Quick answer
Discharge energy recovered divided by charging energy at defined boundaries and conditions, with comparable stored energy at the start and end of the assessed sequence.
What Is Round-Trip Efficiency?
Answer: Round-trip efficiency, or RTE, is recovered discharge energy divided by charging energy at stated measurement boundaries over a defined charge–discharge sequence. Comparable starting and ending stored energy are important. A DC battery-terminal figure, an AC system figure and a solar-to-load pathway can include different losses, so their percentages should not be compared without their test conditions.
RTE (%) = energy out ÷ energy in × 100.
For a hypothetical complete sequence with 10 kWh input and 9 kWh output at the declared boundary, RTE is 90%, with 1 kWh difference. This is input energy, not automatically a battery’s usable-capacity rating.
Define the Boundary Before Comparing Products
| Boundary | Questions to answer |
|---|---|
| Battery DC terminals | Does the figure include only battery losses? What test sequence and operating conditions apply? |
| AC charging input to AC output | Which charger/inverter, wiring and auxiliary loads are included? |
| Solar source to eventual AC load | Which PV conversion stages and direct-to-load flows are inside the boundary? |
| Project-period monitoring | How are changing stored energy, standby operation and other energy sources accounted for? |
SAM’s battery-results documentation labels separate DC and AC quantities and describes its battery RTE metric. Use the correct result variables rather than assuming a metric called “battery efficiency” includes every downstream loss.
DC-coupled and AC-coupled arrangements have different pathways, but architecture alone does not establish a universal efficiency advantage. Compare the actual converters, operating powers, energy routing and included loads.
Starting and Ending Stored Energy Matter
Suppose 10 kWh enters, 8 kWh leaves, and the battery ends with 1 kWh more stored energy than it started with. The simple 80% output/input ratio is not a matched-state round trip. At this declared boundary, the energy balance is:
Loss = 10 − 8 − 1 = 1 kWh.
PVsyst’s battery-loss documentation explicitly accounts for stored-energy change. Use a suitable matched-state sequence or a documented energy-balance method. A SOC display alone may not give an exact kWh correction.
A snapshot of charging or discharging power is not RTE: the charging and discharging energies occur over time. Similarly, charge recovery in ampere-hours, called coulombic efficiency, is different from energy recovery in kilowatt-hours.
Combining Losses Without Double-Counting
For a simplified series pathway, assume 95% charging conversion, 92% battery round-trip efficiency and 96% discharge conversion, all defined over compatible conditions and with no overlapping stages:
Combined efficiency = 0.95 × 0.92 × 0.96 = 0.83904, or 83.904%.
An assumed 10 kWh AC charging input then yields 8.3904 kWh AC output. These are hypothetical stage values, not a vendor specification. Real converters vary with operating point, and auxiliary consumption may need separate treatment.
If a published AC-to-AC RTE already includes both converters, multiplying those efficiencies again double-counts them. Conversely, a battery-only DC figure may omit equipment consumption. SAM’s storage documentation treats converter inputs and ancillary equipment losses separately.
When estimating backup from energy already stored, use the remaining discharge pathway. Applying the entire round trip charges the estimate for a charging process that has already occurred. Check depth of discharge and SOC references before translating a capacity into available load energy.
Operating Conditions and Economics
Power/current, temperature, duration, idle consumption and battery condition can affect observed efficiency. No chemistry label establishes a fixed system percentage or annual decline. Compare supported test data and model assumptions at the intended duty cycle.
For an assumed 10 kWh input every day and matched boundaries, a 90% versus 85% efficiency difference produces 0.5 kWh/day, or 182.5 kWh over 365 days, of extra output. It is a five-percentage-point difference, not a universally fixed annual financial saving.
Value depends on the charging energy’s cost or foregone export value, discharge timing, tariffs, demand effects, losses and degradation. RTE alone does not establish payback or a percentage increase in bill savings.
Practical Handoff
Designers should document the input/output meters, SOC/energy endpoints, duty cycle, converter efficiencies and auxiliary assumptions. Installers should follow supported commissioning procedures and retain the measurement method; do not force an undocumented deep cycle merely to calculate RTE.
Sales teams should state the boundary beside any efficiency figure and explain the assumptions behind savings. When evaluating SurgePV, ask which storage pathways, efficiency inputs and financial assumptions the workflow can represent.
Common Questions
What is a good RTE for a solar battery?
Compare documented figures at equivalent boundaries and conditions. A higher percentage measured over a narrower scope is not necessarily the more efficient complete system.
Why does RTE matter for solar storage?
It affects energy recovered after storage and can influence dispatch economics. It does not by itself determine backup capacity or bill savings.
Does RTE change as a battery ages?
It can change with battery and system conditions. Use product evidence or a declared sensitivity assumption rather than a universal loss per thousand cycles.
Related Glossary Terms
About the Contributors
Editorial contributor · SurgePV
Rainer Neumann is credited as an editorial contributor on SurgePV content. This profile does not assert engineering credentials, project totals, software-testing experience, education, speaking engagements, or media citations because independent verification evidence is not retained in the publication record.
CEO & Co-Founder · SurgePV
Keyur Rakholiya is identified by SurgePV as its CEO and a company co-founder. His SurgePV author page lists only role information that can be tied to the public profile below; credentials, project totals, testing claims, media appearances, and speaking engagements are not asserted without retained evidence.
