I have thought about doing this for a long time. The largest 120V water element I could find is 2000 watts, which by the math will draw 16.6 amps. But reality the wattage will be a closer 1850 watts to because most house voltages are rarely set above 116 VAC. If you decide to do this you need to know what the temperature rise times would be for stepped mashing. But insulating the canister and using a good cooler as a mash tun will help a lot.
I posted this article a few days ago that points to a article I posted on www.instructables.com
that gives details for installing a electric heater in a stainless steel pot.
You could do the exact same thing with a canister, the trick will be to find something that will house the element and provide leak free inlet & outlet fittings. You also need to be able to clean the inside of the canister.
I considered making something out of PVC but if the element were ever to start dry the PVC would melt and I would have a disaster on my hands. You need some sort of metal container, stainless steel preferred, with a removeable lid for cleaning. Something like drawing I attached (I'm no artist). Let me know if you find something that works!