It changes depending on the company that you use. However, most of the time, you can find that you are choosing from a few different power levels. For example, Smart Tint provides you with anything from 28VAC to 240VAC. It depends on what kind of power level you need.
One good thing to note about the company I mentioned is that they comply with all National Electric Code Low Voltage needs. This gives you that smart glass solution without the excess power consumption than other competitor models offer.
In general, this works extremely well if you are looking for a low-cost, easy-to-run solution for adding extra transparency (and opacity) on demand. Voltages, though, will vary on the manufacturer, so try to keep that in mind when shopping around.