Poppy, thanks for your comment. Do you run a shower from the combi boiler?
Combi boilers often fail to produce enough flow/pressure to give a decent shower. It depends on the KW output of the boiler. This is my experience in UK but this may well be on account of the proportion of hot water to cold required is much higher in UK because of the low ambient temp of the mains water coming into the house.
The heat exchangers in gas combi boilers are not designed to give high temps AND high flow, as electric cold-feed showers are. Good for basins and baths but I’ve seen some pretty wimp showers running off combi hot water delivery. You have to add a pump to the system.
So, in my UK house I had a combi boiler for radiators and the bath/sink/basin h.w. and a lekky shower over the bath. This was a prefect set-up for UK and is very common.
But my ignorance remains total, in so far as I still don’t understand what a 5.5KW tariff means. If you have a 4.5KW lekky oven on, plus the devices that may well be also running on a continuous basis [in practical terms] like fridge and freezer and you turn on a 2KW room heater or a washing machine, does the main trip flip?
Do you, as you have to with working out the AmpHrs of 12 volt battery capacity in a boat/camper, have to total up all the devices which might conceivably be running at any one time, and go for a tariff next up from the result total of the calcs?