Convert satellite image data (GOES 15) into solar irradiance values?
It is far too complex a process to estimate surface irradiances utilising satellite data. No software takes just satellite images as input and gives out irradiances. 

I am pasting link to an article that describes algorithm to convert geostationary satellite measurements to surface reaching solar irradiance. 

Gadhavi et al., 2008; doi: 10.1029/2007JD009308

To summarise following are most important aspects in estimating surface radiation budget. 


Calibration constants to convert photon counts measured by satellite sensor to radiance
Parameterisation scheme to convert narrow band satellite radiances to broadband radiances. 
surface optical characteristics as function of wavelength, sun elevation and season
Geophysical parameters like cloud amount, aerosol amount, ozone, etc. (Essentially these are the real satellite input in whole process). 
Radiative transfer model to estimate irradiances using inputs received from satellite and other sources.


There are few web-site where one can directly get final output of irradiance values. See following link. 
https://data.noaa.gov/dataset/surface-radiation-budget-surfrad-network-1-hour-observations