Can I benchmark an open parking lot?

The short answer is: Not really, but with a couple modifications it can be done.

The longer answer is: Portfolio Manager is intended to track the energy use of a building, and an open parking lot doesn't have a building. A property could be a parking garage, but in this case the "Property GFA" (Gross Floor Area) is for the square foot of the Parking structure, it does not include any open parking lots.

So, we have to trick the system a little to track an open parking lot. 

Enter your property as a Parking Garage or Other. When you're adding the property, you will be asked for the Property GFA and Portfolio Manager requires a number greater than 0, so enter 1 sq ft (it actually doesn't matter what you enter here because Portfolio Manager ONLY uses it to double check you entered the right property uses). You'll get an error that your "Property GFA - Self-Reported" does not match your "Property GFA - (Parking Structure)", which you can ignore.

You will not get any energy use intensity (EUI) metrics for an open parking lot, but you can track the energy use and GHG emissions.