Well, have you read the linked spoon theory? I ended a friendship once by not going to their birthday. I did nothing unavoidable and there was no option, no priority (unless loosing your normal even already part time job would be a valid option for you, to go to such a party) given. Yet still I hadn't the energy left.

That's still the difference. I also feel spent and use the battery analogy in my day to day life. My "capacity" is pretty decent, I can afford things, but I have to manage it, and yeah, so much is a cost, even things you'd never expect.

If we say a party costs 50 and your battery is at 20, that sucks. That relative I mentioned has been suicidal over this, and its entirely possible that their inability to manage that battery is directly caused by their mental health struggles, too. I'm not claiming they're a bad person, when I say they're a bad friend. I'm saying they aren't going to be there for those that call them "friend". It has absolutely been a lifelong struggle for them, and that sucks, but they cannot do it. I cannot power a laptop with a standard phone charger, even if they're both USB-C, and this person I know isn't a good friend outside of a very small group they have.

I don't mean to sound like a dick when I say that, everyone's different and we have different struggles, different things take a different amount of battery or take more or less spoons.

The issue of "how do you tell if someone's running on empty or just not willing to put energy into it?" is a whole separate issue than "this costs more battery than I have when fully charged". The solution to the first, is managing your charge. The answer to the second, I dunno. To me, that feels like the mental health equivalent of asking if this mole looks cancery online, and if I was in that situation, I'd be seeing a therapist or a med manager about it. Not saying you haven't tried or anything, I'm just saying I don't know the answer to that, and I have the feeling the answer is way deeper than someone on the internet will be able to go. I sincerely hope things go well for you, though.

In a case like that, just plan to go for 30 minutes. Just making the effort to show up goes a long way. You'll find that often you end up staying longer and it's fine, because the hard part all along was passing through the door.

Sometimes, if you want to nurture friendships, you need to let your battery go negative sometimes.