How To Calculate Energy Use Intensity

Energy Breakdown Table
Energy Source	Raw Usage	Unit	Converted (kBtu)
Electricity	–	kWh	–
Natural Gas	–	Therms	–
Other Fuel	–	–	–
Total	–	–	–

What is Energy Use Intensity (EUI)?

Energy Use Intensity (EUI) is the building performance equivalent of a car’s “miles per gallon” rating. It represents the total energy consumed by a building in one year relative to its size. Specifically, EUI is expressed as energy per square foot (or meter) per year.

Understanding how to calculate energy use intensity is critical for facility managers, real estate owners, and sustainability consultants. A lower EUI generally signifies a more energy-efficient building, while a high EUI indicates potential waste or the need for retrofitting.

It allows for apples-to-apples comparisons between buildings of different sizes and is the primary metric used in benchmarking ordinances (like NYC Local Law 84 or similar mandates in Boston and California) and certifications like LEED or ENERGY STAR.

Common Misconceptions

Myth: EUI depends on the building’s height. Fact: EUI is normalized by floor area, so a skyscraper and a small office can have the same EUI if they are equally efficient per square foot.
Myth: It includes energy generated on-site. Fact: “Net” EUI accounts for on-site renewable generation, but standard “Site” EUI looks at what is drawn from the grid or utilities.

Energy Use Intensity Formula and Mathematical Explanation

The math behind how to calculate energy use intensity is straightforward but requires careful unit conversion. The goal is to convert all different energy sources (electricity, gas, oil) into a single common unit, usually kBtu (thousand British Thermal Units).

The Core Formula:

EUI = (Total Annual Energy in kBtu) ÷ (Gross Floor Area in ft²)

To get the “Total Annual Energy,” you must apply conversion factors to your utility bill readings:

Variable / Fuel	Billing Unit	Conversion Factor (Approx)	Target Unit
Electricity	kWh	× 3.412	kBtu
Natural Gas	Therms	× 100	kBtu
Natural Gas (CCF)	CCF	× 103.7	kBtu
Fuel Oil #2	Gallons	× 138	kBtu
Steam	klbs (thousand lbs)	× 1,194	kBtu

Practical Examples (Real-World Use Cases)

Here are two scenarios demonstrating how to calculate energy use intensity in realistic commercial settings.

Example 1: Mid-Sized Office Building

Scenario: A 50,000 ft² office building in Chicago.

Electricity Usage: 600,000 kWh
Gas Usage: 12,000 Therms

Calculation:

Convert Elec: 600,000 × 3.412 = 2,047,200 kBtu
Convert Gas: 12,000 × 100 = 1,200,000 kBtu
Total Energy: 3,247,200 kBtu
Divide by Area: 3,247,200 ÷ 50,000 = 64.9 kBtu/ft²/yr

Interpretation: This is a reasonably efficient office building, likely close to the national median.

Example 2: Older Elementary School

Scenario: A 25,000 ft² school building.

Electricity Usage: 200,000 kWh
Fuel Oil Usage: 5,000 Gallons

Calculation:

Convert Elec: 200,000 × 3.412 = 682,400 kBtu
Convert Oil: 5,000 × 138 = 690,000 kBtu
Total Energy: 1,372,400 kBtu
Divide by Area: 1,372,400 ÷ 25,000 = 54.9 kBtu/ft²/yr

Interpretation: Schools often have lower EUIs than offices due to shorter operating hours.

How to Use This Energy Use Intensity Calculator

This tool simplifies the process of converting units and normalizing data. Follow these steps:

Input Gross Floor Area: Enter the total interior square footage of the building. Do not include parking garages or exterior spaces.
Enter Utility Data: Refer to your annual summary utility bills. Input the total kWh for electricity and total Therms for gas used over the last 12 months.
Add Secondary Fuels: If your building uses fuel oil, propane, or steam, select the type and enter the volume.
Review Results: The calculator immediately updates your Site EUI. Compare this number against the chart benchmarks.
Use “Copy Results”: Click the button to save the data for reports or emails.

Key Factors That Affect Energy Use Intensity Results

When analyzing how to calculate energy use intensity, remember that the resulting number is influenced by several critical variables:

Building Use Type: A hospital or data center will inherently have a much higher EUI than a warehouse or an empty storage facility due to equipment intensity and 24/7 operations.
Operating Hours: The longer the lights and HVAC run, the higher the numerator in the equation. A school open 8 hours a day differs vastly from a hotel open 24 hours.
Climate Zone: Buildings in extreme climates (very hot or very cold) require more energy for heating and cooling, inflating the EUI compared to buildings in temperate zones like San Diego.
Occupancy Density: More people generate more body heat and require more ventilation and plug loads (computers, screens), increasing energy demand.
Building Envelope Efficiency: Poor insulation, single-pane windows, and air leaks will drive up heating and cooling loads, resulting in a higher EUI.
HVAC System Age: Old boilers and chillers operate at lower efficiencies. Replacing a 30-year-old boiler with a condensing boiler can significantly drop the EUI.

Frequently Asked Questions (FAQ)

What is a “good” EUI score?

A “good” EUI depends entirely on the building type. For an office, an EUI below 50 is excellent. For a hospital, an EUI below 200 might be considered efficient. Consult the CBECS database or ENERGY STAR Target Finder for specific targets.

What is the difference between Site EUI and Source EUI?

Site EUI (calculated here) measures what the building meter reads. Source EUI accounts for the efficiency of the power plant and transmission lines. Source EUI is often used for national policy, while Site EUI is used for local utility bill management.

Does EUI include parking garages?

Generally, no. When calculating EUI for benchmarking compliance, parking garage energy and floor area are often excluded or calculated separately because garages have very low energy intensity compared to occupied spaces.

Can I calculate EUI for a single month?

Technically yes, but it is not recommended. EUI is an annual metric. Calculating it for a single month (e.g., July) would skew the result because seasonal heating/cooling loads are not balanced out.

How does vacancy affect EUI?

If a building is half-empty, its total energy usage drops, often lowering the EUI. However, this doesn’t mean the building is efficient; it just means it’s underutilized. Adjusted EUI metrics exist to correct for this.

Why is my EUI higher than last year?

Check for changes in weather (a colder winter), increased occupancy, extended operating hours, or equipment malfunctions (like a damper stuck open).

Is a lower EUI always better?

Usually, yes. However, an extremely low EUI could indicate broken ventilation systems or insufficient lighting, which negatively impacts occupant health and comfort.

What units are used for EUI outside the US?

While the US uses kBtu/ft², most of the world uses kWh/m² or GJ/m². To convert kBtu/ft² to kWh/m², multiply by approximately 3.15.

Related Tools and Internal Resources

Enhance your facility management strategy with our other tools related to energy use intensity:

Energy Benchmarking Guide – A comprehensive guide to ENERGY STAR Portfolio Manager.
HVAC Efficiency Calculator – Calculate SEER and EER ratings for your equipment.
Carbon Footprint Tool – Convert your energy usage into CO2 equivalent emissions.
Utility Bill Analysis – How to read and audit complex commercial energy bills.
Retrofitting ROI Calculator – Estimate the payback period for LED lighting and insulation upgrades.
Building Automation Systems – Learn how BAS can automatically lower your EUI.