There is a value called the solar constant, a measure of the flux of energy from the Sun. However, in recent years evidence from both the geologic record and our own more recent measurements have shown that the Sun is not constant. Over the course of its 4.5 billion year existence, the Sun has brightened about 25 percent. But that is a very long-term variation.
Recent evidence suggests that short-term variations can be much more rapid.
Significant cyclical variations in the Sun's energy could be called in to explain the long-term ups and downs with terrestrial climate, but we simply don't have enough evidence. However, there are other possibilities. In the 1920's a mathematician in Belgrade, Milutin Milankovitch, got the idea of developing a mathematical forecasting model of the Earth's climate. We know that like a top in slow motion, the Earth's axis is pointed toward a particular direction in space. Currently the axis is pointed toward Polaris, the North Star.