e.g. Parallel resistors, serial capacitors, parallel inductors.

EDIT: Corrected typo. 

newtype RecipSum a = RecipSum {getRecipSum :: a}

instance Fractional a => Semigroup (RecipSum a) where

    RecipSum x <> RecipSum y = RecipSum (recip (recip x + recip y))

    sconcat (x :| xs) = mconcat (x : xs)

    stimes n (RecipSum x) = RecipSum (x / fromIntegral n) 

instance Fractional a => Monoid (RecipSum a) where

    mempty = RecipSum (1 / 0)

    mconcat xs = RecipSum (recip . getSum $ foldMap (Sum . recip . getRecipSum) xs)