This is very similar to the "energy marbles" explanation that I use to describe my on-going battle with the mind-numbing fatigue my MS creates. Like Christine's spoons, I have only so many energy marbles to play in any given day. When they're gone, they're gone, and I'm fairly useless for the rest of the day.
I'm going to print it out and post it on the refrigerator to remind my household that while I don't look sick, I'm not 100% and soup and sandwiches will suffice for supper.