How to get Quantity to collect like terms?

Question

In Mathematica numerical terms collect when their variables are equal.

.2 "SERC" + .7 "SERC"
(* 0.9 "SERC" *)

However, numerical Quantity objects do not collect.

Quantity[.2, "Megawatts"] "SERC" + Quantity[.7, "Megawatts"] "SERC"
(* "SERC" (Quantity[0.2, "Megawatts"]) + "SERC" (Quantity[0.7, "Megawatts"]) *)

I would expect these to collect into Quantity[.9, "Megawatts"] "SERC" but they do not. How can I make Quantity objects collect? My understanding is that they should collect in this instance.

Answer 1

This is weird behavior on the face of it. You can force them to collect explicitly:

Collect[Quantity[.2, "Megawatts"] "SERC" + Quantity[.7, "Megawatts"] "SERC", "SERC"]

(* Out: "SERC" Quantity[0.9, "Megawatts"] *)

... or you can use Simplify:

Simplify[Quantity[.2, "Megawatts"] "SERC" + Quantity[.7, "Megawatts"] "SERC"]

(*Out: SERC Quantity[0.9, "Megawatts"] *)

The latter will also make the necessary unit conversions:

Simplify[Quantity[2, "Megawatts"] "SERC" + Quantity[700, "Kilowatts"] "SERC"]

(* Out: "SERC" Quantity[2700, "Kilowatts"] *)