I use one in my house (it's good for getting the washing dry when we have months of rain).
I am on my second one in 6 years. The first one had the thermocouple pack up, so the plates ended up covered in a block of ice and I'm not sure where to get a replacement from.
They cost around £100. The more capacity you need the more they cost, but £100 one would do a garage.
B&Q tend to label them as for a 2 bed house, a 3 bed house, or a 4-5 bed house.
The issue is that you need the garage to be pretty much sealed. Having a big gap round the door with the wind bowing in, means that it is a waste of time. Unless it is sealed like your house don't bother, or get a big plastic tent to put it in.
From memory I think that use 100-200W, so the heating effect is not much more than a lightbulb. Basically it is a fridge without the bodywork, so the water condenses on the plates and drips off. There is a fan to keep the air flowing over the plates.
The amount of water you can extract is reduced as the air temperature drops. Below about 5 degrees it removes very little.
So on a winters day with at 2 degrees C with fog, the relative humidity is 100% and you can't remove any water.
Heating the air will also lower the relative humidity, but heating a garage will cost a but in running costs, although perhaps a paraffin heater might be more economical during particularly cold spells.
(Should have added that burning paraffin produces water vapour.)
