Residential PV systems cut summer peak demand by more than 50%

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In addition to reducing electricity costs and dependence on fossil fuel power, residential rooftop solar systems can also serve utilities in lowering peak demand in hot summer months, according to a report by Texas-based energy research firm Pecan Street Research Institute.

The study examined original data from a research trial of residential solar panel performance and looked at how much of the rooftop generated energy was used inside the home rather than sent back to the grid. The 50 single-family homes analyzed in the report – all located in Austin, Texas, were randomly selected from 175 homes with rooftop solar PV participating in the research trial.

Roughly half the homes studied have south-facing solar arrays, 14 have west-facing arrays, and 14 have a combination of west and south-facing systems.

"Residential solar systems have understandably raised concerns about their impact on electric reliability," said Pecan Street CEO Brewster McCracken, the report’s lead author. "These findings suggest that rooftop solar systems can produce large summer peak reductions that benefit utilities and customers alike without requiring customers to change their behavior or sacrifice comfort."

West-facing PV systems deserve higher rebates

The study’s authors were surprised to find that west-facing solar systems in the sample produced more electricity than south-facing systems on both an actual and a normalized basis. During summer peak demand hours (3-7 pm), the gap was even more pronounced: west-facing systems produced 49% more electricity during those hours than did south-facing systems. As a result, the report suggests that utilities that offer residential rooftop solar rebates may want to extend rebate eligibility to west-facing systems and even offer higher rebate levels than is provided to south-facing systems.

The analysis focused explicitly on a single period of the year: June 1 – August 31, 2013. The summer months have more daylight hours and higher levels of seasonal electricity use for air conditioning in areas with such demands. Summer months present very different home energy use and solar generation profiles than other seasons.

Key findings from the report include:

Counting only the electricity generated by a rooftop solar system that is actually used in the home (and therefore not counting electricity that was sent to the grid because it could not be used in the home), homes averaged a 58% peak demand reduction for electricity from the grid. South-facing solar systems cut peak demand from the grid by 54%, while west-facing systems reduced their homes’ peak demand from the grid by 65%.

During peak hours, homes used 80% of the power generated from the rooftop systems and returned 20% to the grid. In the homes with south-facing systems, 78% of the power generated was used in the home; 22% was returned to the grid. In homes with west-facing systems, 84% was used in the home; 16% was returned to the grid.

Over the course of the full day, 64% of the energy generated by the rooftop systems was consumed on-site; 36% was returned to the grid. Over the course of the full day, and not including surplus energy returned to the grid, the solar systems provided 36% of the average power used per home. Nearly a third (32%) of the power was generated during peak demand hours.

Headquartered at the University of Texas at Austin, the Pecan Street Research Institute is a non-profit university-based scientific research institute.