ERP is Effective Radiated Power... the power that is equal to the transmitter output, minus line loss, times antenna gain and height advantage. That is the standard used by the FCC for all stations. I have no idea what the KGO transmitter is putting out, but their authorized power is 24,000 watts ERP. That's all the FCC would approve for them at their antenna height.
Remember, High VHF takes a lot less power to cover the same area as a UHF station. KGO at 24 kW digital is equivalent to their 316 kW analog on channel 7, since digital is measured as average power, analog was measured as peak power.
Stations on Low VHF needed even less power to cover the same area.
Unless you've studied electronics, this is all difficult to understand, but it all has to do with the frequency characteristics.