Watt is a measure of power, and is the appropriate unit of measure to use. Joule is a measure of work, and is defined as power over time; one Joule equals one watt for one second. The reason Joules are inappropriate to use is you control the amount of power used in total by how long you push the button. That is if you have the machine set at 40 Watts and hold the button for 1 second, you will use 40 Joules. But if you hold the button for 2 seconds at the same setting, you will do 80 Joules. It's like the difference between a 1500 watt hair dryer, and the Kilowatt-hours you pay on your electricity bill. the 1500 watts is how much the machine uses when it is active. The kWh is how much electricity you used in total while you were using the machine. The only way Joules would be correct is if the mod pulsed exactly that many (wherever you set it) and then shut off no matter whether or not you still have the button pushed.
