Close your eyes for a moment and try to imagine the ideal day for solar production—a day in which your rooftop array is churning out power at its optimal efficiency. What comes to mind? The intuitive answer is a cloudless midsummer day. But, as it turns out, spring is actually the best season for solar!
It is quite common for arrays to have their best single day of the year in April or May. There are three primary reasons for this: angle of incidence, temperature, and site specific shading.
ANGLE OF INCIDENCE
All else being equal, a module will produce the most electricity when sunlight is most direct (i.e., when the sun is perpendicular to its surface). Arrays we install are tilted to a degree where the sun is most perpendicular. Portland is located at 45-degrees North. The sun moves up to 23-degrees above the equator on the summer solstice, leaving us 22-degrees (90-45-23) off the zenith (directly above the ground). That same angle is the corresponding angle of your roof tilt for perpendicular sunshine at midday on the solstice. Most Portland roofs are around 30-degrees, so the sun is more perpendicular approximately 32 days before or after solstice (8 degrees/0.25 degrees of movement per day).
Temperature is the biggest factor in module production/efficiency. A solar module loses about 0.4% of its power for every degree Celsius above 25°C (77°F). While it may be 25°C ambient, the module is much hotter, even up to 40°C hotter than ambient. Roofs are warm places, and given that the module is ~20% efficient, that means 80% of the suns energy is turning into heat, warming up the module and decreasing its efficiency. This is the biggest difference in the performance of the system in May versus June or even later in the summer. Of course, this is all taken into account when we design your solar array to meet your energy needs.
The last factor is each property's unique month-to-month shading. Some locations have more shading in June than in May, which can affect your daily production during different parts of the year.
In summary: Your solar array can still have high production days throughout the summer, but don't be concerned if your highest days are in the cooler, yet still sunny spring and fall months!