Round-trip efficiency calculator

Work out round-trip efficiency from metered energy, the cost of the losses per cycle, and what a DC figure becomes at the AC terminals.

Read this first. Every default is an invented placeholder, not a product's data or a real tariff. Use a full cycle, or a long period that starts and ends at the same state of charge, or the ratio will be wrong. Prices are per kWh in one currency of your choice. Use a dot for decimals; "10,000" is read as ten thousand.

1. From metered energy

Energy in per cycle (kWh, from the grid or source) Energy out per cycle (kWh, delivered) Auxiliary energy per cycle (kWh, cooling, controls; 0 if none) Charge price (per kWh) Discharge value (per kWh) Cycles per year

Result	Value
Round-trip efficiency, out ÷ in	-
Including auxiliary, out ÷ (in + auxiliary)	-
Energy lost per cycle (in + auxiliary - out)	-
Cost of that loss per cycle, at the charge price	-
Margin per cycle (out × discharge value - (in + auxiliary) × charge price)	-
Loss per year: energy and cost	-
Discharge value needed to cover the charge cost	-
Value of one more percentage point of efficiency, per year	-

2. From a DC figure to the AC terminals

Battery DC round-trip efficiency (%) Inverter or PCS efficiency, charging (%) Inverter or PCS efficiency, discharging (%) Energy in per cycle at the AC terminals (kWh) Auxiliary energy per cycle (kWh)

Result	Value
AC to AC round-trip efficiency	-
Energy out at the AC terminals	-
With auxiliary, out ÷ (in + auxiliary)	-
Gap from the DC figure	-

The DC figure is treated as one number for the battery alone. Datasheets differ on where they measure; read the footnote.

Method: efficiency is energy out divided by energy in over the same period, as in the NREL BESS evaluation method (December 2023). The chain in part 2 multiplies the three stage efficiencies, as HOMER Front's manual does for system round-trip efficiency.