That's a really good question. But, I don't have a good answer. The first full month of running the HPS, was my lowest light bill all year. My last light bill was only 90 bucks. My highest that I've had all year was about 200 bucks. But, I live in a very old house, without central heat and air. Plus, I live alone, so I don't have to worry about keeping it comfortable for others. Also, I'm in the south and the winter is usually less electricity intensive for most folks. Except this week. It's currently in the low 20's and is getting down to single digits on Monday. I just fired up my kerosene heater about 20 minutes ago and it's only 57 degrees in the house right now...I'm cold!
Also, since I live alone, that means there aren't very many lights on, other than in my cabinet for most of the day. Like right now, the only light on, is the light on the ceiling fan and it's got CFLs in it. Also, when people come over, they notice that I only have the light on in the one room, and they are good about shutting off the kitchen, bedroom or bathroom light when they leave the room.