Remove this line if you go for the comment i made below.

I think you need to do more, or rather less.
In C++11 making a singleton is really easy! All you need is:

Units &Units::self()
{
  static Units units;
  return units;
}

And done. It gives you a thread safe singleton (which your current version isn't).
I also think the current compiler requirements for plasma and frameworks allow you to use the code I've just given.. So.. Use it :)
Note: i changed the return value to a reference.

Also, why use "self" as getter for a singleton? I think "instance" or "getInstance" is the most used name for this, but i might be wrong here.

Add documentation please.

If this were pre C++11, you'd be done :)
But we have move semantics now. You need to disable moving as well!

Units(Units const&) = delete;                       // Copy construct
Units(Units&&) = delete;                              // Move construct
Units& operator=(Units const&) = delete;  // Copy assign
Units& operator=(Units &&) = delete;         // Move assign

I'd drop the Q_DISABLE_COPY and go for the above "= delete" lines, but that's up for you to decide. You have to add them for move semantics anyway (Q_DISABLE_COPY does not do that for you).

Remove this line if you go for the comment i made below.