While it is in the FCOM 1.34.20 P1, also try looking at the AAIB report (2/2008) of G-EUOB and the electrical failure it suffered. In the descriptive text, there is a good explanation of the power sources of the ISIS. In short, it says that the ISIS has two power sources - Ess DC and HBB with prior standby horizons just being Ess DC powered unless modified to the ISIS state.