There is an area on the template where you enter project area versus entire building area (and the savings is pro-rated...or whatever the opposite of pro-rating is). With a 20-story office building, you probably only need to see an improvement of 1% building wide (on HVAC consumption) for it to equate to something greater than 15% for your tenant. I highly recommend looking at the reference guide for this. There are a couple different ways you can model the baseline building.

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