Round-trip efficiency

What it is

The share of the energy sent into the battery that comes back out after a full charge and discharge. The model splits the loss in two equal halves, one on charge and one on discharge. With AC coupling the discharge half is multiplied by the AC conversion efficiency; with DC coupling the PV inverter's own efficiency converts the delivered power. The product of the two halves is slightly above the input: 0.8836 for 88 %.

Typical values and how to get it

86 to 90 % for lithium iron phosphate systems measured with their converter, from the manufacturer's datasheet or public cost and performance assessments; with AC coupling and such a value, set the AC conversion efficiency to 100 % so that the battery inverter is not counted twice. The default is 88 %. With the AC conversion efficiency left empty, the defaults treat 88 % as the battery without its inverter and add the PV inverter's peak efficiency on discharge: about 87 % of the energy diverted comes back at the grid connection.

How it is applied

$$\eta_c = 1 - \frac{1 - \eta_{rt}}{2}, \qquad \eta_d = \left(1 - \frac{1 - \eta_{rt}}{2}\right)\eta_{AC}$$

where

Symbol	Meaning	Unit
$\eta_c$	the charge efficiency	—
$\eta_{rt}$	this input as a fraction	—
$\eta_d$	the discharge efficiency	—
$\eta_{AC}$	the AC conversion efficiency; 1 with DC coupling	—

References

1. Mongird et al. 2020, "2020 Grid Energy Storage Technology Cost and Performance Assessment", DOE/PNNL
2. DiOrio, Dobos & Janzou 2015, "Technoeconomic Modeling of Battery Energy Storage in SAM", NREL/TP-6A20-64641

API

storage.round_trip_efficiency — sent as a fraction 0–1

Symbols follow the symbol registry.