well a good safe wattage 150, 250 and maybe 400 watts. You should change stuff around your house to more energy efficiant products when growing too. like change regular incandesent bulbs to Compact Floroesents. A 19 watt CFL can replace a 75 watt incandesnt. This way your electric bills stay around the same.

To make an estimate on electric costs, just follow these steps.
1) check your latest electric bill and find out how much it cost for a kilowatt hour
2) http://www.overgrow.com/tools/powercost.php