Solar Derating
The full power generating capacity of your solar panels is achieved only when the sun shines perpendicular to the panel, from straight overhead. Otherwise, as the angle of the sun changes, less power is produced. Some systems use "trackers" which slowly rotate the panels so that they always face the sun. Some systems use mirrors to shine more light onto the same panel to increase the power output. Panels produce power less efficiently at higher temperatures. They produce less as the panels age over their lifespan (which could be as much as 50 years).

Depending on what lattitude your home is at, the length of daylight changes over the year. Furthermore, weather and clouds greatly diminish solar panel output. All such factors must be combined to form a yearly average derating factor suitable for your home location and orientation. The power company consultant will be able to estimate this. For example, he may tell you that you will effectively average 4 hours of full sun per day over a typical year. Thus your derating would be 4 hours divided by 24 hours (hours in a day) giving 16.6%.