This is one of those messy questions. The answer, "It depends"
Consider a 1 candle power heater. It heats 1 cup of water to boiling in 10 minutes. Total energy used: 1/6 candle power hour. Now try to heat 10 liters of water. Due to the much larger pot, it never gets it to boiling. 1 candle power gets it to something above luke warm, and never get any warmer. Total energy used Infinite. And you get people impatient for their tea.
The most efficient water heater is one that works so fast that there are no loses. Few of us have flash boilers in the kitchen. Failing that, one that is insulated and has the element surrounded by water will do better than anything with a separate water vessel and heater.
Practical advice: Heat the water in the pot in the microwave. The microwave is very efficient. It heats the pot at the same time. If you don't need a pot full, use a thermal mug and a teabag in the microwave. Put the tea in first or after.
My suspicion is that this is not as efficient as the insulated immersed elment kettle. The microwave conversion is not perfect, nor is all of the energy absorbed by the items in the microwave. However pouring boiling water into a pot can result in the water dropping 10-15C. The water used to heat the pot is discarded. Overall I suspect that using the microwave is a win, but I cannot back that up with actual tests or data.
The inside of a microwave reflects microwaves. So the waves bounce until they are absorbed. This is one reason you don't run them while empty. I couldn't find actual figures for MW efficiency, but one indication: Energy that isn't absorbed has to show up somewhere. The exhaust temperature from my microwave doesn't get warm until there is steam coming off the food, so emperically it is quite efficient.