If you're using gas or resistive heating anyway this achieves very little.
? This isn't correct. If my kettle boils in 2 minutes when it's half full, and 3.5 minutes when it's full, then by only half filling it you are saving 1.5 minutes worth of (gas/electricity). Low absolute impact, but high relative impact, and adds up if you boil the kettle 3+ times a day
If you're using gas, it depends on how your electricity is generated and how much heat escapes through your boiler's exhaust etc (some very old boilers are less efficient than a gas turbine + transmission losses).
Plus you seemed to have gone on the boilers rather than kettles.
Yeah, I meant using in the present tense, sorry (ie. The system is keeping the room above ambient even if it's presently idle due to the thermostat). If you're cooling the space, it's much worse.
> Plus you seemed to have gone on the boilers rather than kettles.
The boiler would be the gas central heating system. The kettle is making the tea.
So unless you have some quite specific circumstances it is almost always better to boil only as much water as you need at the time.
If your grid is low carbon it might still break even (it will cost you money though).
This is all relatively pointless minuitae though. The main advantage is it's faster so one should do it anyway as there's no downside.
Unrelated, I've always been fascinated by the idea of a solid state heat engine driven kettle. Peltiers are so inefficient that it's not really pointful even as a toy though. I keep waiting for magnetothermal to be a thing.
https://www.wolframalpha.com/input?i2d=true&i=+3Divide%5BkW%...