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Energy Budget Calculator Detailed Instructions

Your energy use in your home is based on thousands of different factors. Energy models take these factors into account. For example, energy models make precise calculations about the transfer of heat through walls and windows. For other factors, energy models have to make assumptions, for example, how many showers do you take each week and for how long. The following EUI are based on energy modeling completed by the University of Arizona, College of Architecture and Landscape Architecture. The energy efficiency strategies used to develop the EUI are documented in table 1.4.3 of the Net-Zero Standard. This table is based on these strategies and was designed so that a building built and occupied based on these strategies will be net zero. However, this table is not a substitute for energy modeling and energy modeling is recommended to ensure net-zero.

Step 1:

The first screen will calculate your energy budget based your entries for the:

A. Roof Type

Each roof type shall be classified as one of the following:
  1. Flat Perimeter Parapets only. A roof that is ≤ 2:12 pitch with a parapet only at the exterior walls
  2. Flat Perimeter and Interior Parapets. A roof that is ≤ 2:12 pitch with a parapet at the exterior walls and vertical changes of level < six inches and/or additional parapets inside the exterior wall parapets
  3. Low Pitch ≤ 2:12. A roof that is ≤ 2:12 pitch without parapets
  4. High Pitch > 2:12
Based on your roof type, the percentage of roof available is an estimate based on experience with solar panel layouts to avoid shading from parapets and other equipment on the roof. The percentage of roof available is determined by the table below.

Roof Type /
Orientation Factor
Flat-Perimeter
Parapet Only
Flat-Perimeter and
Interior Parapets
low pitch < 2:12 high pitch ≥ 2:12
E-W 50% 30% 90% 45%
N-S 45% 30% 90% 44%
Off Axis 40% 25% 90% Performance path

B. Building Orientation

building orientationThe alignment of the longest building axis with respect to geographical compass points shall be classified as one of the following:
  1. E-W is any long building axis that is within plus or minus 15 degrees of true east-west.
  2. N-S is any long building axis that is within plus or minus 15 degrees of true north-south.
  3. Off Axis is any orientation that is not E-W or N-S.
The orientation of the building is important to determine two key energy relation factors:
  1. How well will a solar installation fit on the roof
  2. How well will the building take advantage of passive energy savings due to proper orientation.

Determine Orientation

Determine Orientation

(SECTION 1.3.1.1, 2.3.1.1)
STEP 1: With the building oriented with true north to the top of the paper, layout true north-south and east-west geographical lines over the building footprint.

STEP 2: Draw the long building axis with approximately ½ of the area of the building footprint on each side of the axis line.

STEP 3: Determine the angle of the long axis from the closest of either the north-south or the east-west lines. In the example above the long axis is closest to the east-west line and the angle is 22 degrees.

STEP 4: Determine the orientation of the building
E-W is any long building axis that is within plus or minus 15 degrees of true east-west. N-S is any long building axis that is within plus or minus 15 degrees of true north-south. Off axis is any orientation that is not E-W or N-S. In the example above the angle from the east-west line is greater than 15 degrees there for the orientation is Off axis.

C. Building Shape

Based on the basic shape of the building, how much of the available roof area can be used. Irregular shapes reduce the potential to layout PV in a useful way.

The building plan floor plan shape shall be categorized as one of the following:
  1. Rectangle: The floor plan long building axis is > 1.3 times the length of the short building axis and no more than eight exterior corners.
  2. Square: The floor plan long building axis is ≤ 1.3 times the length of the short building axis of the home and no more than eight exterior corners.
  3. Irregular: Floor plans that are not a Rectangle or a Square.

DETERMINE


Building Shape
(SECTION 1.3.1.3, 2.3.1.3)
STEP 1: After determining orientation per sketch 1, draw the short building axis with approximately ½ of the area of the building footprint on each side of the axis line.

STEP 2: Count the exterior corners of the building. If the building has more than eight exterior corners the building shape is irregular.

STEP 3: If the building has < 8 exterior corners, measure the length of the long axis and the short axis. In the case of the example above, the long axis is 40 feet and the short axis is 23 feet.

STEP 4: Divide the long axis length by the short axis length. If the floor plan long building axis is > 1.3 times the length of the short building axis (and there are < 8 exterior corners) the building is a rectangle.

D. Number of Stories

The number of stories of your building.
The calculator can calculate one or two story residences and up to three stories for apartments, offices and retail.

E. Conditioned Area

The conditioned area of your Building in Square Feet.
Conditioned area does not include Garages (unless it is conditioned) porches or balconies. The full definition is in the 2012 IECC.

Step 2:

With the results of step one and your energy budget, you can select a construction type that meets your budget. There are three construction types in the table with an EUI value for each orientation. The calculator will automatically select your orientation when you select the construction type. If the EUI for the construction type is in red, you must either:

Select the EUI with bonus. This is an additional energy efficiency building strategy, or
  • For homes: adjust your roof type, orientation, shape or number of stories, or
  • For apartments, offices or retail: add covered parking.

Step 3:

Based on steps one and two the calculator will show you the energy efficiency strategies from table 1.4.3 or 2.4.3 of the Net-Zero Energy Standard that you will need to incorporate in your building.