ASK THE EXPERTS: PV System Efficiency & Roof Planes

PV efficiencies at various tilts and azimuths as a percent of ideal for a specific location.

I have a gable-style roof, with roof planes that face due east and due west. I’d like to install a grid-tied PV system that produces electricity at its maximum efficiency. For best performance, should I put an equal number of solar-electric modules on both sides of my roof or favor one side over the other?

James Thunell • via email

PV arrays provide their best efficiency when aimed directly at the sun. When their horizontal or vertical angle to the sun is not straight on, reflection losses increase, and the number of photons hitting the surface decreases. The sun’s horizontal and vertical positions change throughout the day and vary during the year. The sun’s brightness (irradiance) is lower in the morning and evening. However, small deviations in the angle of incidence result in only small efficiency losses.

Maximum power point tracking (MPPT) helps maximize a PV array’s production by electronically determining and adjusting the best balance between voltage and amperage of both the PV array and the battery bank (or utility grid). The efficiency gains of MPPT help offset the losses from a poor angle of incidence.

Your utility may benefit most by production from PV arrays that face east and west, and you may benefit as well. Their load demands are highest in morning and evening, which matches the production peaks of east- and west-facing arrays. In contrast, a south-facing PV array’s production peaks near noon, when the utility doesn’t need as much incoming energy. If your utility uses time-of-use (TOU) billing, your electricity rate per kWh consumed can be substantially higher during the mornings and evenings, but a PV array maxing out its production during those times also means you’ll earn more for the net-metered energy your system produces! For these reasons, east- and west-facing arrays are becoming more common for net-metered grid-tied PV systems.

Dan Fink • Buckville Consulting

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