Reserves (Energy)

When discussing energy sources, it’s important to distinguish reserves from resources.

Resources are the total amount of a primary energy source known (or estimated) to exist; so the total amount of oil thought to exist in a oil field, the total amount of woody biomass in an area of forest or the amount of solar or wind energy available over an area of land.

Reserves by contrast are a subset of resources, in particular the amount of the energy resource that can be recovered in practice, based on current (and near-future) technological and economic realities. Not every drop of oil can be recovered from an oil field: it may be too expensive to drill to some parts of the field, or some of the oil may be too viscous to extract using current techniques. It’s worth noting that you’ll only see the term reserve used with reference to resources that require extraction/recovery (oil, gas, geothermal etc.), not to resources such as solar and wind (which just “arrive” without us needing to do any work).

Even when an energy resource remains unchanged, the amount deemed to be a reserve can change over time: if the oil price goes up, it may become economic to drill deeper; or new technologies may make extractable parts of the resource that were previously not.

Particularly in the context of fossil fuels, you may also hear sub-definitions of the term reserve, specifically that of proven reserves. These are reserves calculated to have (normally) at least 90% confidence of being recoverable under existing economic and technological conditions. Thus they can also be termed P90 (or, less obviously, 1P).

Of the various renewable sources, geothermal is the one with the most obvious parallels with the resource/reserve terminology used in oil and gas (since it involves drilling to tap into an underground resource).

However it’s worth noting that the P90 terminology also arises in the analysis of power generation from sources such as wind and solar. The difference there is that it is used in the probability analysis of energy (electricity) output of the power plants rather than as a subdivision of the primary wind or solar resource into different levels of accessibility.