Arizona Public Service signed a 15-year power-purchase agreement with First Solar for peaking power from a 50-megawatt (MW) solar-powered battery.

Typical peaking units in Arizona are fueled by natural gas and are used on the hottest days of the year when air conditioning demand is highest.

First Solar will build and operate the facility that includes a 65-MW solar field to charge the battery. The field will store power when the sun is high in the sky and deliver it to customers between 3 and 8 p.m. when the sun is on its way down, but energy use is peaking.

On February 15, the Federal Energy Regulatory Commission voted 5-0 to remove what it said were nationwide barriers to the participation of electric storage resources in the capacity, energy and ancillary services markets operated by Regional Transmission Organizations and Independent System Operators.

In a November 2016 Notice of Proposed Rulemaking (NOPR), the Obama-era commission said that market rules designed for traditional generation resources can create barriers to entry for emerging technologies such as electric storage resources.

The Arizona Public Service project adds to the three grid-scale batteries currently on APS’s system. Over the next 15 years, APS says it plans to adopt more than 500 MW of additional battery storage.

The facility is set to begin service to customers in 2021 and will be built directly adjacent to the existing APS Redhawk Power Plant in western Maricopa County outside of Phoenix.