EME 807
Technologies for Sustainability Systems

7.3. Building Energy Use Intensity

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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:

EUI [kbtu/sf/year] = Annual Energy Use [kbtu/year] / Area [sq.ft]

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.

For example, if House A uses grid electricity, natural gas for heating, and has a solar module to provide some of the electric needs, all of those sources need to be included in the equation:

Gross EUI (House A) = (E(elec) + E(gas) + E(solar)) / Area

Even though the solar energy contribution is free (generated on site), it still works to balance the energy demand of the house.

If the neighboring House B has the same design and energy demand, but uses only grid electricity to meet its energy needs, its Gross EUI will be expressed as follows:

Gross EUI (House B) = E(elec) / Area

The value of the Gross EUI of the two houses will be the same or close.

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.

Following up on the example above:

RPI (House A) = E(solar) / Area

RPI (House B) = 0

Then we can express the Net EUI as follows:

Net EUI (House A) = Gross EUI – RPI = (E(elec) + E(gas)) / Area

Net EUI (House B) = Gross EUI – RPI = Gross EUI

In case of House B, since there is no on-site generation, the gross EUI is equal to net EUI. In case of House A, the net EUI will be lower since we exclude onsite renewable generation. In the marginal case, when all energy demand of the house is met by on-site renewable generation, Net EUI = 0, i.e. we have the net-zero energy balance.

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.

Self-check questions:

1. Mr. Morningstar uses 50,400 kbtu of energy a year at his residence of total area 1,800 square feet. What is the Gross EUI of his house?

  • (A) 50,400
  • (B) 32
  • (C) 28
  • (D) 16

Click here for explanation to this question

ANSWER: To find gross EUI, divide the total annual energy demand for the residence by square footage: Gross EUI = 50,400/1,800 = 28 kbtu/sf/yr

2. Mr. Morningstar installed a solar module on his roof, which now supplies 50% of his annual energy need. How did the Gross EUI of his house change?

  • (A) Decreased by 50%
  • (B) Increased by 50%
  • (C) Did not change
  • (D) Impossible to answer

Click here for explanation to this question

ANSWER: Gross EUI will not change, since regardless the source, his house will still require 50,400 kbtu of energy for heating/cooling, appliances, etc. Net EUI will change though.

3. Next, Mr. Morningstar installed additional insulation in his house and new air-tight windows, which decreased the house’s energy demand by half. How did the Gross EUI change?

  • (A) Increased by half
  • (B) Decreased by half
  • (C) Did not change
  • (D) Impoassible to answer

Click here for explanation to this problem

ANSWER: Gross EUI will decrease by half as well because regardless the sources of energy, the house will use less kbtu based on improved efficiency.

4. Based on the conditions described in questions (1)-(3), does Mr. Morningstar have a net-zero house?

  • (A) Yes (or close)
  • (B) No, half way to go from his original point
  • (C) Impossible to answer

Click here for explanation to this problem

ANSWER: Yes actually – His original energy demand 50,400 was reduced twice by insulation and window upgrades: 50,400/2 = 25,200 kbtu. We also remember that the solar system supplied a half of his original energy demand 50,400/2 = 25,200 kbtu. That means that all his energy demand is met by solar generation throughout a year. Hence, on the annual average, he should be close to net-zero.

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.

5. Mr. Morningstar decided to live in the tent all the way through the summer. What is the gross and net EUI of his dwelling? Explain.

Click here for explanation to this question

ANSWER: If he does not use any appliances in the tent, his EUI = 0 (both gross and net). We assume here that using campfire for cooking is outside the boundaries of his tent.  However, if he uses a flashlight or lantern during the dark times inside the tent (those tools are usually battery-powered and use electric grid for charging), that amount of energy needs to be counted, and his EUI will be above zero.

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).

Weather-normalized EUI = EUI / (HDD+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.  

Today’s average outside temperature (mean between the low and high) in my hometown State College PA is 40 F. It is below the standard temperature, so I can count heating degree days as follows:

HDD = (65F – 40F) x 1 day = 25 [deg F.day]

If on a summer day, the average outside temperature is 70 F, which is above the standard temperature, I can count the cooling degree days for that day as:

CDD = (70F – 65F) x 1 day = 5 [deg F.day]

These numbers indicate how much energy I may need to spend for heating or cooling on a specific day. Adding the HDD and CDD for the entire year would give me a measure of energy demand to expect for the heating and cooling season. Typical annual degree day counts for my Middle Atlantic region are HDD 5780 and CDD 877, according to U.S. Energy Information Administration (EIA).

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:

  Location   EUI (house)
[kbtu/sq.ft/yr]
  HDD     CDD     Total DD     Weather-normalized EUI  
[btu/sq.ft/yr/deg.day]
 Minnesota    28   6969   1134   8103    3.5
 California    20   3168   1006   4174    4.8

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.

Self-check question

Calculate weather-normalized EUI for a building located in Atlanta, GA, if its annual energy use is 75,000 kWh and its floor area is15,000 sq.ft.

Tip: first try and see if you can solve it in your notes before checking the answer

Click here for the solution

SOLUTION:

This is a multistep problem. First, you need to convert energy units from kWh to kbtu:

75,000 kWh/yr x 3.412 kbtu/kWh = 255,900 kbtu/yr

Then we divide by square footage of the building to find EUI:

EUI = 255,900 kbtu/yr / 15,000 sq.ft. = 17.06 kbtu/sq.ft/yr

Now we need to relate it to the total degree days in Georgia: based on EIA’s map:

HDD + CDD = 2630 + 2413 = 5043

Finally, weather-normalized EUI = 17.06 kbtu/sq.ft/yr x 1000 btu/kbtu / 5043 deg.day = 3.38 [btu/sq.ft/yr/deg.day]