most street lights are 600w hps (i used to work for the street dept) but they are also sealed in some heavy duty glass, idk if the temp difference would be enough to crack the glass, i would like to think our goods are higher quality than that ya know?

you would be suprised how a stream of 85 degree air can make a difference. you don't have to melt the entire roof, just a layer to allow the rest of the snow to slide right off.
if you have a flat roof, obviously this doesnt happen.