How this instrument works
Energy Use Intensity (EUI) is a building's total annual energy consumption divided by its gross floor area, expressed in thousand British thermal units per square foot per year (kBtu/ft²/yr). It's the headline metric behind ENERGY STAR Portfolio Manager and many city and state building energy-benchmarking laws, precisely because raw annual energy use alone tells you almost nothing about efficiency — a skyscraper naturally uses more total energy than a corner store, but dividing by floor area puts both on a comparable, size-independent scale.
'Annual energy use' means every purchased fuel a building consumes over a year — electricity, natural gas, steam, fuel oil — added together after converting each to the common kBtu unit. Electricity is typically converted at roughly 3.412 kBtu per kWh, and natural gas at roughly 100 kBtu per therm; a building running on more than one fuel needs each converted and summed before it goes into the EUI formula, so the result reflects total site energy rather than any single utility bill.
This is site EUI — energy as delivered to the building's meter — not source EUI, a related ENERGY STAR metric that also accounts for the generation and transmission losses incurred upstream in producing that delivered energy (which raises electricity's effective weight substantially versus on-site fuel combustion). Site EUI is the simpler, more direct figure and the one this instrument computes; whether a given EUI counts as 'good' also depends heavily on property type, since a data center and a warehouse have very different baseline energy intensities for entirely legitimate reasons.
- Enter Annual energy use (kBtu) — total energy consumed over one year, with every fuel already converted to and summed in kBtu.
- Enter Gross floor area (ft²) — the building's total floor area.
- Read Energy Use Intensity (kBtu/ft²/yr) — annual energy use per square foot.
- To combine electricity and gas, convert first: kWh × 3.412 = kBtu, and therms × 100 = kBtu, then add the two before entering.
- Compare your result only against similar property types — a warehouse and a hospital have very different typical EUI ranges, so cross-type comparisons aren't meaningful.
Worked example — a 2,000 ft² building using 50,000 kBtu a year
Enter 50000 into Annual energy use (kBtu) and 2000 into Gross floor area (ft²). Energy Use Intensity reads 50000 ÷ 2000 = 25.0 kBtu/ft²/yr.
That figure now lets this building be compared directly with one ten times its size: a 20,000 ft² building using 500,000 kBtu a year would have the identical 25.0 EUI, and — despite using twenty times as much total energy — would be considered exactly as energy-efficient per square foot as the smaller one in this example.
Questions
Does 'annual energy use' mean just my electric bill?
No — it should include every fuel the building purchases, not electricity alone. If the building also uses natural gas, fuel oil, or district steam, convert each to kBtu (roughly 3.412 kBtu per kWh for electricity, 100 kBtu per therm for natural gas) and add them together first. Leaving out a fuel understates true energy use and produces an EUI that isn't comparable to a properly calculated one.
What counts as a 'good' EUI for my building?
There's no single good number — typical EUI varies enormously by property type, because a hospital, data center, or restaurant inherently uses far more energy per square foot than an office or warehouse for reasons that have nothing to do with efficiency. ENERGY STAR Portfolio Manager publishes national median EUI tables broken out by property type specifically so buildings are compared against genuinely similar peers rather than a single universal benchmark.
What's the difference between site EUI and source EUI?
Site EUI, which this instrument calculates, is energy as delivered to the building — what shows up on the utility meter. Source EUI additionally accounts for the energy lost generating and transmitting that delivered energy upstream, which typically makes electricity's source-EUI contribution notably larger than its site-EUI contribution, since power plants and grid transmission lose a meaningful share of energy before it ever reaches a building.
Is gross floor area the same as a building's rentable square footage?
Not necessarily. Gross floor area for EUI purposes typically means the building's total enclosed floor space, including areas like mechanical rooms, stairwells, and common corridors that a commercial lease's rentable-square-footage figure may handle differently. Using a leasing measurement instead of true gross floor area can shift the EUI result, so it's worth confirming which definition your data source is using.
How do I convert kWh and therms into kBtu?
Multiply kilowatt-hours by roughly 3.412 to get kBtu, and multiply therms by 100 to get kBtu (since one therm is defined as 100,000 Btu). Add the converted figures for every fuel the building uses together to get the single annual-energy-use figure this calculator's formula needs.