What kind of bulb? If its a HID light assembly including an MH or HPS and your on a 24 hour photoperiod then plan on spending 22$ two 27$ i know this because i have exactly two 150 watt HPS light and my electric bill has always been consistant so it was simple to figure the increase. It also says 12$ for 12 hours a day for a month on the box it came in.