Power generation/Great Britain

All except the smallest power generators (say, less than 100kW) should be primarily tagged as power=plant, as this is the main tag for power generation. If you wish, you can then tag the individual generating elements as power=generator.

In most cases, this involves drawing an area around the entire site (generally the fenced boundary) and tagging it with power=plant, plant:source=*, and ideally plant:output:electricity=* and operator=*.

More info on renewable power tagging can be found at Renewable Energy in the United Kingdom.

Wind Farms

Onshore wind farms should be tagged as a dispersed power plant using a type=site relation.

Most offshore wind farms at the moment are tagged as areas, as this allows the appropriate Seamark tags to be applied as well.

Solar Farms

Draw an area around the perimeter (there's almost always a fence), and tag it as power=plant, plant:source=solar. If there are multiple perimeters which comprise a solar farm, use a multipolygon with the same tags.

If you have the patience, you can then draw areas around each individual group of solar arrays and tag them as power=generators, but this is much less important than the power=plant tag.