Round-trip efficiency (RTE) is one of the most commercially consequential specifications in battery energy storage procurement — and one of the most frequently misquoted. A single percentage point of RTE difference between competing BESS systems can translate into tens of thousands of dollars of revenue difference over a 20-year project life. Yet procurement documents routinely accept a supplier's quoted figure without clarifying whether it is measured AC-to-AC or DC-to-DC, at what temperature, and at what depth of discharge. This article explains what RTE means, how to read a supplier's datasheet, and what Econo Solar recommends EPCs and developers demand from suppliers before signing a supply contract.
What Is Round-Trip Efficiency?
Round-trip efficiency measures the ratio of energy recovered from a BESS during discharge to the energy delivered to it during charging. The formula is direct: RTE (%) = (Energy out during discharge, kWh) ÷ (Energy in during charge, kWh) × 100. A system rated at 90% RTE loses 10% of every kWh cycled as heat, distributed across both the charging and discharging passes. Over a project life of 6,000 warranted cycles, those losses compound into a very significant quantity of energy and forgone revenue.
Unlike the cell-level efficiency figures reported in laboratory datasheets, real-world round-trip efficiency captures every stage of the energy conversion chain: electrochemical losses in the battery cells, parasitic loads from the battery management system (BMS) and thermal management system, and the bidirectional conversion losses of the power conversion system (PCS) that connects the battery module to the AC grid. The difference between a clean laboratory figure and a measured field figure can be meaningful — particularly at low ambient temperature or high C-rates — making precise specification at procurement stage essential.
DC Round-Trip Efficiency vs AC Round-Trip Efficiency
The most important distinction in BESS efficiency specifications is between DC-DC RTE and AC-AC RTE. DC-DC RTE measures efficiency at the battery cell or module boundary only — it excludes the conversion losses of the power conversion system. AC-AC RTE captures the complete energy transformation from AC grid input to AC grid output, and is the figure that matters for project yield models, energy revenue forecasts, and performance guarantee calculations. Most suppliers lead with DC-DC RTE in marketing materials because it is numerically higher. Always request AC-AC RTE explicitly.
A typical loss breakdown for a modern LFP BESS illustrates why the two figures diverge: battery cell electrochemical losses account for approximately 1–2% per cycle; BMS and auxiliary system loads contribute around 0.5%; and the bidirectional PCS contributes 3–4% of losses across both directions of energy flow combined. Summing these components explains why a system with strong cell-level DC performance will still land at 90–93% at the AC-AC system boundary. When requesting RTE data from suppliers, specify the measurement conditions: ambient temperature (25°C standard), SOC operating window (typically 10%–90% for LFP), and C-rate (0.5C for most commercial systems).
BESS Round-Trip Efficiency Comparison
The table below compares published AC-AC round-trip efficiency figures for leading commercial BESS systems, including Sungrow's utility-scale and C&I platforms. Performance varies with operating temperature and configuration; figures should be verified with the supplier for your specific project conditions.
| System | AC-AC RTE | Chemistry | Warranted Cycles | Capacity |
|---|---|---|---|---|
| Sungrow ST2752UX-US | ≥93.0% | LFP | 6,000 @ 80% DoD | 2.76 MWh |
| Sungrow MBL160 (C&I) | ≥90.0% | LFP | 6,000 @ 80% DoD | 160 kWh |
| Sungrow MGL060 (C&I) | ≥90.0% | LFP | 6,000 @ 80% DoD | 60 kWh |
| CATL EnerC (utility) | ~90–92% | LFP | 6,000 | Various |
| BYD Battery-Box Prem. | ~90% | LFP | 6,000 | Various |
Published figures; verify with supplier for your specific configuration and temperature range.
How RTE Affects Project ROI
Consider a 1 MWh BESS cycled once per day over a 20-year operating life — a standard configuration for utility-scale and large C&I arbitrage applications. At 93% AC-AC RTE, 930 kWh is delivered for every 1,000 kWh charged; the daily energy loss is 70 kWh, or 25.55 MWh per year. At 90% RTE, the daily loss rises to 100 kWh, or 36.5 MWh per year. The 3% RTE difference accounts for approximately 10.95 MWh of additional energy loss per year, per MWh of installed BESS capacity.
Over 20 years — and before accounting for progressive RTE degradation with aging — that gap accumulates to roughly 219 MWh of energy per MWh of storage. At an electricity value or arbitrage price of US$100/MWh, the 20-year revenue difference between a 90% and a 93% system is approximately $21,900 per MWh of installed capacity. For a 50 MWh utility-scale project, that is over $1 million in energy revenue. In competitive storage tenders where levelised cost of storage (LCOS) is the decision metric, RTE is one of the highest-leverage specification parameters a developer can optimise at procurement stage.
Factors That Reduce Real-World RTE
Four operational factors routinely cause field RTE to fall below a manufacturer's rated figure at standard test conditions. Temperature has the largest impact: LFP chemistry exhibits approximately 2–3% lower round-trip efficiency at 0°C compared to 25°C, because reduced lithium-ion mobility at low temperature raises internal resistance and increases resistive heating losses. Projects in cold climates — Central Asia, Northern Europe, high-altitude desert sites — must model temperature-adjusted RTE in their energy yield forecasts.
Depth of discharge (DoD) is a secondary factor. Cycling to 100% DoD rather than the standard 80% DoD window raises average C-rate slightly and generates more heat per cycle, reducing effective RTE. Most commercial LFP manufacturers specify RTE at 80% DoD (10%–90% SOC) to reflect typical operating practice; be cautious of competitors quoting RTE at narrower, more flattering SOC windows.
State of health (SoH) declines with cycle count and calendar aging, progressively increasing internal resistance and reducing RTE. Procurement contracts should specify both initial guaranteed RTE and the maximum permitted RTE degradation at key cycle-count milestones over the warranted life. Finally, C-rate matters: systems dispatched at 1C or higher for fast-response frequency regulation will exhibit lower effective RTE than systems on slower 0.5C arbitrage cycles, because higher current densities increase ohmic losses per unit of energy cycled.
What to Demand from Suppliers
Econo Solar recommends including four mandatory RTE requirements in any BESS procurement specification or request for proposal. First, require a guaranteed minimum AC-AC RTE at 25°C and at the expected average site ambient temperature — both figures should appear as performance guarantees in the supply contract, not merely in the datasheet.
Second, require an RTE warranty clause with a published degradation schedule, specifying the minimum warranted RTE at defined cycle-count and calendar-year milestones across the warranted operating life. A simple initial guarantee offers no protection against premature RTE decline in years 8–15.
Third, request independent test results from an accredited laboratory — TÜV Rheinland, UL Solutions, or equivalent — using IEC 62933-2 round-trip efficiency measurement methodology. Factory acceptance test data alone is insufficient for project lender technical due diligence.
Fourth, include performance liquidated damages in the supply contract: a contractually defined remedy — typically a price deduction of 0.5–1% of contract value per 1% RTE shortfall below the guaranteed minimum — if measured RTE fails to meet the guarantee at factory acceptance testing or on-site commissioning. These four requirements cost nothing to specify and protect the project against the most common form of BESS performance shortfall identified during project finance due diligence.