Temperate European climates - the Iberian interior, southern France, the Po Valley - sit in the best-case window of 1–2%. Control systems, fire-suppression standby, lighting and communications add a further 0.3–0.7%. None of this shows up in the cell-level 96% DC RTE number.
Academic work [3] has quantified the parasitic load as a function of setpoint and outside temperature. The most energy-efficient cell temperature band for LFP operation is roughly 22–25°C; below or above that band, both degradation and HVAC-driven parasitic load rise.
RTE drifts with SoH and temperature
As a cell ages, its internal resistance grows. Ohmic loss - the I²R term - scales with the square of current and with resistance, so a 10% increase in internal R at a given C-rate is a proportional efficiency hit. Over 8–10 years, DC RTE can drift 1–2 percentage points just from resistance growth, and AC RTE tracks it.
Temperature compounds. Cold cells have higher internal resistance; hot cells need more HVAC work to stabilise. Both directions erode RTE. Industry HVAC/battery studies put optimum energy efficiency in the 21.8–25.2°C range, with measurable losses either side.
The revenue impact of one point
Consider a 100 MWh system doing one full cycle per day at a €80/MWh average capture spread and operating 350 days of the year. A one-point improvement in AC RTE - from 85% to 86% - delivers roughly one additional MWh of net throughput per cycle. Across a year, that is 350 MWh × €80 = €28,000 of additional gross margin, per point, per year.
Scale that over a 15-year operating horizon, and the present value of a sustained one-point RTE improvement on a 100 MWh system is a six-figure number. On a 500 MWh portfolio it is materially larger. This is why operational telemetry matters: an asset whose HVAC runs at the wrong setpoint, or whose PCS firmware is not tuned for partial-load efficiency, is leaking real revenue every day it operates off-optimum.
What to look at
A disciplined operator tracks three numbers continuously: DC RTE at the battery terminals, AC RTE at the POI meter, and auxiliary load as a fraction of throughput. The three of them together tell you whether degradation, PCS tuning, or HVAC is the current leak. ACCURE's report also notes that only around 83% of projects surveyed met their nameplate capacity at the site acceptance test - meaning RTE drift is typically layered on top of an under-delivered baseline. Measure early, measure often.