It is simply a self-management tool that can help you better cope with living with ME/CFS. Utilizing the energy envelope will enable you to live a more balanced life by helping to prevent relapses. As an added bonus, you may find that over time your available energy supply will gradually increase, giving you a bigger energy envelope to work with.

Source: The Energy Envelope Theory: An Effective Self-Management Tool for ME/CFS – Prohealth