First, use this formula to calculate the resistance @
240 Volts
p = e^2
.......p = watts e = volts r = ohms(resistance).....---
.....R
2400 = (240)^2
.........------
...........R
R = ((240) x (240))
.....-------------
.....2400
R = 24 ohms
Then use the same formula to calculate the power at
120 volts:
p = (120)^2
.....------
.....24
p = 600 watts
Here are some more formulas that are helpful:
http://www.the12volt.com/ohm/ohmslaw.asp#pie