Well there are heat pads MADE from heat tape, so that's a hard question to answer! But if its for one cage, I hardly see the sense in wiring up 1 foot of heat tape, just to heat a single cage. I'd go get a heat pad and put it on a dimmer or better yet, a thermostat.
More than 4 or so cages, I'd use 11-inch heat tape down the back of a rack system. Sooo damn efficient it isn't even funny. Seriously.
Best of luck!