Every charge–discharge cycle loses energy to heat and BMS overhead. This battery round-trip efficiency calculator converts energy in and energy out in watt-hours into round-trip efficiency % for solar storage and off-grid planning.
Benefits
- Core formula: efficiency % = (Wh out ÷ Wh in) × 100.
- Quantifies one full charge-to-usable-discharge cycle.
- Benchmarks real packs against 95%+ Li vs. 80–85% lead-acid norms.
How it works
- Log energy in (Wh) from charger or solar into the bank for one cycle.
- Log energy out (Wh) delivered to loads before the next recharge.
- Read round-trip efficiency %—loss is 100% minus efficiency.
FAQ
What is battery round-trip efficiency?
It is usable energy out divided by energy stored in, × 100, for one cycle. Example: 1,000 Wh in, 950 Wh out → 95% round-trip efficiency. The missing 50 Wh became heat, BMS draw, and internal resistance loss.
How do I measure Wh in and Wh out?
Use a battery monitor shunt, inverter cumulative kWh, or charger meter. Wh in is what the pack accepted during charge; Wh out is what loads consumed from that charge window—before the next top-up.
What is a good round-trip efficiency?
Modern lithium often tests 93–98% at moderate C-rates; lead-acid may land 80–88% depending on age and charge profile. Compare your measured % to datasheet values at similar charge amps and depth of discharge.
Technical specifications
- Round-trip efficiency % = (energy_out_Wh ÷ energy_in_Wh) × 100.
- Loss Wh = energy_in − energy_out.
- Output Wh cannot exceed input Wh in passive systems.
- Related: battery-efficiency, battery-charging-time, battery-energy.
Round-trip efficiency is the storage tax
Solar and grid charging put Wh in; loads pull Wh out. A battery round-trip efficiency calculator measures the gap—critical when sizing arrays and banks. A 95% round trip on a 10 kWh bank means you plan on 9.5 kWh deliverable per full cycle, not 10 kWh, before inverter loss downstream.
Chemistry and C-rate move the number
Fast charging and deep discharges heat cells and lower measured efficiency. Lead-acid Peukert effect and lithium BMS balancing both eat Wh. Document charge amps, DoD, and temperature when you log in/out so comparisons to lab sheets are fair.
Chain efficiency through the system
Round-trip battery efficiency is one layer—inverter conversion and cable loss add more. Pair results with Battery Charging Time for refill hours and Battery Energy for nameplate Wh. Multiply efficiencies when stacking charger → battery → inverter → load for whole-system yield estimates.