I use three, 300 watt 12 volt Maulibu type transformers to light my
yard and walkway lights around my home.

I was under the impression the reason one would use 12volt instead of
line voltage was the 12v was less costly to run.

How do I determine how much power each of these is drawing (per hr?)
so I calculate my cost per month. I don't think it will matter, but
one of them is only drawing about 160 watts. Each of the others is
near their 300 watt limit.

Thx