7.3. Building Energy Use Intensity

The Energy Use Intensity (EUI) metric is easy to calculate if you know your building’s annual energy use. The most accurate way is to look at your energy bills. Take the total annual amount of energy used and divide it by the total floor area of the house or building:

Before using this metric in analysis, we need to understand the difference between the gross EUI and net EUI metrics and what they indicate.

The gross EUI reflects the total building’s energy demand and includes all available sources: electricity, natural gas, renewables, and delivered fuels. No matter from what sources your energy comes, the building will require a certain amount of energy for annual operation, and this is what is accounted. Thus, the gross EUI will depend on the efficiency of the building envelope, design, and purpose. At the same time gross EUI will not be dependent on the type of energy you choose, it will only depend on the characteristics of the building itself.

The Net EUI reflects the difference between the gross energy demand and on-site generation. This is the metric that can characterize a building on the net zero scale. In this case we need to define the Renewable Production Intensity (RPI), which is essentially all energy supplied by on-site renewable sources, primarily solar, in kbtu/year divided by the total floor area of the building.

In case of a grid-bound solar system, the electricity bill will reflect the net kilowatt-hours taking into account consumption and on-site generation. So the easy way to calculate the Net EUI would be just using your utility bills for purchased energy:

Net EUI = E(elec) + E(gas) / Area

To calculate EUI in kbtu/sf/year (this is how it is presented in the LEED studies), you need to convert your energy units from all sources to kbtu and present the area in square feet.

The following conversion factors can be used:

• Electricity (both grid and onsite solar): 1 kWh = 3.412 kbtu
• Natural gas: 1 therm = 100 kbtu
• Fire wood for space heating: 20,000 kbtu/cord*

*Note: energy content of fire wood would depend on the type of wood and vary. The given value is an average that can be used as first approximation.

It should be noted, though, that within a certain month during the year, the net-zero condition may or may not be achieved. For example, in winter higher energy demand for heating may not be matched by seasonally decreased solar generation. At the same time, extra energy generated over the summer months would be fed to the grid and can be used to offset the winter deficit.

Weather-normalized EUI

What if we have two buildings of similar size located in different climate zones? One – in Minnesota and the other – in California. The first building has EUI of 28 and the second has EUI of 20. Would it be fair to say that the second building is more energy efficient?

As the matter of fact the first building may require more energy through the year not because of its inefficiency, but due to much higher heating load. After all it is placed in much more severe environment and has to withstand much more drastic temperature gradients, especially in the winter time.

To provide a fair comparison of the buildings in this case, we can use weather-normalized EUI. This is the metric that takes into account the weather, specifically heating and cooling needs, which can be expressed as heating degree days (HDD) and cooling degree days (CDD).

If you never heard of heating and cooling degree days, please check out this link. Those are common measures used to estimate the heating and cooling capacities needed for a building. Degree days indicate for how many days the outside temperature stays below or above the reference point of 65 F (this is the standard temperature by convention!). Degree days can be counted for any time period – a day, a month, or a year. Let me give you a short example.  

Let us come back to the case of two houses placed in different climate zones. We are going to compare data for those two locations in the table:

From this calculation, we see that the house in California in fact spends more energy per degree day than one in Minnesota to keep the temperature at the comfort level. So the ultimate conclusion is that the building envelope of the first house is more energy efficient.

The above-discussed metrics for house energy efficiency will be included in your lesson activity, so you will have a chance to apply those to your own residence and compare it to others.