648 megawatts is peak capacity. A fixed-tilt solar PV farm in a good location, like Topaz Solar Farm (and presumably this one) can output 27% of peak capacity averaged over a whole year: https://en.wikipedia.org/wiki/Topaz_Solar_Farm

You'd actually expect something like 648 * 365 * 24 * .27 = 1.5 TWh/year.