sorry not John T here but anyway the answer is no. 1500 watts is the measure of power used whether is be 120 volt 240 volt or 12 volts. Watts are calculated by multiplying the volts times the Amps. so a 1500 watt heater running on 120 volts draws 12.5 amps on one phase of power. A 1500 watt heater on 240 volts draws 6.25 amps on TWO legs of power, equaling the same total power used. The reason this could save you money is that now you can run a smaller wire to supply your heater if y run it 240 volt. Also with motors they start easier with 240 volt single phase power. A motor will start even easier with 3 phase power, but that is a whole different subject.
Now my question for John T is this. If I use a 100 watt bulb for a heater in one area, and use a resistive 100 watt heater in another, is there any difference in efficiency? Just wondering since when you make light with an incandescent light the heat is a by-product and then potentially wasted(thermal inefficiency?) If used as a heater, then is the light a by-product and also a waste? No real reason for this just wondering