when the eroi is large that means that producing energy from that source is relatively easy and cost effective however when the number is small obtaining energy from that source is difficult and expensive for example when the ratio is 1 there is no return on energy invested according to the world nuclear association the break even number is 7 
in its simplest form eroi is calculated as however there are dramatic differences in how certain steps of the input process are measured this measurement is complex because the inputs are diverse and there is uncertainty as to how far back they should be taken in the analysis in addition to energy costs there are other external costs that need to be considered with respect to energy production such as those associated with the environment and people s health generally we can expect that the highest available eroi energy sources will be used first because these offer the most energy for the least effort a net energy gain is achieved by expending less energy when attempting to acquire and use a source of energy eroi analysis is considered part of a life cycle analysis types of energy sources where eroi is measuredthere are a number of consumable energy sources where eroi is determined for efficiency and cost analysis these energy sources include oil biofuels geothermal energy nuclear fuels coal solar wind and hydroelectric the average eroi across all generating technologies is about 40 for the united states according to the world nuclear association pages cited above the association cites a study by weissback et al 2013 which states that the results show that nuclear hydro coal and natural gas power systems in this order are one order of magnitude more effective than photovoltaics and wind power according to the u s energy information administration fossil fuels such as coal petroleum and natural gas have been the major sources of energy since the late 1800s until the 1990s hydropower and solid biomass were the most used renewable energy resources since then the amount of energy coming from biofuels solar and wind energy has increased the eroi for oil has decreased dramatically over the past hundred years the amount of energy required to produce one barrel of oil has decreased as more efficient methods such as fracking have been introduced 