Wikipedia and IEC definitions

Wikipedia and IEC describe the power generation concepts in these terms:

• A power station (601-03-01, also referred to as a generating station, power plant, or powerhouse) is an industrial facility for the generation of energy (electricity, useful heat...).
• A generator (151-13-35) is a device that converts one form of energy to another (example: mechanical energy or sun light to electricity).
• A wind farm (415-01-03) is a group of wind turbines in the same location used for production of electricity. The land between the turbines may be used for agricultural or other purposes. A wind farm may also be located offshore.

OSM usage

The easy way to map these features in OSM would have been to keep these terms with the same definition.

Instead, the current OSM usage is the following:

• power=generator is used to map power stations (for nuclear, thermal and hydro stations), but also generators (for wind turbines).
• Nothing exists to map generators if power=generator is used for the power station. Some users create new power=generator nodes inside the main power=generator area.
• Nothing exists to map power stations if power=generator is used for power generators. Some users create another surrounding power=generator area around the generators, especially for nuclear power plants. A landuse=industrial could be used, but this tag is not enough to define a power station.
• Wind farms are usually not mapped. When they are, a surrounding power=generator area is created around its wind turbines, as for conventional power stations. This is not adequate, as the perimeter does not strictly exist (there is no fence around a wind farm), and the implied industrial nature of power=generator is not applicable to wind farms: the land between the turbines may be used for other purposes, especially agriculture (landuse=farm).

New approach

Power plants would be summarized as a set of precise kind of elements, generators and infrastructure which allow to produce power.
Such stuff may be inside a perimeter which physically exists, like a fence and mapped as a closed power=plant . Sometimes, power plants don't expose fenced area or any other element which allows to aggregate facilities inside. In this particular situation the power=plant object is a and features are bind to it with no precise roles.
In details, different generators could get involved in the power generation process. Thus we would make the distinguish between intermediate generators which produce usefull power for output generators. Output generators produce power which is send outside the power plant. They are the only generators used to compute the overall capacity of a power plant. Examples are available at the end of this proposal.
Source and output values are specified at the power=plant scale to allow consistency checks.

To have an unambiguous and consistent power generation scheme, it is proposed to:

• Create a new power=plant tag for what Wikipedia calls a power station. A nuclear power station with multiple reactors on one site with a single name (such as Fukushima II) would be contained within a single area outlining the whole facility tagged power=plant. An array of wind turbines known operated together and known by a single name might be represented by a node at some suitable location central to the array tagged power=plant.
• Create a new plant:output=* namespace for the rated output of the facility (which may not be a simple sum of their capacities). Its tagging being exactly the same as generator:output=*.
• Modify the current definition of power=generator to refer only to an individual power generation device rather than the entire power plant. Each turbine in a wind farm would be a node generator; each individual reactor within a larger nuclear power station would be a node or an area. The output of a generator would be the rated output for that particular device.
• Create a new generator:type=* key to distinguish generator types among a single generator:method=*. This is particularly needed for nuclear energy, where no electricity-producing fusion reactor is expected before 2033 (DEMO). All current nuclear reactors use fission. Fission reactors are classified by their design, and power plants can mix different generator types. For example, the French nuclear power plant of Flamanville contain two PWR and one EPR. reactor_type=* is already in use, but only in Chernobyl power plant.
• Create a new generator:plant=* key to optionnaly distinguish output and intermediate generators as describe above. Generators are output by default.

Enclosed power plant

A power=plant is tagged as a around the power plant perimeter (usually bordered by fence). We may use the following attributes:

In this case, spatial capabilities of the database would enable us to retrieve features which are inside the power plant's perimeter. According to the functional chart, we must find object tagged as power=generator + generator:plant=output inside and attributes must be defined consistently with power plant's ones.
Many other stuff (buildings, substations), introduced all along this proposal is recommended or optional.

We can compare the total rating of a power plant, specified in plant:output=* and addition of all the generator:output=* values of its generator members :

• plant:output=* < generator:output=* : Too many generators on the map or current generators have oversized output rating
• plant:output=* = generator:output=* : Everything is all right
• plant:output=* > generator:output=* : We miss generators

Please note that this comparison is made on two axis : The first by comparing all kind of output in both plant and generators (it's a problem to have a plant with yes whereas there is no generator with generator:output:electricity=* for instance). The second axis is on the amount of each kind of power made as above rules.

As discussed on mailing list, we mustn't try to bind stuff (like outside power station for instance) to an enclosed power plant if it is not in the power plant perimeter. We'd better to create two independent objects. More information here: https://lists.openstreetmap.org/pipermail/tagging/2013-April/013321.html
In some situations power plant could expose two or more distinct perimeter. This can be mapped with Relation:multipolygon and definitely fulfill the dispersed power plant model introduced below.

Dispersed facilities power plants

Wind, tidal and photovoltaic power plants may consist in a group of several turbines or solar panels in the same location used for production of electricity. Each turbine or panel would be tagged as power=generator. The land between generators may be used for agricultural or other purposes while it may correspond to the ocean's ground for tidal farms. A wind/solar farm may also be located offshore instead of a tidal farm which is always located on the sea ground.

Such a farm is tagged as a using type=site + power=plant. Computing of total farm rating is done by adding all the generator:output values of its generator members which are described with generator:plant=output (output generator, see Generator chapter for more information).

When there are several fenced areas around the power plants sites, Relation:multipolygon should be used.

All and each generators nodes must be tagged like this:

See common cases below, feel free to adapt:

Plant area/perimeter

As discussed on tagging mailing list, power plants areas and perimeters would be pretty hard to map when many different operators share the same location on a generation site.
http://gis.19327.n5.nabble.com/Power-proposals-td5753173.html

A simple rule to make the distinguishing between operators is to create a separate area for each one. Two or more power plants would directly be created.
In general, only one company is operating a power plant so if two or more operators are present on a site, we could have two different power plants or two different components for power generation. In all cases we mustn't map it in a single one area. Each area will have different operator=* values and potentially as many name=*. The key point is still the ownership of the land occupied by the power plant.

Furthermore and often, room is reserved besides actual power plants for future extensions. This room should be concerned by the power plant area right now since it's fenced as the main production site.

Finally, Relation:multipolygon doesn't match these operator consideration since operator distinguishing MUST be done.

Generator

A generator is a device that converts one form of energy to another.
For example: mechanical energy or sun light to electricity. 

A power=generator is tagged as a or , on the generator itself, if visible (wind turbine, gas turbine, single solar panel...), or on the smallest building containing it (nuclear reactor, hydroelectric turbine...). Large photovoltaic power plants may group solar panels blocks in a single multipolygon.

This proposal does not change its current attributes. See power=generator for its description.

Furthermore, as suggested by polderrunner (on osm, edits, contrib, heatmap, chngset com.), generators should be tagged with frequency=* since they can operate at different frequencies than the common one used on national grids. Indeed, some power plants can fed train transmission lines which often live at different frequency.

Please note that generator:output=* gives the nominal maximum available rating for the given generator. It's the easiest information we can get. Effective rating would be hard to compute or to obtain from power plans operators.

Specific elements

Solar and hydro power plants may contain specific objects, depending on their generator:method=*. Such special objects may vary on each site but here is a list of common cases we would encounter.

Single-site facitilies

'Single-site' power plants are ones where all, or most, of the equipment is found on a single site, often with a clear perimeter such as a fence unlike wind-farms and some other types which are distributed without a clearly demarcated perimeter. There's normally no need to setup relations for such examples, the perimeter allows us to retrieve all features inside.

Dispersed facilities

JOSM

• File request to add/update power related presets.

Potlatch 2

• File request to add/update power related presets. Further, resolve ticket #3779

Mapnik

• File request to render power=plant.