Sean, an important question that no one has asked yet:- What lathe is it?

1/2hp is suitable for a Myford sized lathe, but 3/4 is better for even an ML7 (I speak from experience). A lathe that's sized for a 2hp motor is not going to be happy with larger work or more aggressive cuts if hobbled by a 1/2hp motor.

In terms of power consumption, the losses of a 2hp single phase motor will be greater than those of a 1/2hp single phase motor or 3ph motor + VFD, but the difference is likely to be no more than 25-50W. I.e.. 3/5 of 5/8 of F-all, as one of my colleagues used to say.

The suggestions of getting a plug in power meter are your best bet. They are cheap these days (especially when Aldi or Lidl have got them on sale, but also from a number of online sellers), and can give you a good idea of just what in the house is eating the power.

Before you do anything else though, have you got an immersion heater that someone's turned on?