Hello MJT - On the attached chart you see that the size E (aka M24) tank is the largest portable unit. At 680 liters when filled to pressure, one unit will last about 3.5 hrs at your desired flow (3L* 60min = 180L/hr.). Look here:
https://www.phc-online.com/O2-tank-duration_a/1...
I see you live in a pretty big town. Although there may be some O2x rationing going on due to this Covid, your supplier is not the only show in town. Get your Doc to write the Rx first so you don't have to do any retesting. Then start making some calls to get the backup supplies you need. Stay safe and be Well.

@A MyCOPDTeam Member, you're such a good friend to help everyone like this.

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A MyCOPDT...

yes you can have them bring you some small tanks the kind you can carry around I always keep some because I sometimes like to go out for awhile so I take the portable

I have size D tanks for power outage. you can set it in a roller. They come up to your knee. Not every country call tanks by the same letter. I get 9 to 10 hours with each one.

Your O2 provider should give you several of the big E tanks. Those last a good long time and two or three will last for over 24 hours. You should let the power company know of your situation, and be sure you know where the nearest hospital or fire department is. You should be able to get O2 at either location should you get really low.