For Electric & Natural Gas Customers
Doors often gain and lose heat through direct conduction. The gain and loss of heat is measured by a U-factor, which is the rate a door conducts non-solar heat flow and is usually expressed in units of Btu/hr.
Solar heat gain is the amount of solar energy absorbed through a door and released as heat inside a home. Check the U-factor rating when looking for a door as lower ratings signify the door will be more energy efficient.