Several things can lower the amount of electricity your solar system produces. Dust, dirt, and bird droppings on panels (called soiling) can block sunlight, while cloudy or rainy weather naturally reduces how much sun your system gets.

Even in perfect conditions, you won’t always get the maximum possible output. This is because your inverter, solar panels, and other equipment don’t work at 100% efficiency all the time. Over the years, solar panels also lose a little bit of their power—usually less than 1% per year. So, while your system may be designed for a certain output, real-world factors always bring that number down a bit.