I have implemented Power units on a site and wanted to share. The base is one watt. Got the data from:
http://physics.nist.gov/Pubs/SP811/appenB9.html#POWER

function mymodule_unitsapi_load_alter(&$units) {
  $units['watt'] = array(
    'singular' => 'watt',
    'plural' => 'watts',
    'symbol' => 'W',
    'factor' => '1',
    'kind' => 'power',
    'source' => 'http://physics.nist.gov/Pubs/SP811/contents.html'
  );
  $units['kilowatt'] = array(
    'singular' => 'kilowatt',
    'plural' => 'kilowatts',
    'symbol' => 'kW',
    'factor' => '1000',
    'kind' => 'power',
    'source' => 'http://physics.nist.gov/Pubs/SP811/contents.html'
  );
  $units['us mechanical horsepower'] = array(
    'singular' => 'US horsepower (mechanical)',
    'plural' => 'US horsepower (mechanical)',
    'symbol' => 'hp', 'factor' => '7.456999E+02',
    'kind' => 'power',
    'source' => 'http://physics.nist.gov/Pubs/SP811/contents.html'
  );
  $units['metric mechanical horsepower'] = array(
    'singular' => 'Metric horsepower (mechanical)',
    'plural' => 'Metric horsepower (mechanical)',
    'symbol' => 'hp',
    'factor' => '7.354988E+02',
    'kind' => 'power',
    'source' => 'http://physics.nist.gov/Pubs/SP811/contents.html'
  );
}