[jpk]

With electric resistance heaters, they are all the same as far as efficiency; you get out the amount of heat that's pretty much 100% equivalent to the amount of electrical energy input. You get about 1000 btu's/hour per 300 watts of electric heat (293 watts actually, but 300 is a nicer number to work with).

With 120v heaters, you're limited to about 1500watts per unit since that's just under 15 amps - the limit of most circuits in the house. That means you can get 1500/300 or 5,000 btu's/hour out of one 120v heater.

With 220-240 volts, you typically install a dedicated circuit which can go much higher and you can size the heater to the need and the circuit to the heater.

The next question is how much heat do you need? guidelines say about 1000 btu's per 30-35 square feet. Based on that, you need 15,000 to 18,000 btu's for you garage. But, that's based on normal living space in a Northern climate. You might not need quite so much if you don't need to get the garage up to 72 degrees or if it's 35 degrees outside vs -20.

You might get by just fine with a single 1500 watt 120v heater. Since there's no efficiency difference between the different types (oil filled, ceramic, Amish craftsman built fireplace etc...) I'd suggest trying a $25 milkhouse heater. If that works, then you're golden. If it's not quite enough, you could get a second, just realize you'll have to plug it into a different circuit or you'll just blow the breaker. Even if you had to run a separate circuit back to the panel it might be more cost effective to use 2 (or 3) 120v heaters than buying and installing a dedicated 240v unit.