Take the wattage of your bulb(s) X hours/day run X 30 days X .07(average cost per KWH, your local price may vary) divide by 1000= monthly cost
Example: (1) 250watt MH X 10 hr/day X 30 X ..07 / 1000 = $5.25 month...
You can also add up all the wattage of pumps, heaters, lights,skimmer, filters etc. to get total cost/mo. to run your tank...