With each conversion of energy, part of the energy from the source is converted into heat energy. E.g. rotors, gearboxes or generators are never 100 per cent efficient, because of heat losses due to friction in the bearings, or friction between air molecules. Most of us have the sensible notion, however, that as we e.g. burn fossil fuels, somehow, loosely speaking, the global potential for future energy conversion becomes smaller. That is absolutely true. By that they mean that our ability to perform useful work converting energy decreases each time we let energy end up as heat which is dissipiated into the universe.
Useful work is called exergy by physicists.
Since the vast majority of wind turbines produce electricity, we usually measure their performance in terms of the amount of electrical energy they are able to convert from the kinetic energy of the wind. The rest of us may still call them wind turbines. Mixing up the two units is a very common mistake, so you might want to read the next section on power to understand the difference. Power is energy transfer per unit of time . Power may be measured at any point in time, whereas energy has to be measured during a certain period, e.g. a second, an hour, or a year.