So let's do a little calculating
Electricity costs about 25 cents per kwh
A gadget requires about a minute (1/60 hr) to heat a cup of water
Each gadget draws abut 1 kw ( adjust as needed)
So the cost to heat is 25 x 1 /60 < 0.5 cents
At about a penny per tea cup... really who cares.
The comments about auto shutoff are important, but it's easy enough to set the microwave for 1 minute, most come with a 30 sec duration per button push
That said, I mostly use an electric tea kettle for the liter of tea I make once a day for just the reasons Spikey said..it does just enough but no more
OTOH, to reheat a cup of coffee, I use the microwave
If you do get the tea kettle going.. let the old guy fill it to the brim... at the very most you are out a nickel, and he's happy. If I could make my spouse happy for less than a nickel, I know I'm in