The biggest factor is your latitude, which affects the insolation at the surface by the cosine of your latitude. At high latitudes, the solar rays are more steeply inclined to a square meter of the surface, and so the power delivered for heating is lower by the cosine of the latitude.
The next factor is the time of day, with the maximum possible insolation occurring at local Noon.
After this comes the time of year which determines how high up in the sky the Sun can get, and again, the cosine of the altitude angle determines how much power flows into a square meter of surface.
Finally, there are small effects due to the fact that in the Northern Winter, the Earth is a few percent closer to the Sun than in Summer so that the absolute flux in space at the Earth orbit is slightly different. And on top of this is a very slight 0.01 percent modulation due to sunspot coverage.