BESS Round-Trip Efficiency Calculator
Round-trip efficiency (RTE) is the percentage of energy you get back out of a battery compared to what you put in — energy discharged divided by energy charged. It captures every loss in the system: cell chemistry losses, inverter conversion losses, and auxiliary loads like cooling and the battery management system. This calculator takes energy charged, energy discharged, the charging electricity price, and cycles per year, then reports RTE with a health flag, energy and cost lost per cycle, and annual energy and cost lost. Pair it with our Battery Arbitrage Revenue Calculator to see how efficiency losses cut into arbitrage spreads, our LCOS Calculator to fold RTE into levelized storage cost, and our Battery Thermal Management Calculator — this calculator's heat-loss concept is applied to cooling costs there.
Total energy put into the battery system per cycle, in MWh.
Total energy delivered back out of the battery system per cycle, in MWh.
Typically an off-peak charging price when modeling arbitrage-focused BESS.
Number of charge/discharge cycles per year. 1 cycle/day ≈ 365.
Healthy — typical range for modern lithium-ion BESS
energy charged − energy discharged
energy lost per cycle × charging electricity price
energy lost per cycle × cycles per year
cost of losses per cycle × cycles per year
Results update live as you type. For planning and field-check estimates — always verify against applicable standards and equipment ratings.
How we calculate this →A few efficiency points add up fast. At 87% RTE, a 100 MWh daily cycle loses 13 MWh to charging and discharging losses — worth $390 at a $30/MWh charging price. Cycle that 350 times a year and it's $136,500 in lost value, before accounting for the fact that RTE typically declines further as the battery ages. Small efficiency differences between vendors — or a degrading system — compound into real money over a project's life.
How round-trip efficiency is calculated
Round-trip efficiency (RTE) is the percentage of energy you get back out of a battery compared to what you put in — energy discharged divided by energy charged, expressed as a percentage. It captures every loss in the system: cell chemistry losses (internal resistance), inverter/power conversion system losses (often 5-8% alone), auxiliary loads like HVAC and thermal management, and step-up transformer losses. Five quantities tie the calculation together.
Round-Trip Efficiency (%) = (Energy Discharged (MWh) ÷ Energy Charged (MWh)) × 100. At 100 MWh charged and 87 MWh discharged, RTE is 87%. Energy Lost per Cycle (MWh) = Energy Charged − Energy Discharged — 13 MWh at the defaults — the energy purchased but never recovered. Cost of Losses per Cycle ($) = Energy Lost per Cycle (MWh) × Charging Electricity Price ($/MWh); at $30/MWh, that is $390 per cycle.
Annual Energy Lost (MWh/year) = Energy Lost per Cycle × Cycles per Year — 13 MWh × 350 = 4,550 MWh/year. Annual Cost of Losses ($/year) = Cost of Losses per Cycle × Cycles per Year — $390 × 350 = $136,500/year. The health flag grades the calculated RTE against the typical 85-92% AC range for modern utility-scale lithium-ion BESS: 92% or above is excellent (top of the range), 85-92% is healthy, and below 85% may signal inverter losses, thermal management issues, or battery degradation worth investigating.
A useful distinction: DC-level (cell-only) efficiency is higher, usually 92-98%, but AC system-level efficiency is what actually matters for revenue and project economics because it captures every loss between the grid connection and the cells. RTE also typically declines as a battery ages — internal resistance increases alongside capacity fade — so tracking it over time is one signal (alongside capacity and state-of-health metrics) for spotting degradation trends. Data sources: NREL battery storage efficiency studies; utility-scale lithium-ion BESS vendor datasheets (Tesla Megapack, Fluence Gridstack, LG Chem RESU, Saft Intensium Max); IEEE standards for AC/DC efficiency measurement.