When people with ME/CFS talk about capacity, they mean the amount of demand — of any kind — that may be manageable at a particular time. It is a description, not a measurement. It cannot be read off a meter, and two people with the same diagnosis may have very different capacities, as can one person on two different mornings.

Capacity is usually both limited and variable. Limited, in that there is a point at which continuing costs more than the body comfortably repays. Variable, in that the point moves — hour to hour, day to day, week to week — sometimes with an identifiable reason and often without one. Someone may feel able in the morning and not by mid-afternoon, or manage a full Thursday and not a quiet Sunday.

The most useful thing to understand about capacity in ME/CFS is how many different kinds of demand draw on it:

• Physical demand — movement, standing, washing, carrying
• Cognitive demand — reading, forms, decisions, concentration
• Sensory demand — light, noise, screens, busy environments
• Social demand — conversation, visitors, responding appropriately
• Emotional demand — worry, conflict, grief, supporting someone else
• Upright or postural demand — sitting up for long periods, standing, travelling

These are not separate accounts that refill independently. Different types of demand compete with one another: someone may use much of what is available on an appointment — travel, sitting upright, talking, processing — and then have little left for cooking that evening, or even for conversation. A day with no physical tasks at all can still end up fully spent after forms and phone calls.

One of the stranger features of capacity in ME/CFS is its unevenness. Someone may be able to read for half an hour but not stand at the sink for ten minutes; able to cook with a stool but not to sit through a film; able for a quiet visit but not a phone call. This is common, and it is one reason outside observers — and sometimes the person themselves — get confused: how can she manage that and not this?

The answer is that demand is about the nature of the activity, not its visible size. An hour with a close friend may be manageable while an hour with a demanding one is expensive. Completing paperwork may leave nothing for physical tasks afterwards, while a slow walk might be followed by a normal evening. When capacity for one thing exists alongside no capacity for another, that is information about the illness, not inconsistency in the person.

A number of things may influence what feels manageable on a given day:

• Poor or unrefreshing sleep
• A recent illness or infection
• Recent activity — yesterday, or several days ago
• Hormonal cycles, for some people
• Environment — heat, noise, how much the day requires
• Emotional load, whether visible or not

Sometimes a hard day has a clear cause; sometimes it does not, and the search for one becomes its own burden. Poor sleep may explain one difficult morning and be irrelevant to the next. A better day does not necessarily mean a permanent improvement — it may be a window rather than a trend. And rest may reduce ongoing demand without guaranteeing restoration: lying down for an afternoon can lower what is being spent without returning what was already spent. Capacity is weather in most people's experience — something to observe and work around, not something that always yields an explanation.

Because limited capacity is so often misread from the outside, it is worth being direct about what it is not:

• Not motivation — the willingness is usually there; the capacity is the part that fluctuates
• Not willpower — pushing through does not create capacity, and often borrows against what is left
• Not fitness — exercise conditioning does not behave the same way in ME/CFS as it does in healthy bodies
• Not laziness — nobody chooses a day spent choosing between a shower and a meal
• Not a fixed daily allowance — it cannot be budgeted precisely in advance, because the day itself has not been spent yet

Treating capacity as a fixed allowance that a person should simply use better is where most misunderstanding comes from — from professionals as much as from family. A fluctuating allowance is genuinely harder to plan with, and the person living with it is usually the one doing the most careful planning.

The same ideas apply across the spectrum, at different sizes. At milder levels, capacity may cover much of an ordinary day with careful choices around demanding commitments. At moderate levels, whole categories — work, social life, driving — may sit outside it on most days. At severe and very severe levels, the competition between demands can be stark and constant: a few minutes of communication may be what the day holds, and choosing between a wash, a message and a quiet hour with a voice recording is the real daily decision. At every level, the underlying fact is the same — available capacity is limited, and deciding what to do with it is genuine work, whether or not the decisions are visible.