Although not as thorough in the extent of listings in the power category (Yottawatt? What's that? Power ratings on planet Tattooine?) the link below leads to a conversion assortment that I've used for years and have found to be quite useful.
One drawback is that inversions are not automatic. Volume per fixed distance isn't automatically converted into distance per fixed volume, for example. An extra step and a hand held calculator is needed when converting miles per gallon to liters per 100 km, or just remember the 235.2 constant, as in: