One of the hassles you'll have living in Toronto is that the wintertime incoming cold water temperatures can be quite chilly...potentially approaching freezing. That cools off the tank faster, and would make the needed temperature rise on an electric assist more problematic. A gallon of water is a bit over 8# (sorry, not metric!). Many of the heating devices are rated in BTUs. One BTU will raise one pound of water one degree F. Should you want to do this in watts, one BTU/hour=0.293W. Problem comes when you want hundreds of pounds of water hot in a short time. Got a car wash shower, and it could easily get into the tons of water if you lounge in there for a bit. Now, converting everything into units more useful over time involved can get tricky, but can bring some enlightenment.