The term “Potential to Emit” under the Clean Air Act is defined under 40 CFR 70.2 as “the maximum capacity of a stationary source to emit any air pollutant under its physical and operational design”. You use the PTE to calculate your expected air emission when you are applying for a permit.
For example, if you have a piece of equipment that is designed to operate 24 hours a day and 365 days a year, you will have to calculate your air emission based on these maximum of hours. The agency is going to assume you will be running that equipment 24/7 every day of the week. Now if the equipment requires an hour of down time for maintenance, you can then use 23 hours per day for your PTE.
If you have a paint booth and you need so many hours per day to switch paints and set it up, you can deduct those hours when calculating your PTE. These are operational designs.
If you voluntarily restrict your operating hours in order to apply for a FESOP (federally enforceable state operating permit), you can use your reduced hours to calculate your PTE. In this case, your voluntary restriction on your operating hours is enforceable by EPA (hence federally enforceable). If you exceed those hours, EPA can take enforcement action against you even though you are operating under a state operating permit. So be very careful when you apply for a FESOP. Make sure you can meet your own operational restrictions.