You can buy a device from bunnings that plugs into the power point in between the socket and the lead going in. It shows Amps, Volts, watts and you can program in your power price and it will show you how much the running cost is and the overall cost for a set time. Grab one and set it for a month then either show or pay for the minimal power you will be using.
I checked how much power 3 2fters were using the other night and there was a total of 58 watts at the time. There is a big 4 outlet air pump, hang-on filter, 2 50w a 1 100w heaters. It was approx 7pm and all the tanks have 2cm poly on back and sides.