Calculate how much energy needs to be input into acriculture and transport to provide that wattage.

To be fair we'd have to consider how much of this same secondary energy would be required to build, operate and maintain the power grid. The grid itself is not 100% efficient either so we'd need to calculate how much power is directly wasted every single day just in inserting and extracting power from those overhead lines.

That's way off the envelope though.