Your utility bill is the best measure of energy efficiency.
It measures energy use intensity, which is the amount of fuel used per sq ft.
NRG can save over 50% per sq ft of floor space, compared to conventionally insulated buildings.
Example 1: Don Reed's is a 6800 sq ft NRG block grocery store in Genesee, PA (climate zone 6-cold).
The store heats and cools for 33 cents per sq ft (2009 prices) annually, or 28 cu ft of natural gas per sq ft.
Open up the DOE CBEC (Commercial Buildings Energy Consumption Survey) pdf below.
The CBEC Survey Manager recommends using climate zone and building size for energy use intensity comparisons.
You can see that the natural gas energy energy intensity for a 5k- 10k square foot building in the Middle Atlantic census region is 64.8cu ft per sq ft, more than double the 28 cu ft that Don Reed's store uses.
Example 2: Ridgewood Bible Church, a 38,000 square foot NRG block building near Buffalo, NY.
It is cold in Buffalo. The church has a natural gas heating energy intensity of about 23 cu ft per sq ft.
Compare that with the CBEC below, where you can see that a 25k-50k sq ft building in the Middle Atlantic region has a natural gas energy intensity of 47.4 cu ft per sq ft, double the amount of the Ridgewood Bible Church.