On Tue, 14 Aug 2018 09:18:44 +0100, Brian Gaff wrote:

So how does a slow heater make any difference. Take the kettle if it
takes longer to boil the water surely you have probably used the same
power as you would have with a more powerful heater for a shorter time.

Slight confusion of "power" and "energy" but yes if you are heating 1
l of water from 15 C to 100 C it requires a certain amount of energy.
If a 1.5 kW kettle delivers that energy in 2 minutes and 3 kW one
will do it in one.

It can probably be argued that the 3 kW is slighly more effcient than
the 1.5 kW as there is probably less radiation from the kettle in
the 1 minute to 100 C compared to 2 minutes to 100 C.

Same goes for room heaters.

Room heaters are not quite the same. They have to balance the heat
loss of the room. If their power rating is less than the heat loss
they won't keep the room warm.

--
Cheers
Dave.