Trying to work this out (I know nothing of these things), I find out:

The specific heat of water is 1 calorie/gram °C = 4.186 joule/gram °C.

My microwave is 800 W output with 10 power levels so I presume it would put out 80 W on power level 1... I know I'm on the right track here...

So I boil the water in kettle first then try to use the microwave to maintain the boil. I've learned watts are joules per second.

A litre I want to maintain at 100C, so I need to know the temperature of the surrounding environment it is going to lose heat to but I don't know what that will be inside a microwave and that's getting me stuck. I assume the steam would fairly quickly bring the temperature up in the microwave to around the same temperature as the water or am I wrong?

Now I think a litre of water contains 418600.0 joules of energy. I guess to get it perfect I need to know the surface area of the container. There's no way I'm going to do that with a tea-pot so I can calculate how much will be lost by radiation. Perhaps I will give up and just start on power 1 and trial and error what it takes to keep the water at a boil