Facility Water Cycle and Emissions Crosswalk

Water consumption by end-use is measured in thousand gallons (kgal).

FEMP's Water Balance Tool is an online software tool that can be used to estimate water use by major water end-uses. Water use can also be determined through metering or comprehensive water assessments. 

Use Table 2 and Table 3 to look up your energy-water equivalence metric. This metric describes the energy intensity of water consumption by end-use or process, measured in kilowatt-hours (kWh) per thousand gallons.

Table 2 includes the energy-water equivalence related to heating water for specific end-uses.

Table 3 includes the energy-water equivalence related to water treatment and distribution, and the energy-water equivalence related to wastewater treatment.

The emission factors needed to convert energy to emissions are measured in pounds of carbon dioxide equivalent (lb CO₂e) per kilowatt-hour.

If the energy source is electricity—Use eGRID subregional emission factors based on your location. 

If the energy source is not electricity—Natural gas, fuel oil, or propane energy from heating water can be converted to emissions using EPA emission factors. 

Now that you have water end-use consumption, kilowatt-hours per thousand gallons value by end-use, and your emissions factor, you can use the following formula to calculate water-related emissions, measured in pounds of carbon dioxide equivalent.

initial emissions (lbs CO2e per year)

$$\text{<em>= kgal initial water <strong>consumption </strong>per end use per year </em>}x\frac{\mathrm{kWh}}{\mathrm{kgal}}x\frac{\mathrm{lbsCO2e}}{\mathrm{kWh}}$$

Subtracting the final from the initial emissions yields the emissions avoided.

final emissions (lbs CO2e per year)=

$$\text{<em>= kgal initial water <strong>consumption</strong> per end use per year </em>}x\frac{\mathrm{kWh}}{\mathrm{kgal}}x\frac{\mathrm{lbsCO2e}}{\mathrm{kWh}}$$

$$\text{<em>Initial emissions - Final emission = Emissions avoided</em>}$$

Water savings can be converted to emissions avoided using the same equation as in Step 4 by focusing on the amount of water saved instead of consumed.

initial emissions (lbs CO2e per year)

$$\text{<em>= kgal water <strong>savings</strong> per end use per year </em>}x\frac{\mathrm{kWh}}{\mathrm{kgal}}x\frac{\mathrm{lbsCO2e}}{\mathrm{kWh}}$$

$$\text{<em>= lbsCO2e avoided for kgal of water saved per year</em>}$$

With emissions categorized by each water end-use scenario, the next step is to find ways to reduce these emissions by reducing the amount of water consumed.

Example Calculation in Practice

A building manager is looking to install high-efficiency bathroom and kitchen faucets. The building is in California and the energy source is fully electric. 

1. The building water meter that shows their total water consumption is 2,000 kgal/year. They estimate that the faucets make up 5% of the total water consumption. So, the faucets’ water consumption is roughly 100 kgal/year.
2. Using Table 2, they determine that faucets typically use 148.0 kWh/kgal.
3. Using eGRID for the region around California, the emissions associated with energy use in the building are found to be 531.68 lb CO₂e/MWh, or 0.532 lb CO₂e/kWh.
4. Calculating the initial emissions for the current faucet fixtures results in 7,873.6 lb CO₂e/year.

$$\text{<em>kgal water consumption per end use per year </em> }x\frac{\mathrm{kWh}}{\mathrm{kgal}}x\frac{\mathrm{lbsCO2e}}{\mathrm{kWh}}\text{<em>= lbsCO2e per year</em>}$$

$$\text{<em>100 kgal/year </em> }x148\frac{\mathrm{kWh}}{\mathrm{kgal}}x0.532\frac{\mathrm{lbsCO2e}}{\mathrm{kWh}}\text{<em>= 7,873.6 lbsCO2e per year</em>}$$

5. The new, high-efficiency faucets decrease water consumption by 30%, meaning that the new faucet water consumption would be 70 kgal/year. With this new faucet water savings, the final emissions would decrease by 30% of the initial emissions, resulting in 2,362.1 lb CO₂e avoided.

$$\text{<em>kgal water final <strong>consumption</strong> per end use per year </em> }x\frac{\mathrm{kWh}}{\mathrm{kgal}}x\frac{\mathrm{lbsCO2e}}{\mathrm{kWh}}\text{<em>= lbsCO2e per year</em>}$$

$$\text{<em>= lbsCO2e final emissions</em>}$$

$$\text{<em>70 kgal </em> }x148\frac{\mathrm{kWh}}{\mathrm{kgal}}x0.532\frac{\mathrm{lbsCO2e}}{\mathrm{kWh}}\text{<em>= 5,511.5 lbsCO2e per year</em>}$$

$$\text{<em>Initial-Final: 7,873.6 lbsCO2e - 5,511.5 lbsCO2e = <strong>2,362.1 lbsCO2e saved per year</strong></em>}$$