A washer-dryer is pretty power hungry (~3kWh/load), and takes several hours (4-6hrs) to do a full wash/dry cycle. If you don't want to be running an engine/generator for hours to provide the power -- expensive and inefficient, especially using the engine -- you'll need a lot of solar and a decent-sized battery bank/inverter . Perfectly doable on a narrowboat so long as you don't mind covering a lot of the roof with it, which many people don't want to do. LFP batteries are not essential, but are much more user-friendly with high-power loads, and make much better use of any power you do get from solar (or an engine/generator) with better efficiency and shorter running times. I do have a washer-drier on board, also induction hob, electric oven, toaster, microwave -- but then my boat is a series hybrid so I've already got the big/expensive LFP battery bank/inverter/solar/generator needed for propulsion, so high-power electric appliances like a washer-drier are "free", and their power/energy use is considerably less than propulsion -- about the same as an hour's cruising. OTOH on a diesel boat a lot of this doesn't really make sense, the cost to provide the high-power infrastructure just for this (going gas-free, washer-drier etc) would be very high -- which is why most boats don't do it... 😉
