But what is that requirement?
CH?
Or DHW including a shower?
In the main, DHW is cheaper from electric sources, unless the boiler is already running, the cost to heat a whole boiler up to get enough water to wash one's hands, is silly.
So this house, main room, around 4 kW of radiators, rooms which we always want some heat, toilets, hall, landing, maybe another 3 kW and that's before the TRV's start to throttle back the radiators, at night even less just for bedrooms, bathroom and landing. So my 20 kW boiler is way oversized.
OK my boiler will not modulate anyway, but in the main, this house could be heated with a 6 kW boiler, when I return from being away, so all rooms need heating, we should hear the boiler running for a few hours before it starts the mark/space ratio, but it starts after just an hour to cycle off/on, so either radiators too small, the pipe work is too small, or the boiler is too big.
I know when we have had a breakdown, I can keep living room warm with a 750 watt oil filled radiator, so it is very clear, the whole central heating is oversized.
So if I had a central heating sized correctly for the house, how much less oil would it burn? We see an oil or gas boiler of 20 to 35 kW being replaced with a 7 kW heat pump, and told how much more efficient the heat pump is, but we have not replaced like for like, so in real terms not a clue how well a heat pump would work if grossly oversized like the boiler it replaced.