At what time of the year are the differences in total solar radiation between the north and south

At what time of the year are the differences in total solar radiation between the north and south of our country greater and why?

Under the total solar radiation. We understand the radiation that has reached our planet. It must be remembered that the heat will go back. That is, the heat itself will be reflected: plowed lands, reservoirs, snow cover and sands. And what remains is considered already warm. Spring, summer and fall can make a difference. As in the number of warm days in a year. Yes, yes, in winter, the heat will be less than in summer due to the snow cover.