Solar & Reliability

Different technologies make different contributions to meeting peak demand.

APS plans and prepares year-round for the hot summer months in Arizona, when customer demand for energy soars along with the temperatures.

Providing our customers with high levels of reliability means not only keeping the lights on and air conditioners running during this busy season. It also means safely and efficiently managing all of the resources connected to the energy grid, including variable resources like solar energy.

Not all solar is created equal
There is no question solar power is an important and growing part of Arizona’s cleaner energy mix. But solar can make widely varying contributions to meeting customers’ peak demand, depending on the technology. The difference was evident during our peak day for energy demand in 2015, Aug. 15.

Data from the nine universal grid-scale plants in APS's AZ Sun Program and more than 25,000 residential customers with private rooftop solar showed that while universal grid-scale solar plants kept generating energy when customers needed it most, rooftop solar was producing little or none at all.

Energy production from the AZ Sun plants and rooftop systems peaked at about the same time in late morning that day. As the afternoon wore on, the grid-scale plants continued to generate at high capacity while rooftop solar’s contribution diminished more rapidly, as this Peak Day video details.

The differences in technology also extend to their value in reducing the need for APS to secure high-priced power to meet demand. When rooftop solar production was at its highest around noon (see chart), APS could buy power on the wholesale market for less than 3 cents per kilowatt-hour. At 7 p.m., when wholesale power was the most expensive (and valuable to customers), rooftop solar had stopped generating power.



Our customers did receive value from solar into the evening because of Solana. The plant generated power until 11 using the sun’s stored energy.

In addition to their diverse energy contributions, universal grid-scale and private rooftop solar also vary in cost. Studies from the MIT Technology Energy Initiative and energy consulting firm The Brattle Group in 2015 showed that universal grid-scale solar costs at least 50 percent less than private rooftop solar to build equivalent capacity. This also makes universal grid-scale solar the least-cost technology for growing capacity and achieving economic, environmental and policy benefits, such as reducing carbon emissions.

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