Question
When batteries are used, there is a loss of electricity associated with the roundtrip charge and discharge, resulting in fewer generated kWh. Why does the Commission provide a compliance credit for a battery storage system that is coupled with a PV system if there is a loss of energy?
Answer
Battery storage systems store the PV generated electricity in the middle of the day when solar resources are generally plentiful and electricity prices are low. The system discharges the stored electricity later in the day, during peak hours when solar resources are diminished and electricity prices are high. Battery storage systems have a roundtrip charge and discharge loss of 5 to 15 percent, depending on the type of battery technology and the inverter efficiencies. A compliance credit is available because the electricity price differential between the middle of the day and peak hours is greater than the battery charge and discharge losses. This means that even with the relatively small loss of electricity, it is still cost effective for a consumer to store electricity generated onsite around midday and use it later on instead of purchasing additional electricity from the grid.
To calculate the compliance credit of a battery storage system coupled with a PV system, the Energy Commission’s compliance software on an hourly-basis accounts for the PV generation, losses, storage capacity remaining, charge and discharge rates, cost of electricity, house loads, and hourly exports. Similar calculations are also performed to calculate the benefits of storage for CO2 emissions.
Not all battery storage systems are eligible for compliance credit; the system must comply with the requirements of Reference Joint Appendix 12 (See References). These requirements ensure that the battery storage system operates in a way that allows residents to take advantage of variable electricity costs associated with periods of clean energy availability throughout the day. Static batteries that remain mostly in backup mode have little to no value to the homeowner, the grid, or the environment.