When you buy a panel it will be rated in Kwp. p??? The p is the
peak power output ie if it was at the equator & the sunlight was
striking it at 90deg. Not likely to happen for long if at all. The
actual power output is much less of course.However there is a useful
number which I think for California is 1100. This the number of Kwh
your solar array will generate in a year for every Kwp installed.So if
you hade a 5Kwp array you would get 5500Kwh every year. Here in the UK
where I live, it's only 670. It takes into account weather, lattitude
and so on. However if it's shaded for certain times of the day it's
gonna be less.
Panel deteriorate as they get older. They work less well the hotter
they get too.
Don't go thinking it's gonna be cheap. Solar energy cost about ten
times your mains electricitysupply.