Editing Net energy gain (section)

==Sources of energy==
The definition of an energy source is not rigorous. Anything that can provide energy to anything else can qualify. Wood in a stove is full of potential [[thermal energy]]; in a car, [[mechanical energy]] is acquired from the [[combustion]] of gasoline, and the combustion of coal is converted from thermal to mechanical, and then to [[electrical energy]].
Examples of energy sources include:

* [[Fossil fuels]]
* Nuclear fuels (e.g., uranium and plutonium)
* [[Solar power|Radiation from the sun]]
* Mechanical energy from wind, rivers, tides, etc.
* Bio-fuels derived from [[biomass]], in turn having consumed soil nutrients during growth.
* Heat from within the earth (geothermal energy)

The term net energy gain can be used in slightly different ways:

===Non-sustainables===
The usual definition of net energy gain compares the energy required to extract energy (that is, to find it, remove it from the ground, refine it, and ship it to the energy user) with the amount of energy produced and transmitted to a user from some (typically underground) energy resource. To better understand this, assume an economy has a certain amount of finite [[oil reserves]] that are still underground, unextracted. To get to that energy, some of the extracted oil needs to be consumed in the extraction process to run the engines driving the pumps, therefore after extraction the net energy produced will be less than the amount of energy in the ground before extraction, because some had to be used up.

The extraction energy can be viewed in one of two ways: profitable extractable (NEG>0) or nonprofitable extractable (NEG<0). For instance, in the [[Athabasca Oil Sands]], the highly diffuse nature of the tar sands and low price of crude oil rendered them uneconomical to mine until the late 1950s (NEG<0). Since then, the [[price of oil]] has risen and a new steam extraction technique has been developed, allowing the sands to become the largest oil provider in Alberta (NEG>0).

==={{anchor|Energy payback time}} Sustainables===
{{details|topic=the EPBT for solar PV|Crystalline silicon#Energy payback time}}

The situation is different with [[sustainable energy]] sources, such as [[Hydroelectricity|hydroelectric]], [[Wind power|wind]], [[Solar power|solar]], and [[geothermal energy]] sources, because there is no bulk reserve to account for (other than the Sun's lifetime), but the energy continuously trickles, so only the energy required for extraction is considered.

In all energy extraction cases, the life cycle of the energy-extraction device is crucial for the NEG-ratio. If an extraction device is defunct after 10 years, its NEG will be significantly lower than if it operates for 30 years. Therefore, the '''energy payback time'' (sometimes referred to as energy amortization) can be used instead, which is the time, usually given in years, a plant must operate until the running NEG becomes positive (i.e. until the amount of energy needed for the plant infrastructure has been harvested from the plant).

===Biofuels===
Net energy gain of [[biofuels]] has been a particular source of controversy for [[ethanol]] derived from [[Maize|corn]] ([[bioethanol]]). The actual net energy of biofuel production is highly dependent on both the bio source that is converted into energy, how it is grown and harvested (and in particular the use of petroleum-derived [[fertilizer]]), and how efficient the process of conversion to usable energy is. Details on this can be found in the [[Ethanol fuel energy balance]] article. Similar considerations also apply to [[Biodiesel#Efficiency and economic arguments|biodiesel]] and other fuels.