HELP LOL

Can anybody help me please. This is all new to me. I'm rebuilding an rv roof. I have built wood rafters, which are each arched in the middle. There is a 30 inch space between each rafter,with 4 inch thick rigid styrofoam between each rafter. I want to slice the styrofoam down to the slope of the rafters. I am going to use the rafters as a guide to slice the styrofoam. I need to have the whole roof level so l can place a covering over it to weatherproof it. I need to have a wire 30 inches long, so it will reach from one rafter to the other. How much power do I need to heat the wire?