I'm sorry to tell you, but those 2 scenarios mean sh*t to 1/4, even 1/2 tank.
The power goes out? It's likely so localized that 1/8th or 1/16th of a tank gets you to fuel.
If not, it's likely so widespread and 1/2 tank isn't getting you to gas in time.
(This happened near where I lived a long time ago, a guy made a hand-sump, and a killing.)
A gas shortage with rationing?
1/4 tank isn't going to go far and neither is 1/2 tank.
You can sit with your car off with less than 1/4, and if you're talking about "having some" to last you it isn't stretching that far.
In a crisis, if you don't have an actual tank/supply somewhere you aren't getting far at all, it doesn't last 1, 2, even 3 days, a shortage is a long minute brother.
Cases like this ^ make me glad I can work remotely from home.