Freeman:
I've always taken an electric process immersion heater as being essentially 100% efficient. In other words, all the electric power you apply with it is coverted to heat and this is directly imposed on the fluid you are heating. Unlike a heat exchanger, you don't have a return electric flow with unused heat. I don't recall ever having to apply the power factor to a resistance heater. I have always calculated the Btu/hr required as the heat transferred and directly converted this into kW. There are, of course some losses, but I always leave that up to the fabricator and they are usually small.