Solar panel output is the actual power generated by a solar panel. Due to factors like Sun angle, shade, and panel temperature, your panels will rarely (or more likely, never!) generate as much power as their nominal rating ("100 watt panel"); the rated wattage would be difficult to achieve even if the panel was located on the Equator at solar noon on a completely clear day. The nominal rating can be reached under standard test conditions in the lab.
Sun angle– The closer you are to the Equator, and the closer to directly overhead the Sun is, the more power your solar panels will produce. The Sun angle changes with the seasons too; you'll get much less solar power during the winter than you will during the summer.
Shade–Depending on your solar panel type, shade over just a small portion of a panel can significantly reduce output for the entire panel. For example, a shadow that covers just one cell of the panel surface could result in an output reduction of 80%.[1] Anything that physically blocks or reduces sunlight will reduce panel output:
- shade from nearby objects: trees, buildings, mountains, etc.
- objects on your vehicle roof: roof vents, roof racks, or other equipment
- snow, dirt, or dust on the panels[2]
- clouds, haze, fog
Panel temperature– Somewhat counterintuitively, solar panels produce more power in cold temperatures than they do in warm temperatures. In the winter, this offsets some of the seasonal losses due to Sun angle, but you'll still get much less panel output in the middle of winter than in the middle of summer. It's generally helpful to have panels mounted with an air gap underneath so that excess heat can escape.