It only multiplies three numbers...

calculator.fs

open FSharpx.Option

/// hour, a unit of time
[<Measure>] type h

/// megawatt, a unit of power
[<Measure>] type MW

/// pounds sterling, a unit of currency
[<Measure>] type ``£``

let energy() = 
  // ... an expensive computation ...
  None

let price() = 
  // ... an expensive computation ...
  Some 10.0<``£``/MW/h>

let factor() = 
  // ... an expensive computation ...
  Some 2.0

let revenue (energy: float<MW h>) 
            (price: float<``£``>) 
            (factor: float) =
    energy * price * factor

maybe {
  let! energy = energy() // computation exits here
  let! price = price()
  let! factor = factor()
  return revenue energy price factor
}