You can imagine your energy as money stored in a bank account. Because of your CFS or FM, your account has a very low balance (a small number of “energy dollars”). 

While healthy people are able to store up energy for a day’s activity with seven to eight hours of rest at night, people with CFS or FM may get only a few "energy dollars" from a night’s rest. The small number of energy dollars makes it easy to spend more energy than your have and overdraw your account. 

There is often a big service charge (intense symptoms) if you overspend. Once you’re overdrawn, you have to deposit more to your account in the form of rest. The alternative is to budget your time and spend your energy dollars carefully, in order to avoid service charges (post-exertional malaise). 

For an example of how one person used this idea, see the article My Energy Bank Account.

This approach uses a similar idea with a different image. Imagine your available energy as marbles in a bowl. Each marble represents a small amount of energy. You estimate your energy level each morning and put an appropriate number of marbles in the bowl. (Some people take this idea literally, using marbles or coins stored in a bowl. Other people do calculations in their head.) 

With every activity, you take one or more marbles out of the bowl: one for showering, one for dressing, etc. Some tasks take more marbles than others. Also, the same task may require more marbles on bad days than on good days.

Physical activity uses up your supply, but mental and emotional activity consume marbles as well. For example, if you feel frustrated about how few marbles you have, your frustration will use up some of your marbles. Stress, tension and fear are all big marble-users. Whatever you can do to lessen them will preserve your supply of marbles for other uses.

Another way to think about living within limits is called the Fifty Percent Solution, described in a book by William Collinge in his book Recovering from Chronic Fatigue Syndrome. He suggests you estimate how much you think you can accomplish each day, then divide that in two and aim to do the lesser amount.

Rather trhan challenging your limits, you keep your activitry to a safe level. The unexpended energy is a gift of healing that you give your body. The Fifty Percent Solution is a clever way of addressing our tendency to overestimate what we can accomplish. Another benefit is that it gives you permission to take care of yourself. 

A popular way to explain the idea of limits to others is called the Spoon Theory, created by Christine Miserandino, a woman with Lupus. She describes how she once explained to a friend what it is like to have a serious illness by using spoons to symbolize available energy. 

She gave the friend 12 spoons to symbolize her energy allotment for the day. With each task the friend imagined doing, she lost one or more spoons. The friend used half her spoons just getting ready to go to work.