The first variable is dependent on the type of material you wish to melt, and is called the heat of fusion 'L'. For ice, L is = 335 kJ/kg.

The next variable is the mass 'm' of the material you wish to melt measured in kg.

The last variable is the effect 'p' at which heat is transferred to the material, measured in watt (W).

Now lets try an example. If I wish to determine how much time 't' it will take to melt two kilos of ice on a stow at 1000 W, i will have to use the following expression:

t = L·m / p

t = 335000 J/kg · 2 kg / 1000 W

t = 670 seconds = 11.2 minutes.