In this country (UK), stamp duty land (the tax you pay when you buy a new house) is higher if you are buying a house that won't be your main residence, so taxing vacant homes is, in principle, a workable tax policy.

I'm sure there would be people that would try to work around the system, same as with every other law.

What do you think would be more expensive: a criminal conviction for fraudulently disguising property status, or this tax?

There's nothing criminal about owning a second house and using electricity. Maybe there's staff members keeping it clean. Maybe there's a pool pump that needs to run. You completely ignore the spectrum of utility consumption for completely vacant, minimally occupied, half occupied, etc. homes that exist on the market.

Investigating and prosecuting would be more expensive than either. Plus proving intent over “oh I forgot to check all my taps before I went out, whoops” is difficult.