It's like trying to shop with a limited budget, when prices can change (usually up, sometimes down) at any time, without notice. Oh, and the budget can change without notice too.
You often have to abandon items or switch items for cheaper/lesser alternatives. The luxuries might be first to go, but even essentials can become priced out of reach. The only other option is to borrow money at extortionate levels of interest.
At that level of interest, borrowing has to be a one off thing for emergencies and special occasions. You'll go bankrupt if you keep borrowing every time the prices go up or budget goes down part way through your day.
The most important skill becomes being able to change plans without warning. Being able to adapt around whatever is happening and to prioritise so that you can a) afford your shopping and b) you keep the right items.
In the context of living with a variable chronic condition or disability, the shopping list becomes a To Do list or schedule, and the energy 'cost' of any activity will vary, as will the energy 'budget' available, due to a range of factors.
These factors will vary between people. Some of mine include: how well I've paced, hydration, temperature, what I've eaten, whether I've been able to exercise regularly recently, hormone levels, amount of sleep/insomnia, needing to do an unexpected task (anything from clearing up a spilled drink, to a journey taking longer than expected), or the body fighting an infection.
Bearing in mind that the 'right items' will also vary (sometimes personal hygiene might be the most important, other times socializing or doing something that helps mental health, other times physical health or exercise might take priority) this process is pretty exhausting and turns a simple shopping trip into an epic lateral thinking, problem solving challenge. It's not just the very limited energy levels that are the problem. It's the variability that makes life a constant balancing act of decisions, and makes every day a step into the unknown.
These changes of plan are the only way forward - the alternative is to run off adrenaline and then crash out. This might look fine from the outside because the crash tends to happen behind closed doors, when I've relaxed, so others often don't realise the toll it's taken. Crashing out takes far longer to recover from. The more I push, the bigger the crashes and longer the recovery. Trying to keep pushing through my fatigue hospitalized me for 5 weeks. Totally energy bankruptcy. It's taken years to get back to close to where I was pre-crash. I really don't want to repeat that!
But sometimes things can't be abandoned - perhaps I'm out and can't go straight home, or it's an emergency, or a special occasion. Then I need to do what I can to reduce debt. For me this will include: rehydration salts, lying down whenever I can (park benches, quiet corners of shopping malls, the floor of the train, the back seat of a car) eating little and often, accepting help, periods of zoning out and not talking to anyone, and generally pacing. And then taking a day or more afterwards trying to spend as little energy as possible/spend energy on things that will improve long term energy like nutrition,exercise, and pacing. Then the 'spare' budget at the end of the day can go towards paying off the debt - i.e. getting myself to a point I am less symptomatic and more able to function.
At times I've had to reassess and rearrange my whole lifestyle (career, exercise, social life, diet etc) to get things back within budget. It felt horrible, like a failure, but it turned out to be the best thing I could have done, and something I now see as one of my most precious achievements - why? because by having a more flexible lifestyle I've been able to live mostly within budget, and that has triggered a slow increase in my budget over time. Things I thought I'd given up for ever are now becoming possible again.
I rarely (if ever) get my budgeting spot on, but getting it 'more or less right-ish' most of the time, and taking the time to recharge properly if I've gone totally over budget has made a huge difference to my overall energy levels and ability to do things.
The irony is, that the process of making sensible decisions so that I stay within energy budget also takes energy! Thankfully, in my experience, the energy saved far outweighs the energy it costs - and it's got easier over time, although I don't think it will ever stop being a challenge.
A worthwhile and rewarding challenge, but a challenge nonetheless.