About the Round-Trip Loss
Round-trip loss is the energy lost in a battery storage system during a complete charge-discharge cycle — expressed as a percentage of the energy put in. It directly reduces the effective value of stored energy and must be accounted for in all BESS economics, sizing, and dispatch optimisation.
Formula
Round-trip efficiency (%) = E_out ÷ E_in × 100
- E_out
- AC energy delivered on discharge (kWh)
- E_in
- AC energy consumed on charge (kWh)
Round-trip loss (%) = 100 − Round-trip efficiency
- RTE
- Typically 88–95% for modern LFP BESS; 75–85% for lead-acid
How to calculate it
- 1Charge the battery from a known SoC, measuring AC energy consumed.
- 2Discharge to the same SoC, measuring AC energy delivered.
- 3Divide E_out by E_in — the ratio is RTE. Losses include battery internal resistance, BMS overhead, and PCS conversion losses.
- 4For planning: use 90% RTE (10% round-trip loss) for LFP; 80% RTE (20% loss) for lead-acid.
- 5Apply to sizing: to store 100 kWh net, the battery must absorb 100 ÷ 0.90 = 111 kWh gross.
Worked example
A 200 kWh LFP BESS charged with 210 kWh (from solar) and discharged to deliver 189 kWh (to load). RTE = 189 ÷ 210 = 90%. Round-trip loss = 21 kWh per cycle — at ₹7/kWh and 365 cycles/year, this loss costs ₹53,655/year in energy value. It's a fixed cost of storage that must be recovered by the value spread between charge and discharge.
What you can calculate
- Loss kWh/cycle
- RTE breakdown
- Auxiliary loss
- Net throughput
Frequently asked questions
What causes round-trip losses in a BESS?
Round-trip losses come from: (1) battery cell internal resistance (heat generated during charge and discharge — 2–4% each way for LFP), (2) power conversion system (PCS/inverter) losses during AC→DC conversion on charge and DC→AC on discharge (1–3% each way), (3) BMS and auxiliary power consumption (cooling fans, control boards). Total round-trip loss: ~7–12% for LFP; ~15–25% for lead-acid.
Does round-trip loss affect BESS profitability?
Yes — round-trip loss means every kWh stored costs more to deliver than it cost to charge. For arbitrage to be profitable, the price spread between charge (cheap) and discharge (valuable) must exceed the round-trip loss × charge price. At 10% loss and ₹5/kWh cheap electricity, you need at least ₹5.55/kWh revenue on discharge just to break even on energy cost — before accounting for capex.
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