An electrical power system (10) for an aircraft includes an ac generator bus (16), an air conditioning system ('ACS') generator (42) and a main engine generator (46). The ACS generator (42) supplies primary ac power to the bus (16) while the frequency of the ac power is within a frequency range. If the frequency of the ac power goes outside the frequency range, the main engine generator (46) supplies backup ac power to the bus. The frequency of the ac power generated by the ACS generator (42) is maintained between upper and lower limits. Such ac power can be used directly by certain ac loads onboard the aircraft. Other ac loads onboard the aircraft are supplied with fixed frequency ac power by reduced-size inverters.