The gravitational potential energy (GPE) of an object can be calculated using the equation:
GPE = m * g * h
where m is the mass of the object, g is the acceleration due to gravity (9.8 m/s^2 on the surface of the Earth), and h is the height above a reference point.
So for a 0.8 kg box on a 2.5 m high shelf, the GPE would be:
GPE = 0.8 kg * 9.8 m/s^2 * 2.5 m = 19.6 Joules