Wikipedia cites 652,375 MWh of "solar only" production in 2015, with natural gas consumption listed separately. That works out to an average 74.4 MW output from the sun alone.
It's not clear how the natgas output is being measured. It's used to bring the working fluid up to temperature. Either the true solar output is lower than stated or the plant is using natgas extremely inefficiently (getting 0.9 instead of 9 MW of it). In the latter case, it's still a cost of obtaining the solar energy that should be accounted for.
Wikipedia has the total and solar-only figures switched, as can be seen by visiting the citations given. The solar-only figure is 644,506 MWh.
https://en.wikipedia.org/wiki/Ivanpah_Solar_Power_Facility#I...
Out of curiosity, why did you subtract the potential natgas from the actual solar output, instead of the actual gas output you just cited?