regarding 120v/240v
Every house gets two phases piped to the house (out of the available three, your neightbour most likely only shares one of your phases for load distribution)....they are both 120V but the frequency is offset slightly so the peaks are not at the same time. Connect two phases together on the Live side and voila, you have your 240V.
And remember kids, wires dont give a rats *** about the voltage thats going through it, only the amperage. Thats the whole reason high draw appliances step up to 240V, so the lines dont have to go nuts in size.
Watts = Amps X Voltage. So if a heater draws 1500w it could either draw 12.5Amps at 120V or it could draw 6.25 Amps at 240V. Frequency comes into play as well but since all house AC is 60Hz it doent matter for this topic but FYI aircraft A/C generation systems are 400Hz!! This allows for smaller components outputting huge amounts of power and the ability to run lots of power through smaller lines.
With that being said, i currently just use two spot lights for heat. I turn them on anyways for better light when working on something and they get the garage pretty warm, we'll see how they do when it really gets cold as my garage is high ceiling and unfinished.
Until you surpass the voltage rating of the insulation.