Overview
This page covers the methodology used in the calculation of PaloAltoGreen’s environmental benefits and equivalencies. Using recent, publicly available government and non-profit supported data, our goal is to provide customers with an explanation of the environmental impact of the renewable energy available through PaloAltoGreen.
Carbon Dioxide Emissions Avoided
The CO2 reduction value of PaloAltoGreen renewable energy is calculated by multiplying the number of kilowatt-hours (kWh) purchased by 1.218, the average number of pounds of CO2 per kWh prevented from entering the atmosphere in the WECC, the NERC region where PaloAltoGreen gets its energy (EPA 2008).
Example: the average Palo Alto household uses 650 kWh per month. 650 x 1.218 = 791.7 lbs. of CO2 avoided monthly, or 9500 lbs. annually.
Cars or Passengers Vehicles Removed from the Road
Passenger vehicles are defined as 2-axle 4-tire vehicles, including passenger cars, vans, pickup trucks, and sport/utility vehicles. In 2005, the weighted average combined fuel economy of cars and light trucks combined was 19.7 miles per gallon (FHWA 2006). The average vehicle miles traveled in 2005 was 11,856 miles per year. In 2005, the ratio of CO2 emissions to total emissions (including carbon dioxide, methane, and nitrous oxide, all expressed as CO2 equivalents) for passenger vehicles was 0.971 (EPA 2007). The amount of CO2 emitted per gallon of motor gasoline burned is 8.81*10-3 metric tons.
To determine annual greenhouse gas emissions per passenger vehicle, the following methodology was used: average vehicle miles traveled was divided by average gas mileage to determine gallons of gasoline consumed per vehicle per year. The number of gallons of gasoline consumed figure was multiplied by CO2 per gallon of gasoline to determine CO2 emitted per vehicle per year. CO2 emissions were then divided by the ratio of CO2 emissions to total vehicle greenhouse gas emissions to account for vehicle methane and nitrous oxide emissions. When simplified, the result is 5.46 metric tons CO2 equivalent per vehicle per year. (1 metric ton = 2,204.6 lbs.) Note: due to rounding, performing the calculations given above may not return the exact results shown.
Example: the average Palo Alto household in PaloAltoGreen avoids 9,500 pounds of CO2 annually, or 4.3 metric tons. 4.3/5.46 = 0.79 cars removed from the road for one year, or 1 car removed for 9 months.
Acres of Trees/Pine or Fir Forest Storing Carbon for One Year
Growing forests store carbon. Through the process of photosynthesis, trees remove CO2 from the atmosphere and store it as cellulose, lignin, and other compounds. The rate of accumulation is equal to growth minus removals (i.e., harvest for the production of paper and wood) minus decomposition. In most US forests, growth exceeds removals and decomposition, so there has been an overall increase in the amount of carbon stored nationally.
The estimate of the annual average rate of carbon accumulation is based on two studies, one on Douglas fir in the Pacific Northwest (Nabuurs and Mohren, 1995), and the other on slash pine in Florida (Shan et al. (2001)). These two studies represent commercially important species from different regions and with different rotation periods (i.e., time between planting and harvesting). The calculation addresses only above-ground carbon; although carbon accumulates in roots, leaf litter, and soils, these below-ground carbon pools are not included.
For each of the two studies, the average annual rate accumulation is calculated as (a) the total carbon production (per acre) at the end of the rotation period divided by (b) the number of years in the period. Shan et al. (2001) reported total biomass production, rather than carbon production; the calculation assumes that carbon comprises 50 percent of biomass. Our methodology uses the average of the two values from the studies, i.e., an annual rate of 1.2 metric tons of carbon per acre, which translates to 4.4 metric tons of carbon dioxide. (1 metric ton = 2,204.6 lbs.)
Example: the average Palo Alto household in PaloAltoGreen avoids 9,500 pounds of CO2 annually, or 4.3 metric tons. 4.3/4.4 = 0.97 acres of trees storing carbon for a year.
Sources:
- Nabuurs, G.J., and G.M.J. Mohren. 1995. Modeling analysis of potential carbon sequestration in selected forest types. Canadian Journal of Forest Research 25(7):1157-1172.
- Shan, J.P., L.A. Morris, and R.L. Hendrick. 2001. The effects of management on soil and plant carbon sequestration in slash pine plantations. Journal of Applied Ecology 38(5):932-941.
- More information…
Garbage recycled instead of landfilled
To determine how much CO2 is prevented from entering the atmosphere by recycling waste rather than landfilling it, greenhouse gas emission factors from EPA's WAste Reduction Model (WARM) were used (EPA 2006). The emissions from landfilling all waste were compared to emissions from waste landfilled without mixed recyclables (e.g., paper, metals, and plastics). For every 2,000 pounds of waste recycled, the result was an emission reduction of 0.79 metric tons of carbon equivalent (MTCE). (1 metric ton = 2,204.6 lbs.) To convert 0.79 MTCE into metric tons of CO2 equivalent, it was multiplied by 44/12, the ratio in weight of carbon dioxide to carbon. When simplified, the result is 2.90 metric tons of CO2 equivalent per 2,000 pounds of waste recycled instead of landfilled.
Example: the average Palo Alto household in PaloAltoGreen avoids 9,500 pounds of CO2 annually, or 4.3 metric tons. 4.3/2.90 = 1.49 metric tons (or 3,276 pounds) of waste recycled instead of landfilled.
Carbon Footprint Calculator
For users who only know the cost of their average electricity bill, the PaloAltoGreen Carbon Footprint Calculator calculates kilowatt hour (kWh) consumption by dividing the dollar amount by the average price per kWh for the average Palo Alto resident. The average residential price per kWh is $0.117433.
Example: A customer’s average electricity bill is $75.00. 75/0.117433 = 638.66 kWh per month.
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