Calculations

POTABLE WATER REDUCTION – USING OCCUPANTS AS BASELINE

Information about “typical” occupancy patterns reflected in Table 5-1 and when adjustments for atypical use should be made is provided under Creating the Baseline in the PoTable Water Reduction Clarifications.

Step 1 – Determine the following:

• Number and kind of occupancy types in the building.
• Number of each type of occupant per occupant type
  • For commercial / retail: Full time equivalent (FTE) occupants = total number of hours worked by all occupants / number of hours in a full-time week
  • For residential: Assume one occupant per bedroom
• Baseline water usage for each occupancy type per person per day (see Table 5-1).
  

Step 2 – Using the above information, calculate:

• Annual baseline water use by occupant type.
  • Typical usage:
    • Annual baseline water use by occupant type = Number of occupants per day per occupant type x Baseline water use per day by occupant type x typical usage (days)
  • Atypical usage (i.e., a residence used only during the summer):
    • Annual baseline water use by occupant type = Number of occupants per day per occupant type x Baseline water use per day by occupant type x days per year on site
• Total annual baseline water use for the project.
  • Total annual baseline water use for the project = Sum of the annual baseline water use of each occupant type
• Required poTable water reduction for the project.
  • Required poTable water reduction for the project = Annual baseline water use for the project x Percentage reduction by project Typology

EXAMPLE CALCULATION – USING OCCUPANTS AS BASELINE

Assume a new mixed use building with 10 offices with 8 full time staff in each and 110-units of multifamily residential including 60 single room occupancy units (SRO) and 50 two bedroom units. Assume the offices are open 40 hours per week.

Step 1 – Determine the following:

• Number and kind of occupancy types in the building.
  • 10 Offices
  • 110 Residential Units
• Number of each type of occupant per occupant type. See Calculations under I02, Urban Agriculture for additional information on calculating number of occupants in commercial and residential buildings.
  • For the offices:
• Full time equivalent (FTE) occupants = total number of hours worked by all occupants / number of hours in a full-time week = 10 offices x 8 employees per office / 40 hours per week = 80 FTE
  • For the residential units (assume one person per bedroom):
• 60 Single Room Occupancy (SRO) units = 60 occupants
• 50 two bedroom units = 50 × 2 = 100 occupants
• Total number of residential occupants = 100 + 60 = 160 occupants
• Baseline water usage for each occupancy type per person per day (see Table 5-1).
  • Office = 10.61 gallons per employee per day
  • Multifamily Residential = 52.56 gallons per person per day

Step 2 – Using the above information, calculate:

• Annual baseline water use by occupant type.
  • For the offices (assume they are typically occupied)
• Annual baseline water use by occupant type = 80 occupants x 10.61 gallons per person per day x 270 days per year = 229,176 gallons per year
  • For the residential units (assume they are typically occupied)
• Annual baseline water use by occupant type = 160 occupants x 52.56 gallons per person per day x 365 days per year = 3,069,504 gallons per year
• Total Annual baseline water use for the project.
  • Total annual baseline water use for the project = 309,812 gallons + 3,069,504 gallons = 3,298,680 gallons per year
• Required poTable water reduction for the project.
  • Required poTable water reduction for the project is 50% = 3,298,680 gallons per year x 0.50 = 1,649,340 gallons per year

This mixed-use project will need to reduce the poTable water used for all uses excluding irrigation by 1,649,340 gallons per year to meet the Imperative requirements.

POTABLE WATER REDUCTION – USING BUILDING AREA AS BASELINE

Step 1 – Determine the following:

• Area in square feet (sf) of each occupant type in the building.
• Baseline water usage for each occupancy type per sf per day (see Table 5-1).
  

Step 2 – Using the above information, calculate:

• Annual baseline water use by occupant type.
  • Annual baseline water use by occupant type = Area in sf of occupancy type x Baseline water use per day by occupant type x days per year in use (assume 365 unless not typically occupied)
• Total Annual baseline water use for the project.
  • Total annual baseline water use for the project = Sum of the annual baseline water use of each occupant type
• Required poTable water reduction for the project.
  • Required poTable water reduction for the project = Annual baseline water use for the project x Percentage reduction by project Typology

EXAMPLE CALCULATION – USING BUILDING AREA AS BASELINE

Assume a new 1,500 sf restaurant, with typical occupancy (6 days a week).

Step 1 – Determine the following:

• Area in square feet (sf) of each occupant type in the building.
  • Restaurant = 1,500 sf
• Baseline water usage for each occupancy type per sf per day (see Table 5-1).
  • Restaurant = 0.35 gallons per sf

Step 2 – Using the above information, calculate:

• Annual baseline water use by occupant type.
  • Annual baseline water use by occupant type = 1,500sf x 0.35 gallons per sf x 365 days per year (because the project has typical use) = 191,625 gallons per year
• Total Annual baseline water use for the project.
  • Same as above
• Required poTable water reduction for the project.
  • Required poTable water reduction for the project = 191,625 gallons per year x 50% = 191,625 gallons per year x 0.50 = 95,812.5 gallons per year

This restaurant will need to reduce the poTable water used for all uses excluding irrigation by 95,812.5 gallons per year to meet the Imperative requirements.

EXAMPLE CUSTOMIZED BASELINE BY STANDARD END USE

Assume a new 10-floor office building with a design occupancy of 1730 occupants. The work hours of the prospective occupants are not known, so a full-time typical 40 hour week is assumed for all. The building has no water-consuming equipment in its heating and cooling systems.

Step -1 Determine the following:

• Full time equivalent (FTE) occupants = 1730 employees (See Calculations under I02, Urban Agriculture for additional information on calculating number of occupants in commercial and residential buildings).
• Baseline water usage per person per day (see Table 5-1)
  • Office = 10.61 gallons per employee per day

Step 2 – Using the above information, calculate:

• Annual baseline water use = 1730 occupants x 10.61 gallons per person per day x 270 days per year = 4,955,931 gallons per year
• Reduction based on end use categories (see Table 5-3)
  • Because the project has no end uses in the heating and cooling category a reduction of 36% is applied = 4,955,931 gallons per year x .36 = 1,784,135 gallons per year.
• Adjusted baseline = 4,955,931 gallons per year – 1,784,135 gallons per year = 3,171,796 gallons per year.
• Required poTable water reduction for the project.
  • Required poTable water reduction for the project = 3,171,796 gallons per year x 50% = 3,171,796 gallons per year x 0.50 = 1,585,898 gallons per year

This office building will need to reduce the poTable water used for all uses excluding irrigation by 1,585,898 gallons per year to meet the Imperative requirements.

EXAMPLE CALCULATION – AFFORDABLE HOUSING PROJECT USING HANDPRINTING

Assume a new multifamily affordable housing project being developed by an affordable housing developer with multiple existing affordable housing buildings in their portfolio. This new building has 250 occupants.

As a first step, project teams would determine the required poTable water reduction for the project based on occupants or area as described above. The result, based on number of occupants for this example, is a required poTable water reduction of 2,398,050 gallons per year, 50% from the total water use of 4,796,100 gallons per year.

Through the installation of low-flow fixtures including sink faucets and toilets, the project team has been able to reduce the on-site poTable water use from 52.56 gallons per person per day to 40 gallons per person per day or a total of 3,650,000 gallons per year, a savings of 1,146,000 gallons per year. In order to meet the Imperative requirements, the project team needs to reduce poTable water use by an additional 1,251,950 gallons per year.

The project team has looked into doing an on-site rainwater or greywater reuse system, but the local jurisdiction is reluctant to permit the system and the cost is prohibitive within the project budget. Instead, the project team decides to employ the handprinting option.

The project’s developer has several other properties, one of which has 500 occupants and toilets that have not been replaced in many years and use 3.6 gallons per flush. The project team decides to replace these toilets with water sense rated toilets that use 1.28 gallons per flush as their handprinting strategy. The project team calculates the water savings resulting from the handprinting strategy using a conservative estimate that each occupant uses the toilet 3 times per day. The following calculations apply:

• Business As Usual Water Use in the Existing Building: 3.6 gallons per flush x 3 flushes per day x 500 occupants x 365 days per year = 1,971,000 gallons per year for toilet flushing
• After Handprinting: 1.28 gallons per flush x 3 flushes per day x 500 occupants x 365 days per year = 700,800 gallons per year for toilet flushing
• Handprinting Impact = 1,971,000 – 700,800 = 1,270,200 gallons saved per year

Since 1,270,200 gallons per year is greater than the remaining required water use reduction of 1,251,950 gallons per year for the LBC project, the project team successfully offset the required poTable water use. Note that if utility data or metering data is available from the existing building that data should be used to determine the actual water savings resulting from the handprinting strategies.

EXAMPLE CALCULATION – ONE PROJECT WITH MULTIPLE TYPOLOGIES

Example efficiency target calculation for a project combining New and Existing Building Typologies. Assume this building has the following features:

• Building Type: Education
• Total Future Occupancy: 1,000 students
• Total Gross Building Area: 9200 sm
• Area of Existing Building: 6000 sm
• Area of New Building: 3200 sm
  • The project is 65% Existing Building and 35% New Building

Step One – Calculate baseline

• 1,000 students * 32.16 liters per student per day = 32,160 liters per day (lpd)
  

Step Two – Calculate reduction requirement for each Typology

• Existing Building reduction requirement = 32,160 lpd – (32,160*0.3) = 22,512 lpd
• New Building reduction requirement = 32,160 lpd – (32,160*0.5) = 16,080 lpd
  

Step Three – Calculate reduction requirement based on Typology percentage

• 22,512 lpd * 65% = 14,632.8 lpd
• 16,080 lpd * 35% = 5,628 lpd
• 14,632.8 lpd + 5,628 lpd = 20,260 lpd allocated for the entire project