Electric Heaters and heating elements become hot due to a voltage being pushed through a wire. The resistance in the wire measured in Ohms creates the heat. The more resistance the hotter the wire gets. The more voltage pushed through the wire, the hotter the wire gets. Simply the smaller the wire (or the longer the wire run) the more resistance it gives to the voltage passing through.
So as voltage increases the wattage will increase four-fold. As the voltage decreases the wattage will decrease by a factor of 4 as well. The below table shows the percentage of drop or gain with different applied voltages to a heating element.
Rated at 277 volts the expected wattage % is
277 applied 100% (rated Wattage)
240 applied 75%
230 applied 69%
208 applied 56%
120 applied 19%
Rated at 240 volts the expected wattage % is
277 applied 133%
240 applied 100% (rated wattage)
230 applied 92%
208 applied 75%
120 applied 25% (1/4 wattage)
Rated at 120 volts the expected wattage % is
240 Applied 400% (4 times wattage - Dangerous)
208 Applied 300% (3 times wattage - Dangerous)
120 Applied 100% (rated Wattage)
110 Applied 84%
This is all based on Ohm's Law.
Simple Ohms Law equations are:
Amps = WATTS / VOLTS
Watts = VOLTS x AMPS
Volts = AMPS x OHMS
Ohms = VOLTS / AMPS
A Conductor Wire's resistance is constant based on wire size and length.
A heater's watt rating is based on the wire used in the heating element and the voltage applied.
To get a 9.5 kilowatt rated heater run at 240 volts; a wire with a resistance of 6.6 Ohms is used. (If there are two elements in the heater, then each is rated at 4.75 kw and the resistance of the wire would be 12.12.)