For us, Wattage = Voltage Squared divided by Coil Resistance
=> W = (V^2)/R
Throwing a little Algebra on it gives you…
V = ( W x R)^1/2
Or, Voltage = The Square Root of Wattage times Resistance.
So Plug in what you know, your Wattage = 10, whatever the Resistance is of your Coil, and then Multiple them together. Then take the Square Root of what the product was.
Wattage = 10 and Resistance = 1.2 Ohms. => 10 x 1.2 = 12
The Square Root of 12 = 3.46. Thus, V = 3.46
Hence, if you set a 1.2 Ohm Coil at 3.46 Volts you will produce 10 Watts of Power.