I plan on having electricity at all times
If you can plan on that then you should probably also plan on having bullets at all times too. Thats kinda easier to do than creating a plan B, just make more of a plan A.
On one of those budget-gun-prepper videos where they were suggesting guns to buy for a certain price, one of the people in the comments section said “A Mosin for $100 with $2400 worth of ammunition beats any of these suggestions” (it was a budget video for 2500). He does have a point.
I dont care what people say about having a Bolt verses a semi auto, its better to have a gun that can blow the head off of something at least 1 time rather than 0 times.
Pellet gun is cheap, thats a standby for later on when or if you run out of bullets. Then again so is muzzleloading or cartridge reloading.
I dont have any experience with pellet guns bigger than .177cal but I can say that Pumping is your friend. Avoid having to use CO2 or pre-charged canisters. If you cant pump it up by hand it might as well be as limited as a firearm.
You can also make custom ammo to shoot out of a pellet gun if you run out of bbs or pellets. It needs to have a deformable ring of some kind to absorb rifling spin, and the best type will be some kind of light rod-shaped projectile (basically a 4.7mm Flechette).
A Nail could do the job, if you took a dremil and cut it to about the length of a rifle bullet. Wrap it with something (like a piece of tissue secured by tape) that will buffer it with the remaining space. Even despite possibly having a rifled spin, it should be Front Heavy so that way it does not start tumbling in the air.
Take this lesson from the Lawn Dart . . . front heavy means the back of the bullet will try to tumble and the air current will just keep pushing it back on course. Back Heavy means the front of the bullet will try to tumble, then you get it spinning hopelessly out of control.
Regardless of what you think, a Projectile which is slower and heavier is more effective (compared to faster and lighter). The trajectory isnt as flat but it will be more reliable. And it will penetrate deeper. Shooting a bunny is good, but putting a hole through one side and out of the other is 2 wounds not 1.
I would suggest getting some reloading slugs for 5.45mm to use in a .22cal Pellet Gun (instead of pellets) … but the minee-effect of the expanding pellet in the barrel may be required to achieve spin.