Not always that directly - the old DHW / space heating question again. We, most of us, have been around the dwellings on this, our passive house design model shows we will need around 2Kw of heat, worst case - peak in a typical winter and max of 3Kw once every 10 years. BUT following a conversation elsewhere with @Nickfromwales we are going, as things stand, for a 35Kw Combi because we want to run two simultaneous showers at 10+ L/min and don't want the standing losses from a tank which @JSHarris has shown, and is discussed above, can be a problem in the well insulated home. The conversations above make the same point in that heating a well insulated home consumes a lot less energy than the DHW demand if we are all showering frequently as has become the norm or at the very least demands the ability to expend energy at a hi rate to recover hot water in standing volumes quickly in readiness for the next hose down! This is a classic 'no one right answer' problem - just an optimal one for each according to their circumstances....