I have a 240V electric heater in my 20X20 garage and I cannot even notice it on my bill - whatever it costs me is less than the tiny monthly fluctuation in power usage prior to installing the heater. I set it around 10-12C and leave it on indefinitely. Garage is insulated but detached so it isn't getting heat from my home. It was dirt cheap and I'm pretty happy with it so far. Might be a different story if I wanted my garage at 20C all the time but that isn't necessary unless I'm working out there, then I turn it up.