The AC system is not consuming the same energy in an overcast day versus an intensely sunny day, even if the outside air temperatures are comparable. The direct

The AC system is not consuming the same energy in an overcast day versus an intensely sunny day, even if the outside air temperatures are comparable. The direct radiant heating of the structure with sun exposure introduces much more heat than the mere conductive transfer from outside air. When people say that AC costs correlate with solar power, they are pointing out that the same solar energy that increases heat gain for buildings during the bright hours also increases photovoltaic output which can be used to compensate for that heat gain.