What Is Energy Use Intensity (EUI)? Formula & Example

What Is Energy Use Intensity (EUI)? Formula & Example

Photo by Doris Morgan on Unsplash.

Energy use intensity (EUI) is a building's total annual energy consumption divided by its gross floor area. In the US it's expressed in kBtu per square foot per year; almost everywhere else it's kWh per square metre per year. It's the single most common way to compare how much energy a building uses relative to its size, and it sits underneath nearly every benchmarking score, green lease clause and audit baseline in commercial real estate.

The calculation itself isn't complicated. A property manager with twelve months of utility bills and a floor plan can produce it in a spreadsheet. What trips people up is what the number does and doesn't tell them, and how it fits with the compliance score their city or state may already require them to file.

What EUI measures, and why it matters

EUI collapses a year of utility bills into one figure that's comparable across buildings of wildly different sizes. A 20,000-square-foot branch office and a 200,000-square-foot headquarters tower can't be compared on raw kWh consumed, the tower will always use more energy in absolute terms, but their EUIs sit on the same scale.

That comparability is why EUI shows up everywhere: green lease clauses that tie rent incentives to performance, appraisal and underwriting models that price energy risk into a building's value, capital planning that ranks which properties in a portfolio need attention first, and the growing list of municipal disclosure ordinances that require an annual filing. Most energy management systems, the metering and software layer a building runs its energy program through, calculate and track EUI automatically as one of their core outputs, precisely because it's the metric everyone downstream expects to see.

The EUI formula

EUI = total annual energy use ÷ gross floor area

Two details matter more than the arithmetic. First, "total annual energy use" means every fuel the building buys, electricity, natural gas, steam, district heating, converted into a single common unit before you divide. Using electricity alone and ignoring a gas bill produces a number that looks like an EUI but understates the real figure, sometimes by a lot. Second, gross floor area means the whole building envelope, not just conditioned or leasable space, which is a common source of disagreement between an owner's figure and a tenant's.

The two common unit systems:

• US: kBtu (thousand British thermal units) per square foot per year

• Everywhere else: kWh per square metre per year

Converting between them: 1 kWh = 3.412 kBtu, and 1 square metre = 10.764 square feet, so 1 kWh/m² works out to roughly 0.317 kBtu/ft².

A worked example: calculating EUI step by step

Take a 45,000-square-foot (4,181 m²) all-electric office building. Its meter shows 950,000 kWh consumed over the past twelve months.

Step 1: convert energy use to a common unit (kBtu).

950,000 kWh × 3.412 kBtu/kWh = 3,241,400 kBtu

Step 2: divide by gross floor area.

3,241,400 kBtu ÷ 45,000 sq ft = 72.0 kBtu/ft²/year

That's the building's site EUI: 72.0 kBtu per square foot per year.

The same building, worked in metric units: 950,000 kWh ÷ 4,181 m² = 227.2 kWh/m²/year. Same building, same energy use, two equally valid expressions of the same number.

A building with more than one fuel source runs the identical process for each: convert every fuel's annual consumption to kBtu (electricity ×3.412, natural gas ×100 per therm, and so on for steam or district heat), sum them, then divide once by total floor area.

EUI and the ENERGY STAR score: site vs. source, and weather normalization

In the US, ENERGY STAR Portfolio Manager is the tool behind almost every benchmarking mandate, and EUI is the raw material its 1-100 score is built from. Two distinctions matter once EUI leaves the spreadsheet and enters that scoring system.

Site EUI is what we calculated above, the energy delivered to the building, measured at the meter. Source EUI goes a step further and accounts for the energy lost generating and transmitting that power before it ever reaches the building. A kWh of grid electricity carries a larger source-energy footprint than a therm of natural gas burned on-site, because of transmission and generation losses upstream. ENERGY STAR uses source EUI, not site, to calculate its score specifically so a building isn't rewarded or penalized just for which fuel it happens to use.

The second distinction is weather normalization. A building's raw EUI in a brutally cold winter will run higher than in a mild one, for reasons that have nothing to do with how well the building is run. Portfolio Manager adjusts for this using degree-day data, producing a weather-normalized EUI that estimates what the building would have used under long-run average conditions. That's the figure most disclosure ordinances actually reference, not the raw annual number.

We cover how the full 1-100 scoring process works, and where it runs out of road as a diagnostic tool, in our guide to energy benchmarking for commercial buildings.

Typical EUI benchmarks by building type

EUI varies enormously by property type, which is why comparing a warehouse's EUI to a hospital's tells you nothing useful. ENERGY STAR's national median site EUI figures, drawn from CBECS survey data, give a workable reference point for a few common commercial categories:

Multifamily housing isn't part of CBECS, which covers commercial buildings only; ENERGY STAR draws that figure from the Fannie Mae multifamily industry survey instead.

Property types with denser energy loads run well above these figures. A full-service restaurant or an acute-care hospital, both running heavy equipment loads for most of the day, routinely posts an EUI several times an office's, not because either is poorly run but because the underlying use is more energy-dense. That's the core rule for using EUI correctly: it's only a meaningful comparison within the same property type.

What counts as a "good" EUI, and where the metric runs out of road

There's no universal good EUI, only a good EUI for your building's type, climate zone and operating hours. As a working rule: sitting below the national median for your property type (52.9 for an office, from the table above) is a reasonable definition of "better than most peers." The top-quartile threshold that qualifies for ENERGY STAR certification sits meaningfully lower still.

EUI's real limitation is that it's a single annual number covering the whole building. It doesn't distinguish between a building that's fully occupied and running efficiently and one that's half-empty and running wastefully at the same energy level, occupancy and usage intensity simply aren't in the formula. Two buildings with identical EUIs can be in completely different operational health.

That gap is where a static, once-a-year figure starts to show its limits. A utility bill tells an owner their EUI in arrears, months after the year that produced it has already closed. Continuous sensor and meter data can't change what EUI is, it's still energy over floor area, but it can decompose that single annual number into how the building is actually behaving week to week: which system is driving consumption, whether a shift is seasonal or a genuine fault, and whether this month's trend is heading somewhere the annual filing won't catch until next year. That's closer to the territory energy management for commercial buildings is built to work in, tracking the same fundamentals a Portfolio Manager filing reports, just continuously rather than once a year.

FAQ

What is EUI?
Energy use intensity (EUI) is a building's annual energy consumption divided by its gross floor area, typically expressed as kBtu per square foot per year in the US or kWh per square metre per year elsewhere. It's the standard way to compare energy use across buildings of different sizes.

How do you calculate EUI?
Add up a building's total energy use for one year across every fuel it consumes, converted into a common unit such as kBtu, then divide by the building's gross floor area in square feet. The result is EUI in kBtu per square foot per year.

What is a good EUI for an office building?
In the US, the ENERGY STAR national median site EUI for office buildings is 52.9 kBtu per square foot per year. A building below that figure is performing better than half its peers; the top-quartile threshold that qualifies for ENERGY STAR certification is lower still. "Good" always depends on climate, operating hours and property type, so compare EUI only within the same category.

How does EUI differ from an ENERGY STAR score?
EUI is a raw measurement: energy per square foot. The ENERGY STAR score is a 1-100 ranking that takes a building's source EUI and compares it to a peer group of similar buildings nationally, adjusted for climate and operating characteristics. EUI is an input to the score, not the score itself.

Does EUI account for occupancy?
Not directly. EUI is a whole-building, whole-year figure and doesn't distinguish between a fully occupied building running efficiently and a half-empty one running wastefully at the same energy level. Some benchmarking tools weight EUI by occupancy or operating hours, but the raw metric on its own doesn't.

What's the difference between site EUI and source EUI?
Site EUI measures the energy delivered to a building, essentially what shows up on its utility bills. Source EUI adds in the energy lost generating and transmitting that power before it reaches the building, giving a fuel-neutral comparison. ENERGY STAR uses source EUI to calculate its 1-100 score.

What units is EUI measured in?
In the US, EUI is almost always kBtu per square foot per year (kBtu/ft²/yr). Outside the US, it's more commonly expressed in kWh per square metre per year (kWh/m²/yr). Both measure the same thing; converting between them uses the factor 1 kWh/m² ≈ 0.317 kBtu/ft².