The power the EP500 needs doesn't change ... double the voltage then you'll need to halve the current.
I think Trevor was trying to calculate available power at the plug ... the numbers will be different depending on how you want to approach it. What Trevor presented was power used into a resistive load when the voltage is doubled. Although the math is correct, I do not think he's getting the answer he's after ...
Home wiring is only allowed to carry so much current for a certain wire size (for safety). For the same wire size doubling the voltage (i.e. only changing one variable, in this case voltage) gives you double the power. Now, if you want to change both current (wire size) and voltage ... which is valid in that standard receptacles are available for a number of voltages and amperages ... going from a 15A 120V outlet to a 30A 240V outlet will give you 4x the power.
