Efficiency percent tells one story; lost watt-hours tell another. This guide shows how to calculate battery energy loss during charge/discharge from measured Wh in and Wh out across a full storage cycle.
Benefits
- Loss Wh = energy_in − energy_out for the cycle.
- Loss % = 100 − round-trip efficiency %.
- Translates abstract efficiency into kWh you cannot deliver to loads.
How it works
- Record Wh accepted during charge (solar, grid, or generator).
- Record Wh delivered to loads before the next recharge.
- Subtract: loss Wh = in − out; pair with efficiency % from the tool.
FAQ
How do I calculate battery energy loss during charge/discharge?
Loss Wh = Wh in − Wh out. Example: 1,200 Wh charged in, 1,080 Wh used out → 120 Wh lost (10% loss, 90% round-trip efficiency). The lost Wh mostly becomes heat in cells, cables, and BMS electronics.
Is charge loss separate from discharge loss?
The tool models one combined round trip—typical field logging sums both directions in one in/out pair. For lab splits, measure Wh from charger to pack (charge loss) and pack to load (discharge loss) separately, then add losses.
Why track Wh loss instead of only efficiency %?
Percentages hide scale—a 5% loss on 500 Wh is 25 Wh; on 20 kWh it is 1 kWh. Loss Wh feeds solar offset math and generator fuel planning when every kilowatt-hour counts off-grid.
Technical specifications
- Loss_Wh = energy_in_Wh − energy_out_Wh.
- Loss_% = (loss_Wh ÷ energy_in_Wh) × 100.
- Efficiency_% = 100 − loss_% (round-trip).
- Related: battery-round-trip-efficiency-calculator, battery-charging-time.