| Scenario | Charge Energy (kWh) | Discharge Energy (kWh) | Charge Eff (%) | Discharge Eff (%) | Aux (kWh) | System RTE (%) |
|---|---|---|---|---|---|---|
| Lab test, mild temperature | 12.50 | 11.10 | 96 | 96 | 0.20 | 87.30 |
| Field site, cooling active | 25.00 | 21.90 | 95 | 94 | 0.90 | 84.88 |
| High load, inverter stressed | 10.00 | 8.50 | 93 | 90 | 0.10 | 84.16 |
System roundtrip efficiency (includes auxiliaries):
RTEsystem(%) = 100 × Eout / (Ein + Eaux)
Use when your measurements are at the meter or system boundary.
Battery-only roundtrip efficiency (adjusts one-way path losses):
Ein,batt = Ein × ηch
Eout,batt = Eout ÷ ηdis
RTEbatt(%) = 100 × Eout,batt / Ein,batt
Useful when you want the cell/pack behavior separated from converters.
- Pick Energy in/out if you have meter readings in kWh.
- Pick Voltage/current/time to compute kWh from logged data.
- Enter one-way charge and discharge path efficiencies if measured outside the battery.
- Add aux energy for cooling, heaters, pumps, or controls during the cycle.
- Press Submit to show results above the form, then export CSV or PDF.
What roundtrip efficiency represents
Roundtrip efficiency quantifies how much usable energy returns after one complete charge and discharge cycle. The calculator reports system RTE using measured energies plus auxiliary consumption, and battery-only RTE after correcting one-way conversion efficiencies. Use the split to separate cell behavior from chargers, inverters, cabling, and protection devices.
Required measurements and common sensors
For energy mode, log kWh from a revenue-grade meter or inverter telemetry at the same boundary for both directions. For voltage-current-time mode, capture average DC voltage and current during charge and discharge and record duration in hours; the tool converts to kWh using V×A×h/1000. Keep sampling intervals consistent when loads vary.
Include start and end timestamps, ambient temperature, and SOC endpoints in notes for traceability. When comparing systems, ensure identical measurement boundaries; mixing AC input with DC output can overstate efficiency. For grid-tied sites, exclude standby consumption outside the test window. Repeat tests and average results to reduce noise from transient loads. and from continuous communications equipment power draw.
Loss components and interpretation
Charge path loss is the difference between measured charge energy and the estimated energy that reached the battery. Discharge path loss is the difference between estimated energy leaving the battery and measured delivered energy. Battery internal loss captures electrochemical, resistive, and thermal losses between those corrected values. Total system loss also includes auxiliaries such as cooling fans, pumps, heaters, and control electronics.
Using efficiencies and auxiliaries correctly
Enter one-way path efficiencies when your measurements are taken outside the battery, for example at AC terminals. If you measure directly on the DC bus at the battery terminals, set both path efficiencies near 100% and treat converter losses separately. Auxiliary energy should cover any loads that run during the cycle and are not already included in Ein or Eout.
Comparing scenarios and improving results
Track RTE versus temperature, power level, state-of-charge window, and cycle age. Higher C-rates typically increase resistive losses, reducing battery-only RTE, while inverter clipping and partial-load operation reduce system RTE. To improve performance, shorten cable runs, optimize charge/discharge power to efficient regions, maintain thermal management, and verify calibration of meters used for both directions.
1) Which efficiency should I report to stakeholders?
Use system RTE for operational reporting because it reflects metered input, delivered output, and auxiliary loads. Use battery-only RTE for engineering comparisons between cell chemistries or pack designs.
2) Why can battery-only RTE be higher than system RTE?
Battery-only RTE removes one-way converter and wiring losses by correcting measured energies using charge and discharge path efficiencies. System RTE keeps those losses and any auxiliaries inside the boundary.
3) What if I only have power readings, not energy?
Use the voltage, current, and time mode. Enter representative average values and the duration for each direction; the calculator converts to kWh using V×A×h/1000.
4) How do auxiliaries affect the result?
Auxiliary energy increases the denominator of system RTE, lowering efficiency when cooling, heating, or pumps run during the cycle. Add only the auxiliary consumption within the same test window.
5) My result is above 100%. What should I check?
Verify that input and output are measured at the same boundary, confirm units, and review path efficiencies. Overestimated discharge efficiency or mixed AC/DC measurement points can inflate calculated battery-only RTE.
6) How can I improve roundtrip efficiency in practice?
Operate in efficient converter regions, reduce resistive losses with proper cable sizing, keep temperatures in the optimal range, and minimize unnecessary auxiliary runtime. Regularly calibrate meters and validate telemetry scaling.