What is round-trip efficiency?
Round-trip efficiency is energy delivered by a battery divided by the energy used to charge it, expressed as a percentage [S1]. In an illustration, 90 kilowatt-hours returned from 100 kilowatt-hours supplied means 90% efficiency. Compare equivalent system boundaries and operating conditions before using a figure in your site's battery model. Checked October 2026.
What is round-trip efficiency in a battery system?
Round-trip efficiency measures the energy returned after storing electricity. Divide discharged energy by charging energy and multiply by 100 to express the ratio as a percentage. SAM’s published battery model uses total discharge and charge energy for its round-trip metric [S1].
For your site, ask where those energy readings are measured. A figure for battery cells alone should not be silently substituted for the electricity returned to your building. Request a description of what the quoted efficiency includes and which operating conditions were used.
How do you calculate round-trip efficiency?
Illustration, not a quote: assume you measure 100 kilowatt-hours (kWh) going into a storage system and 90 kWh coming back out over a complete cycle, with the same stored-energy level at the start and finish. The calculation is 90 ÷ 100 × 100 = 90%. The difference is 100 − 90 = 10 kWh.
If purchased charging energy costs an assumed 10p per kWh, the input costs £10. Spread over 90 kWh delivered, that is £10 ÷ 90 = about 11.1p per delivered kWh, before other costs. These are assumptions, not a tariff or performance forecast. This calculation alone does not establish a financial return.
Which losses should your site proposal include?
Request separate treatment of conversion, auxiliary equipment and standby consumption. As an example of a published accounting boundary, the government’s hydrogen-standard annex includes charging, discharging, electrical equipment and cooling losses in its storage round-trip values [S2]. Its conservative accounting values are not product performance ratings for your site.
Ask the installer to provide the proposed system’s expected input and delivered energy under your operating schedule. Compare that with battery tariff arbitrage and solar and battery returns. Keep depth of discharge separate from efficiency when comparing usable energy.
Next step
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Common questions
Is round-trip efficiency the same as usable battery capacity?
No. Efficiency compares charging and discharging energy. Capacity describes how much the battery can hold. SAM reports efficiency and remaining capacity as different outputs [S1].
Should a storage proposal include cooling energy?
Ask whether it does. The government's hydrogen-standard annex includes cooling and electrical equipment losses in its storage efficiency method. That method illustrates why the system boundary matters [S2].
Can one quoted efficiency figure establish your annual battery return?
No. Request an operating model showing charging energy, delivered energy and losses. SAM defines its round-trip metric using total discharge and charge energy over the project, rather than capacity alone [S1].
Ask about your site
Sources
- Battery Results, National Laboratory of the Rockies - System Advisor Model,Last checked 16 October 2026
- UK Low Carbon Hydrogen Standard - data annex, Department for Energy Security and Net Zero,Last checked 16 October 2026