An object's gravitational potential energy is equal to the product of the object's mass times the gravitational field strength times its height (PEgrav = mass * gravity * height). The strength of Earth's gravitational field is 9.8 meters per second, and height is measured starting from a specified surface.
Gravitational potential energy is stored energy that acts upon an object as a downward force. When the supporting force of an object that is at some height is removed, its gravitational potential energy begins being transformed into kinetic energy by the downward gravitational force. Gravitational potential energy, being a measurement of energy or power, is measured in joules.