These sensors work by absorbing all of the visible and infrared light in the sunshine.
Unlike photodiodes and light sensitve resistors, they collect all of the wavelengths instead of just one range of colours of light.
The surface of the device gets warm, the same as anything you leave lying around outside in the sun.
The thermocouple junction generates a voltage because of the difference in temperature between the warm side of the junction and the cold side.
The temperature of a thermocouple follows a known behaviour which depends on the type of metals used, but is the same for every thermcouple using those metals.
How you would actually calculate the incident solar radiation, I don't know. Presumable you would have to match it to another known device.