How much insolation is received at 0° (equator) on the summer solstice? on the winter solstice? how much does insolation vary from summer to winter at this location?

The equator experiences equal day and night  as the summer solstices has longest days in northern side and the winter has longest in the southern side.

Explanation:

As the summer have the solstice period in the northern hemisphere, the areas in the north gets most of the heat and sun days as that of the southern hemisphere and the reverse is true for the winter solstices that has maximum day in the south and less in the northern hemisphere as sun rays don't pass the 66.5 degrees latitude.