It's called the "solar constant", which turns out not to be constant. Except that the variability over the last 50 years doesn't seem enough to amount to anything. Except .. the last "Little Ice Age" seems to have coincided with the "Mauder Minimum", a period when there were 3-4 orders of magnitude fewer sunspots than we see now. Coincidence? Important? That's the kind of thing people still do their dissertation on...