[returningfory2]

Maybe I'm missing something obvious, but by conservation of energy if all of the energy of the sun was converted to heat then no electricity would be produced? Conversely, that fact that panels do produce electricity means that not all of the sun's energy is heating the panel?

[etrautmann]

solar panels are only partially efficient. if the solar -> electric conversion efficiency is 21%, then 79% of the incoming solar radiation is converted to heat.