What percentage of a household electricity bill typically comes from lights?

I always try to get my roommates to turn off the lights when they leave (to no avail). I've tried some other things that seem to have made a small dent on my energy bill (washing clothes only in cold water, turning off the auto-dry on my dishwasher...this seems to have reduced my bill by about $5 a month). Now I'm wondering, how much do lights comprise my energy usage, anyway? Isn't the fridge the thing that uses the most energy in my house?