system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't we use microwaves for heating the water in our household water heaters?

Would it use more energy than a standard electric water heater?  How would a large scale microwave work?
gpt
It's rather unnecessary and more costly. 


Microwaves are not a better or faster way to heat water compared to conduction. The reason they cook food faster than an oven is because the radiation causes dielectric heating in the food, rather than heating a metal element which then through some air gap heats the food. Compare a microwave oven to an electric kettle, where the heating element is in contact with the water, or an induction stove top, where the stove heats the pot directly in contact with the water, and you are looking at near comparable heating times for these conduction heating devices. The faster one is the one that has a higher power rating, the method is rather irrelevant. Where as if you stuck the water in an oven and turned it on, it would take significantly longer (and waste a bunch of energy on heating air). Not that speed even matters that much, tank heaters are more common than tankless so it doesn't matter how fast it is it's just going to sit there and insulation is the real consideration. There really isn't any advantage to microwaving a tank full of water compared to sticking a heating element inside, which is basically 100% efficient. So if you are going electric, there is no point. In fact, the magnetron and resonant cavity tank design are only going to cost more and be more complicated. As well, the magnetron is going to make heat and that heat probably isn't going into the water and some of the microwaves are going to escape losing some efficiency but also making electrical interference. 



On the other hand you have natural gas, which is the most common source for water heaters when available. The answer here is just a matter of economics, natural gas is a lot cheaper than electricity, though that is location dependent. In most places you get at least two times the energy per dollar with natural gas, so any small efficiency differences really don't matter. So if you have natural gas, you are going to go with it. If you have electricity, you are going to use the simpler and cheaper heating element method.