Wh is a measure of energy.
W is a measure of power.
Energy = Power x Time.
It is quite possible for something to have more power and less energy by delivering stored energy at more power for less time.
The 778Wh seems to have a max of 500W. Meaning if you did drain 500W it would last 778/500 hours. (about 90 minutes).
The 256Wh seems to have a max of 200W so would last 256/200 hours (so about 75 minutes).
As to a recommendation for CPAP or not, this is my opinion. CPAP is primarily for sleeping, to inflate the air passage while it is squished via being horizontal and unconscious, so needs to last 8+ hours so it doesn’t run out of power while asleep. As such you need to look at the CPAP’s power requirements.
From this screenshot you can see their estimate of how many hours a CPAP would operate.
My opinion on this is if you don’t measure you are guessing and for something like a CPAP I’d not guess, I’d seek an owner’s measurements from real use. That 521 256Wh using 40W CPAP is assuming 75% efficiency. That 778Wh is assuming the same 40W. I’d say 75% is a reasonable efficiency so I’d not expect wildly different measured elapsed time for performance.
If it matters for CPAP and if you cannot find a credible independent measurement is simply go for the most Wh as it will last the most h for a given W.
My take of saying not recommended for CPAP was it was < 8 hours and you’d be probably asleep when it ran out. That is not “power” (Energy/Time) but just energy (Power x Time) but more related to Energy / Time = Hours.
So for a CPAP context you’d want the highest Wh number (highest number of hours of CPAP usage, being at least 8 hours). But for other contexts, non-CPAP, power outages in general, I’d give a guarded comment that technology can fail and given these prices are roughly simple $ / Wh is two independent stores of energy is better than one larger store of energy as there’s less chance two or three independent things all fail than one thing fails. i.e. more quantity is more reliable. I don’t need AC power for an outage, and I just have multiple Powercore for this reason, I store then deliberately around the house and one in the car, as during an outage, or an emergency, you cannot be assured of your route out being via collecting something this big, heavy and not waterproof.
