There's no point in keeping an inventory because almost every project will require a parts order anyway. Just order what you need per-project.
That assumes that everything one does is a "project". I don't know about anybody else, but sometimes I just sit down at my lab bench intent to tinker or experiment, or do some ad-hoc repair on something from around the house. And I've found having a modest stash of components on hand is very useful for those activities.
It's a balancing act, of course. There's a limit to how much one would want/need to spend on stocking components, but I posit that the limit should probably be "greater than 0". I can't tell you what the upper limit is though. My component stockpile has been building for many years, and there's no way I could even tell anyone how much is invested in it.
All of that said the one other thing I've often heard quoted, and tend to agree with, is this sentiment:
"When you order components for a project, always order more than you need, and put the unused ones in your stash". The thinking being, something along the lines of "If you needed this component once, you'll probably wind up using it again."
Of course it is, again, a judgment call. Order a few extra $0.50 comparators? Sure. Ordering a few extra of some multi hundred dollar FPGA or ADC or something? Yeah, no so much. :-)
For the beginner you can apply this principle to your first few breadboard projects and you're off to the races with some power-of-10 resistors, one color of LED, a few tactile switches, etc.
The main point is that you shouldn't try to anticipate what parts you will need more than a couple weeks into the future.
No doubt. You could easily wind up spending a TON of money if you do that! Not to say that I've never bought something on totally speculative basis, of course... :-)
If you mostly do digital, I'd say just get the E6 resistors and capacitors, common connectors and switches, and but there's as needed for projects.
Unfortunately multi packs usually have the whole E12 series full of odd sizes, like as if I'm going to need a 68k resistor rather than 100k, in a situation other than a project I'm ordering stuff for anyway, but having the extra selection isn't the worst thing.
A couple of notes:
1. E12 kits plays well with +/-5% tolerance. You're covering the entire spectrum of resistances possible with your kit, within the error-specs of your resistor.
2. Trimmer potentiometers (cheap pots designed for only ~200 cycles of use or less) are the key component needed for most analog / accurate circuits.
-------
You can usually design a circuit with +/- 5% accuracy (nominal), and then have one or two key trimmer-pots tune to 0.1% accuracy at the end. Ex: You might have one trimmer-pot control an a voltage offset (ex: from an instrumentation amplifier), and a second pot control the scale / multiplier (ex: op-amp multiplier and/or resistor divider).
You may have 20 to 30 resistors all across your analog circuit, all within +/- 5% tolerances for various reasons. But these two control points at the very end can "trim" away your error and reach 0.1%, or better, accuracy, in practice.
But you have to be "close enough", and have "linear enough" behavior. E12 (aka: resistors all within 10% of each other) seems to be enough for most such projects.
---------
Digital circuits on the other hand, don't really care too much about this stuff. So you end up with a bunch of 100nF capacitors with 1k resistors just all over the project. That's fine too. You should build a stockpile of the components you plan to use.
E12 resistor kits + a set of trimmer pots are for analog circuits. At least if you want 0.1% or 0.01% tolerances (3 or 4 digits).
For example, if I need specific resistor (and don't have any high precision/tolerance/current requirements — i.e. 90% of hobby projects) I could buy online:
- 10 of them for maybe £3
- 100 of them for around £6
- a kit containing 1000 resistors of 25 different values for <£12
Not only is the last option exponentially better value, but it means that I almost certainly don't need to buy anything the next 10 or more times I need a resistor. There are a good number common components, fittings, etc. that I've found this works for.
If you're doing specific high frequency, RF, hifi, etc. stuff, this approach might not work so well, but for other hobby electronics, messing around with microcontrollers and the like it's served me well.
If you are using substantial discrete digital logic in 2023, we can be friends if we're not already, but you shouldn't be giving people advice on stocking their workshop.
Electrolytic capacitors have a shelf life, and you'd be amazed how time flies. I've thrown out old electrolytics (>10 years) at more than one employer now.
But analog tinkerers do things that are very different from what I do!
Spend on some decent ones. Tayda sell decent quality Royal Ohm ones for not much money.
Better to just DIY an E6 kit on there.
I do most of my stuff on microcontroller boards though, and I keep a small stock (maybe $30 worth of ESP32, ESP8266, and something-that-can-pretend-to-be-a-USB-keyboard dev boards).
Sort of like buying complete cat5 cables instead of adding the connectors yourself from bulk cat5 - bumps your thoughts up an iso layer or two.
And really a nice stock of components doesn't cost a fortune.