It's the watts, not the lights. I run 440 watts of VHO with an Icecap ballast for 10 hours a day. That is 4.4 kilowatt hours a day at 0.065 cents per kwhr in my town or about 29 cents a day. That is a little over $8.00 a month. It's actually less as icecap claims that it runs 440 watts of lights using only 330 watts of electricity.
You can estimate the same for your pumps and powerheads also if you know the watts of each one. There is also an online calculator somewhere that you can plug all this in and it will tell you your estimated monthly bill.