Is the variac just for motor rotation, fan running or heating too?
If it's just for a motor rotating, then 500 Watt should be fine. If you are controlling heat with this, then it's way too small.
I would personally go for 1.2 to 1.5 kW. That's 1200 to 1500 watts then. You don't want to push a variac to it's upper limit, especially these cheap chineese pieces of $#$t out there.
If you are looking at amps. For 110 / 115 / 120 Vac however they list it, Id go for 15 Amps to be safe. 10 Amp is kind of pushing it. If you are looking at 220 V. Personally Id go for 10 Amp, you could probably get away with 8.
If you are just running motors, then 5 amps would be fine.
Now keep this in mind, Ive seen voltages drop as much as 10 volts or more on some houses when a heater kicks in. There are a lot of reasons for this, which I won't get into here. This means you will be pulling even more amps through your variac to make up for this voltage loss to get the energy to your roaster. More amps is more unwanted heat. Now if you live in Florida and are roasting in your garage and it's 95 degrees outside and 115 degrees in your garage, that's even more unwanted heat stressing your variac. Bigger is better, and safer when it comes to variacs.
Aaron
