Really good question… Useful forms of energy are things like electrical energy, kinetic energy, chemical energy. Heat energy comes in different forms… if it is lower temperature (low grade) heat energy then it is less useful.
Making all the useful forms of energy always produces low grade heat energy through things like friction with the air, heating up of electrical wires. You also often get sound energy coming out of power stations or wind turbines.
The efficiency of solar panels is more complicated and to do with quantum mechanics .. but there is a theoretical limit of a bit less than 35% efficiency.
As Jack says, energy has a sneaky way of leaking out when you aren’t looking. There is always resistance to change – inertia – and it is this that gets in the way. So – you can never win in this life, the best you can hope for is to break even. However, you can only ever hope to break even at Absolute Zero………..but it is impossible to go down to Absolute Zero. 🙂
Ha ha @laurence. Great answer. Jack and Laurence have answered this really well – suffice to say that even with a really, really efficient system some of the energy will esacpe as noise or heat or motion – it’s because the world has gravitational laws and matter.
As everyone here says and from electronic engineering prospective … It is rarely that we get exprimental tests show the same thing as theory and the problem with energy providers or circuits that they always face irregularities and also loss of energy at edges or when both ends are not matched well … Well the thing is such life … nothing really is 100% 😀