So when it comes to Gravitational Potential Energy the higher an object of mass is the more energy it has as the equation for EPG is = MGH so the M is Mass of the object , the G is the gravitational constant which on earth is roughly 9.8 m/s and the H is the height of the object. So the greater the H value is the more energy you'll have. An example would be using your 30kg mass say at a height of 10 meters would have the EPG of 2940 Joules of energy since EPG= MGH so EPG= (30)(9.8)(10) . Now if the object was at a height of 20 meters the EPG would be greater as EPG=(30)(9.8)(20) which would be 5880 joules of energy