Tankless whole house units don't usually save energy or water since you have to deal with the 'cold slug' that comes between the warmish hot water and the cold water that comes before the unit can heat the water. I've experienced one of these units at someones house, it used a fair bit of water before the temperature settled enough to take a shower. For point of use, an electric kettle is probably a better and cheaper option both for running costs and intial cost versus a point of use water heater. Where I live the cheapest option is a tank heater running off natural gas, ideally one that is well insulated and condensing(once we get a good one available). Heat pump units come next but now we are a little off topic. To heat up a pot of water an electric kettle after pulling from the cold tap would be your best bet for saving energy and water.
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