What sort of size is the garage etc ?

If you're trying for some heat for yourself, quartz/infra red type heaters will be good and work instantly. But they dont really heat the environment as such.

http://www.tansun.com/gb_en/infrared-heaters/apollo/

And they aint cheap lol.

Not really to be confused with some smaller more domestic type units which just dont seem to transmit heat as well. I've a Sealey one which claims quartz/IR and unless you're close to it, it really doesnt to much at all.

But if it is cold and no other heat, any electric heater will use a lot of electric. Having a couple of smaller simple fan type heaters you could move about might be as handy as anything.

Although the likes of oil filled rads with timer/shutoff might be safer for a workshop environment in these days of H&S lunacy.