What if the power source runs out? When you know you could have fixed the problem with a simple spare part, you will get pretty frustrated pretty fast.
What spare parts should you have on hand? That depends on your equipment and what you plan to use it for. Your list could look different from mine because we might have different items. The items mentioned in this post are general items. Most of these things are basic items and would be able to fix or repair your broken-down item. I am also thinking about needing to recharge or refuel items because your generator or camp stove will be worthless if you run out of fuel.
Batteries are always a must. While having hand-cranked flashlights and radios are great, most emergency equipment works better and faster with batteries. I would keep a lot of batteries in sorts of sizes. Most battery powered objects take either AA, AAA, C, D, and 9-volt batteries. I keep mine in a storage container similar to this. You could keep multiples of these storage containers in the house, garage, shop, and wherever you may need to use them. If you need specialty batteries, I would make a list of those and pick those batteries up the next time shopping. For specialty batteries, I would keep them near the object that takes them so you aren't hunting them down in an emergency.
With so much technology and solar power these days, chargers and charging cords have become a must. I personally do not get rid of an old charger or charging cords until I absolutely know it will not work for a device in the home. A lot of charging cords work for multiple items. I would test the cords periodically and dispose of the ones that do not work or cannot be fixed.
Extra fuel cans are a must to have on hand. Some may not consider them a spare part, but you will regret not having enough fuel on hand when a situation happens. I would keep your fuel cans full and rotate the fuel every 3-6 months. I would also keep a fuel stabilizer either in the fuel or have it on hand to keep the fuel from going stale. I would also keep extra propane cylinders on hand and full in one-pound and twenty-pound cylinders. Propane does not go bad. If you have kerosene heaters or cookers, keep some kerosene on hand too. Likewise, if you have a propane or butane torch, you will need extra cylinders on hand.
Other items to keep on hand:
Extra screws of all sizes
Scrap Lumber to fix broken doors, frames, and windows
Hitch pins, cotter pins, clevis pins, and lynchpins
O-rings and rubber gaskets of all sizes
Wiring Kit (for autos) and/or an Electrical Repair Kit
Light Bulbs for your flashlights and regular lights
Like I said before, you might consider other spare parts essential for your preps. You might want to keep spare parts for:
Your vehicle (bug-out or daily driver)
Camper, ATVs, and boats
Tractor or Semi (if you have one or more)
Guns, Bows, and other weapons
Water filtering systems
Tillers, Snowblowers, Lawn Mowers, and other such equipment
Log Splitters, Wood Chippers, Chainsaws, and Trimmers
Wood stoves, Cookstoves, Grills, and other cookers
Any other equipment you have that is not listed
Without sounding dire, these items could be the difference between life and death. If you have these spare parts on hand, you could be living a much easier life than if you did not. However, having spare parts on hand will not do you a lot of good if you don't know how to fix or repair something in the first place. So you should be working on your skills and learning how to repair your own equipment.