Dual Power Supply Consumption in the event of a site power fail

"Looking to make sure our data center has enough power in reserve in the event of a power failure. Our standard setup is a server with 2 power supplies, one connected directly to the main power, the other to a UPS. My question is if either the main power or UPS fails for the entire site (not just one server), does the power consumption of the Power Supply still running double, or are they always running at 100% and no additional consumption is required? To make sure we have enough power in the event of a power failure I'm trying to find out if we basically need double the resources in use on both the main powerr and UPS in the event either fail. Thanks!"