The output of both solar electric and solar thermal panels is related to the level of radiation falling on them. Obviously the sunnier it is the better, but they will give some output whenever there is some level of solar radiation (i.e. except at night).
In the case of solar electric (PV) systems, the output is proportional to the level of light (not heat) on the panels. They are receptive to a broad spectrum of light so will produce power throughtout daylight hours. They are actually slightly more efficient at lower temperatures.
Solar thermal panels respond more to radiation at the infra-red (heat) end of the spectrum.
It is heat produced from a sustainable source such as solar, biomass or heat pumps. Heat from fossil fuels like gas, oil and coal is not renewable. There is a range of renewable energy sources.
It is power produced from a sustainable source such as solar, wind, or biomass. Electricity from fossil fuels like coal, oil and gas or from nuclear stations is not renewable.