Yup killowatt meter and an electrical calc
http://reefcentral.com/index.php/tank-electrical
They sell kilowatt meters at Lowes now.
But depending on how you setup the tank you could rack up 100/mo easily. I saved a fortune on my electricity after I got a kilowatt meter.
If you had a
Return pump running at 250watts
Halide 500watt 10hrs/day
skimmer pump 100watts
@.18/kw it would run around $ 75/mo
If your even close to that you could easily add up to 100/mo but you need to actually measure as some equipment uses less or more(this is more common) than the manufacturer states.
I have seen pumps using almost double what the manufacturer states. This can also be as the pump is worn and is not performing as well as it should.
Make sure you know the correct per kilowatt price and input that correctly into the calculator