Capacity

What it is

The energy the battery holds when full, in kilowatt-hours; the duration is the capacity divided by the power. The stored energy stays between the floor and the ceiling of the state-of-charge window.

Typical values and how to get it

The rated energy on the datasheet. Utility batteries hold 2 to 4 hours of their power rating; 4 hours is the usual choice for evening shifting. Ratings are usually net of a reserve, so the floor stays at 0 % unless the datasheet states a gross capacity.

How it is applied

$$s_{min}\,C \le S_i \le s_{max}\,C$$

where

Symbol	Meaning	Unit
$s_{min}$	the floor of the state-of-charge window, as a fraction	—
$C$	this capacity	kWh
$S_i$	the stored energy at the end of timestep $i$	kWh
$s_{max}$	the ceiling of the state-of-charge window, as a fraction	—

References

1. NREL 2019, "Grid-Scale Battery Storage: Frequently Asked Questions", NREL/TP-6A20-74426
2. NREL, Annual Technology Baseline, Utility-Scale PV-Plus-Battery (atb.nrel.gov/electricity, the utility-scale PV-plus-battery technology page)

API

storage.capacity_kwh

Symbols follow the symbol registry.