EROEI - Energy Returned on Energy Invested - a measure of of embedded energy costs versus energy yield
The effectiveness and efficiency of machines that consume fuel is measured in terms of Miles per Gallon or Kilowatt hours - something has to be consumed in order to give a return.
Renewables don't consume any fuel at all in use but they still have an energy cost - embedded in the structure of the equipment and the construction of the project. The EROEI isthe ratio of the projected lifetime output of a system that exploits renewable energy - compared to its embedded energy. So something with an EROI of 4:1 (the return for typical PV installations in the UK) will only deliver 4 times more energy than it took to manufacture and install - something with an EROEI of 80:1 (a large wind turbine in the UK) delivers 80 times it's embedded energy cost.
It is also possible to apply EROEI's to fossil energy - by calculating the energy used in extracting and transporting a fuel. For example, hard to extract oil may have an EROEI of only 20:1. To calculate the true carbon cost of fuels both the direct cost of use and the extraction and processing costs need to be calculated.
By applying an EROEI to fossil fuels it's possible to demonstrate that hard to extract fuel is less efficient than renewables.