A reader writes, “The sun comes over the top of a ridge in the morning at about 8 a.m. and sets at about 8 p.m in Ocean View, Hawaii—12 hours of usable light. So I’m confused when some PV system users claim that there are only 4.3 to 5.2 hours of “available light.” Is this due to the fact that their modules are fixed in place? What else may be the cause?”
The daylight you are witnessing (“12 hours of usable light”) and the available sun-hours for solar energy are indeed different. For making electricity, we use the way solar-electric modules work, and how they are tested and rated, to help us calculate what our site’s solar resource (solar irradiance) will provide.
Because the sunlight has to go through more atmosphere, morning and afternoon sun is less intense (lower irradiance) than clear sun at solar noon. To plan for solar generation, we consider how modules are tested for maximum output—at an irradiance of 1,000 watts per meter squared (W/m2)—and estimate how much of that intensity of irradiance is provided to a site throughout the day. We calculate the periods of low insolation and translate those numbers into the equivalent of 1,000 Wh/m2, what we call peak sun-hours (1 peak sun-hour equals 1 kWh/m2/day). The site’s total peak sun-hours give us a multiplier for calculating what a fixed array will generate at that site over the day (for example, 5 kWh/m2/day = 5 peak sun-hours). Peak sun-hours are not a casual observation or timing of sunshine on your site; they are a scientifically measured unit of total solar energy available at a specific orientation and tilt angle.The peak sun-hour values account for all factors, including the ever-changing hours of sun each day and cloudy weather throughout the year, for the location at which the information is gathered.
The National Renewable Energy Laboratory suppliespeak sun-hour data for locations throughout the United States, which assists in planning solar projects. To find the data for a specific site, you can use the NREL PVWatts calculator found at pvwatts.nrel.gov. Put in your location on the calculator and follow the simple instructions to find monthly peak sun-hours data and how much energy your proposed array would generate.
The peak sun-hour range of daily irradiance that the “solar users” referred to (4.3 to 5.2) reflects the difference during different months of the year. Peak sun-hours vary, and are usually greatest in the summer months and fewest in the winter. NREL provides the average monthly peak sun-hour values, along with an annual average.
The great advantage of being able to use the NREL data and calculator is that the numbers at NREL have been generated by irradiance measurements at sites in the vicinity of your site, and therefore take into consideration variances due to cloud cover and other site-specific solar effects. These numbers are assuming unshaded access to the path that the sun takes across the sky (the “solar window”). Shading effects must be measured and included in the calculations to provide an accurate assessment of generation potential. A Solar Pathfinder can be used to accurately determine a site’s shading elements and calculate their effect on the available peak sun-hours of that site.