[throwway120385]

I have a hydronic heating system in one of my houses, but the electrical service can't provide enough amperage to heat the water up, so it's running on propane. Unless you're willing to spring for an upgrade to a 300A service, usually you'll have some combination of gas for heat or for water heating or for clothes drying. There are electric versions of all of these but you don't normally have enough budget on a 200A service to run them all at the same time because you also have to have budget for a lot of 120VAC kitchen and garage appliances. Houses built 20 years ago also had weird circuit configurations to accommodate separate lighting and power circuits because incandescent bulbs were so inefficient. So you'll have a bunch of lighting circuits that are only pulling an amp or two and then a bunch of appliance circuits that are pulling 10's of amps, but your panel gets filled up because of the unnecessary lighting circuits.

At least, in my experience this is how it goes as a homeowner.

So to get to an on-demand water heater you'll probably have a lot of electrical work that needs to be done(potentially thousands of dollars).