How can energy efficiency be calculated?
As the ratio of energy output to energy input.
By definition, efficiency of a system is the ratio of power output to power input.
But since power is the rate of doing work, or the rate of energy transfer,
Since in all cases
useful energy / total energy
You need to know the value of the useful energy given out by the device, this may be one type of useful energy or more than one type, and then divide it by the total energy going into the device.
The unit of energy is J (Joules)
For efficiency you can either leave the number as a decimal or multiple by 100 and give the number as a percentage.
Potential energy vs. usable energy.
HydoCarbon based energy burns at different rates of efficiency. Each gives off water and carbon monoxide. It is the level of carbon monoxide given off which dictates how efficient it is.
Philosophically speaking, all forms of energy, carbon based, water, wind, sun, each have inherent inefficiencies. In sun based energy systems the inefficiency is measured in the available light vs. the amount used.