A couple of inches of water above my element will give me a height of 350mm
So if I use a 3” shell that had 350mm of water in it ( LOL here we go mixing units again ) , that gives me a volume of 1.76 litres in the boiler .
Running that through an online calculator using my 6kW element shows a darn fast time from switch on to boiling at 100% efficiency .
Even at 80% efficiency , it’s way better than I had hoped .
Bolverk , you were pretty much bang on the money

I am trying to find a calculator that will show the rate of water loss but I’m coming up blank . Lots of brew calculators but can’t seem to find one that tells me what I want to know .
IE , at 6kW , what is the top up flow rate going to be ?