There should be a Rating Plate on the heater showing the Power in kilowatts (kW)
Watts divided by Volts = Current - have the fuse a little higher else it will blow...
For instance...
2kW divided by 230 Volts = 8.7A so a 10A fuse will do
In 110V land the current will be roughly double
The fuse should not exceed the current-carrying capacity of the house cables supplying the equipment.
The combined maximum load (Current) should be safely within the range of cable.
The fuse should be sufficient to supply all the equipment on the circuit.
The main purpose of the fuse is to prevent the cables overheating and catching fire - not to protect the appliance.
Good luck...
Peccavi
January 2010