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.
Lost Wh is the planning number installers quote
Homeowners hear efficiency percent; designers budget lost Wh. Calculate battery energy loss during charge/discharge to see how many kilowatt-hours never reach the load each cycle. On a 48 V off-grid bank cycling 8 kWh in daily, 5% loss is 400 Wh—enough to run LED lighting for hours if it were not heating the battery room.
Where the watt-hours go
I²R in conductors, cell internal resistance, BMS quiescent draw, and taper-charge heat all consume Wh. Cold weather and high C-rates inflate loss. Log conditions with in/out readings so a rising loss trend flags aging cells or undersized cabling—not mysterious load growth.
Shrink loss before upsizing the bank
Before adding modules, verify loss Wh is not dominated by fixable factors—charger mismatch, long DC runs, or chronic 100% DoD. Pair loss totals with Battery Depth of Discharge and Charging Time so cycle depth, refill energy, and waste appear on one commissioning line.