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The amount of incoming solar radiation (insolation) that reaches the earth is modified on its way to the earth's surface. The atmosphere itself reflects roughly 5% of the incoming radiation, and clouds reflect an additional 20%. Another 20% of the insolation is absorbed by the atmosphere and cloud cover, leaving about 55% of incoming energy from the sun to reach the earth (and our rooftop sensor).
Since the earth spins on its axis, the maximum theoretical amount varies by latitude and season. So while the total insolation decreases in the wintertime, the day to day variations measured are primarily due to cloud cover.