As an alternative, check out how Go does this. It's a nice compromise that lets you declare kilogram to be a float, and when used alone it treats it as a separate type, but it doesn't get in the way when you use it in an expression.
--------
Per [1] it seems that since both would be named types you wouldn't be able to assign one to the other without explicit conversions. If my reading is correct, then that addresses my concern. It's things like storing a `float` to a `inch` type that can happen without explicit conversion as long as `inch` has float as its base type.
[1] http://golang.org/ref/spec#Properties_of_types_and_values
type synonym/alias.
newtype Kilogram = Kilogram Double
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
newtype Kilogram = Kilogram Double deriving (Show, Eq, Ord, Num, Real, Fractional, Floating, ...)