Microwave heating or cooking of larger quantities is not the same as heating the same amount of a material with a heat source like an electric element or gas flame. One potato might bake in 4 minutes, but 2 potatoes are going to take 7 to 8 minutes of more. So the 4 or 5 quarts of water in a dishwasher are going to take quite a while to heat with microwaves which do not convert all of the electrical input into usable energy, unlike an electric heating element, especially one bathed in water.
While we can look for more efficient ways to heat water and should, it would be good to remember that the energy laws conceived under the Carter administration would have mandated, among other things, that all new housing construction from around 1980 would have to have the capability to heat at least part of the dwelling's domestic hot water with solar panels or by some other renewable resource. Those were all scrapped when the Westerners took over the government after Carter, who is no shining light in my book. The trouble with inline or demand water heating is that at a constant heat input, the temperature rise is inverse to the rate of flow. If you tried this up north in the winter time, you would not have much more than a trickle of hot water at the tap if you tried to heat incoming water at 40 degrees F to 120 or 140 F. While point of use water heating can be more economical, if it is something the country wants to use, appliances that heat water with any speed will need to be on higher amperage 120 volt circuits or on 220 volt circuits. And then you have to compare the costs and advantages of heating much of the water for point of use in a residence that has domestic water heating with natural gas, which historically has been cheaper than electricity. Another plus for heating more water near the point of use is that it will eliminate a lot of water that is wasted bringing warm or hot water to more distant faucets, but a demand circulator pump could bring hot water to a point of use within seconds without wasting water. While retrofitting older construction is more costly, new construction would not be as difficult. Using insulated hot water pipes, very little heat would be lost during the minute or two it would take to move the cold water in the pipes to the storage water heater. But sophisticated application of recirculating pumps is not widely used in standard construction nor, as far as I know, is insulating hot water pipes.
While we can look for more efficient ways to heat water and should, it would be good to remember that the energy laws conceived under the Carter administration would have mandated, among other things, that all new housing construction from around 1980 would have to have the capability to heat at least part of the dwelling's domestic hot water with solar panels or by some other renewable resource. Those were all scrapped when the Westerners took over the government after Carter, who is no shining light in my book. The trouble with inline or demand water heating is that at a constant heat input, the temperature rise is inverse to the rate of flow. If you tried this up north in the winter time, you would not have much more than a trickle of hot water at the tap if you tried to heat incoming water at 40 degrees F to 120 or 140 F. While point of use water heating can be more economical, if it is something the country wants to use, appliances that heat water with any speed will need to be on higher amperage 120 volt circuits or on 220 volt circuits. And then you have to compare the costs and advantages of heating much of the water for point of use in a residence that has domestic water heating with natural gas, which historically has been cheaper than electricity. Another plus for heating more water near the point of use is that it will eliminate a lot of water that is wasted bringing warm or hot water to more distant faucets, but a demand circulator pump could bring hot water to a point of use within seconds without wasting water. While retrofitting older construction is more costly, new construction would not be as difficult. Using insulated hot water pipes, very little heat would be lost during the minute or two it would take to move the cold water in the pipes to the storage water heater. But sophisticated application of recirculating pumps is not widely used in standard construction nor, as far as I know, is insulating hot water pipes.
