Good luck with that Cuppa , I'm dodging the cpap machine like the plague which is probably not a good idea but if I go in my sleep I won't know about it .
As for solar , it's like horsepower and towing something , the heavier the load your towing the more horsepower you need .
Tested my 600 watt system on the van with an overcast sky and gave the numbers to the company I bought them off
( batteries were going flat for some reason ) , they wanted them back because they weren't supposed to work that good under a cloudy sky .
Turns out one battery was shagged the one I didn't replace when I set the dual system up .
You lose a lot of efficiency with solar as you've pointed out Cuppa , depending on conditions .
A panel rated at 100 watt , will only give 100 watt in a controlled lab not in the real world , but a decent set of panels with the right wattage will still give something useful if the day is overcast , how long for depends on usage .