Adding to @Oscarmax useful link above. I also think Delta T 60, 50…… is an important factrften overlooked.
Rather than trying to explan about it, I found this on Stelrad that’s easy to understand
https://www.stelrad.com/news-events/blog/the-importance-of-delta-t-in-calculating-heating-output/
I use the zapper if making 1 or 2 cuppa’s for me and the wife. I also cook alot of meat, meals in a slowcooker to cut down on oven use.