+ micro-wave a cup -- cold versus hot tapwater fill?
+ range-top kettle -- cold versus hot tapwater fill?
+ a plug-in water heating kettle-- filled with cold versus hot tapwater?
Efficiency very much depends on how your tap water is heated.
Absolute best way: if you have an undersink "on demand" hot water heater, fill up direct from the hot tap; microwave it a bit more if you want it even hotter.
Second best: if the dishwasher is already running or, if someone in the same hot water piping loop is showering, draw direct from the hot water tap and microwave in ceramic cup, as needed, to boost temp.
Third best: use a counter top, plug-in small water heating kettle (the heat is directly transferred to the water and, unlike a range-top tea kettle, little energy goes into heating up the device itself). Make sure its an auto-shut off model for safety and efficiency.
Tied-for third: microwave a ceramic cup that was filled with cold tap water. Issues with this distant third are: you frequently compete with other's use of the wave, it takes two or three more minutes more than the "absolute best" method, it can boil over, and the cup becomes unpleasantly hot on the handle.
Absolute worst: (and grossly wasteful of both water and energy) running the water for a length of time to bring it up to "hot", just for one cup of water, per choices two and three above.
NOTE: Hot water pipes vary in diameter and length so an exact calculation of wasted water and energy is not straightforward for the worst case. But you can get an idea from a hypothetical one. Assume that each meter of hot water supply pipe...that's the pipe that goes from your basement or closet-held hot water heater to your tap... holds aproximately 0.4 liters, and your tap is located 10 meters from the heater. Waiting for it to get hot means you have to waste 4 liters (about a gallon) of water before you even begin to feel the heat at the tap. You'll probably waste a similar additional amount before the pipe reaches thermal equilibrium with the hot water flowing through it, and reachs "hot to the touch" status at the tap: so that's 2 gallons wasted total. Then, you're going to fill up just one cup with that hot water, wasting also the heat in the gallon of hot water laying in the pipe. The only scenario that voids this waste is if you have set up the diswasher to run just as you leave home with a hot cup of coffee or tea. The most pathetically wasteful setting of all is when you live in a third floor flat, and the landlord has all hot water originating in the basement. That's at least a 20 meter (14 gallon + energy) shot at the climate changed future per caffeine buzz.
by: John Laumer