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Frequently Asked Questions

The questions below and the answers provided come from Misha Sarkovich, Ph.D., program manager, SMUD. If you have a technical question that is not on this list, please contact him.

1. What if I do not see my tree on the list?
Since there are thousands of tree species, it is unrealistic to expect that all tree species can be listed. Currently, there are 127 trees listed in the estimator. If your tree is not listed, please, select a tree specie from the list provided that is similar in terms of type (deciduous, conifer or broad leaf evergreen), and tree size (small tree, which grows up to 25 ft in height, medium tree, which grows up to 45 ft in height, or large tree, which grows up to at least 46 ft in height). You can get the tree size information from any arboricultural resource, including the Internet. This estimator uses the Western Garden Book, which is published by Sunset Publishing Corporation, Menlo Park, CA, which has 2,000 plant entries. You can check www.treelink.org and www.arborday.org for more specific information about types of trees. To get a tree added to this estimator, contact TreePower.

2. What if my tree is less than 1-year-old?
A tree that is just planted (and thus it is less than 1 year old) is considered in this model to be 1-year-old tree. The benefits are rather small for 1-year-old tree. The methodology is based on the "standard" nursery raised trees which are typically sold in 5-gallon containers, and which are usually 1 inch in diameter at the tree base (1 foot above the ground). This methodology assumes that the standard (5 gallon) trees are "0" age, regardless how long the trees have been growing in the containers.

3. What if I plant trees in a forest or park, but not near buildings?
Regardless of whether the tree provides any direct shading benefits to mitigate the summer cooling load, the program will continue estimating environmental benefits such as the estimates of carbon and CO2 sequestration for the specified tree species.

4. Can you calculate carbon sequestration for shrubs? If so, how can this be done with the estimator?
You could use small tree species (such as Japanese Maple) if you believe that the shrubbery is big enough to provide environmental benefits such as the carbon and CO2 sequestration.

5. What is the definition of a "Mature Tree"?
For the energy savings benefits, the "mature tree" means that the tree has a well-developed or established tree canopy (i.e. mature tree canopy). Smaller trees are considered "mature" at 30 years; medium size trees are "mature" at 40 years, and for larger tress, 50 years. In other words, trees will continue to age but tree canopy will not get bigger (just like adults.) However, a different or longer life tree growth curve is used for the carbon sequestration calculations, since trees continue to grow in terms of the tree volume (the trees continue to store more carbon). Smaller trees are considered "mature" at 60 years, medium size trees are "mature" at 80 years, and for larger tress, 100 years. Trees will continue to age, after the certain age the tree canopy will not get bigger but the tree volume (trunks, branches, etc) will increase (just like adults.)

6. How is the climate data incorporated into the analysis?
To take into account different climate zones in the US, the Tree Benefit Estimator© estimates the impact of climate on the summer cooling load and winter heating requirements by using the Cooling and Heating Degree Days and the relative humidity data. In other words, the summer cooling load and winter heating requirements are essentially a function of the cooling and heating degree days. The KWh impacts of the tree evapotranspiration effect (or indirect effects) is a function of the Latent Enthalpy Hours (LEH).

Heating degree day (HDD) and cooling degree day (CDD) are quantitative indices designed to reflect the demand for energy needed to heat or cool a home. The number of Heating Degrees Day (HDD) is defined as the difference between a reference value or baseline of 65°F (18°C) and the low outside temperature below that point for that day. The value of 65°F is taken as a reference point because experience shows that if the outside temperature has value then no heating is normally required. Cooling Degrees Day (CDD) is defined as the difference between a reference value or baseline of 65°F (18°C) and the outside high temperature above that point for that day. The value of 65°F is taken again as a reference point because no cooling is required if the outside temperature is below this value. The Latent Enthalpy Hours (LEH) measures the amount of moisture that must be removed from outdoor air to bring it to 77°F and 60 pct relative humidity.

Regardless of whether your climate or utility has any summer cooling load, the program will continue estimating environmental benefits such as the estimates of carbon and CO2 sequestration for the specified tree species.

7. Do I need to determine how many of the planted program trees survived?
No, there is no need to inspect and ascertain program tree survival rate for any utility sponsored tree plantings programs. The program tree benefits results already takes into account the expected tree mortality rate or the tree survival rate. The age of the tree from the planting date will determine automatically the tree survival rate factor and the tree growth rate factor, which will then determine together the level of benefits for any year. In other words, the combination of the tree growth rate and the tree survival rate will determine the final multiplier factor that will estimate the appropriate level of tree benefits for any year. To see the underlying assumptions about the tree survival rate factor and the tree growth rate factor, please check the following link /treeben/data/growthmortalitygraph.asp

In order to estimate tree benefits for any tree age (between age 1 to 30 years), the program will automatically multiply the energy, capacity and carbon sequestration benefit values of MATURE trees with the appropriate Tree Growth and Survival Rate FACTOR. You do not need to make any adjustments or any additional calculations.

8. My utility has planted trees for the past 12 years, but never tracked the benefits. Is there a simple way to estimate past savings?
Yes. In order to dramatically reduce the required data inputting workload, it is possible to use a Random Sample of the past tree-plantings. A utility could randomly select at least 380 trees (or more trees) for a sample of its tree planting effort. That would provide +- 5 percent error term. Then the utility would collect the required tree-siting data (such as the year a tree was planted, tree species, tree distance from the building, tree orientation) and then use the estimator for the sample data. That would give the utility an AVERAGE benefits per tree - average energy savings (KWh saved), average capacity savings (KW saved) and average carbon and average CO2 sequestration (lbs) values resulting from trees planted individually in urban and suburban settings. The next step would be to multiply the AVERAGE tree benefits with the total number of trees planted to derive TOTAL benefits.

There is no need to measure or establish mortality rate for any particular utility tree-planting program because the estimator already takes that factor into account automatically through the application's tree survival rate (it is already embedded and part of the calculations). In other words, you do not need to go back and visit every tree planted to determine if tree(s) survived. If there is no energy savings (kWh saved) or capacity savings (kW saved) because trees are planted to far from the building to have any direct shading impact on air conditioning load or trees are planted in open space (such as parks and streets) then there will be no need to collect the tree distance and tree orientation data. You could input for tree distance data (since it is a required field) but you would select "over 45ft" at the bottom and the estimator will give you ZERO values for energy savings (KWh saved) and capacity savings (KW saved) and POSITIVE values for carbon and CO2 sequestration (lbs).

9. What is the difference between KW saved and KWh saved; and how can I explain to customers the benefits of KW and KWh savings?
Kilowatt (KW) represents a unit of electrical power equal to 1,000 Watts. It represents the measure of instantaneous demand for electrical power. If ten customers each have a light bulb with the 100-Watts rating, then it will require 1 KW of electrical power demand to light up all ten light bulbs instantaneously. The utility is required to provide sufficient power in the grid with adequate voltage to provide enough power to light up all ten light bulbs. Kilowatts saved (KW saved) represents a measure of instantaneous demand for electrical power that is avoided or did not have to be generated as a result of the tree plantings.

Kilowatt-Hour (KWh) represents a common unit of electrical energy consumption or the basic unit of electrical energy. One KWh equals 1,000 Watts consumed for one hour. It equals the total energy consumption taken from an electric current steadily for one hour. If ten customers have each a light bulb with a 100-Watts rating, then it will require 1 KWh of electrical energy to light up all ten light bulbs steadily for one hour. For instance, a utility would sell from its electrical grid sufficient energy with adequate voltage (1 KWh) to ten customers to provide enough energy to light up all ten light bulbs for one hour. Similarly, a 2-KW (2,000 Watts) air conditioner operating for 30 minutes uses 1 KWh of energy. Kilowatt- Hour savings (kWh saved) represents a measure of energy use avoided as a result of the tree plantings.

10. What if you plant two (or more) different trees in different areas? How do you calculate the benefit?
You would have to run two (or more) calculations (for example, one for a tree planted in the south; and one for a tree planted in the west) and then add the benefits. The total benefits are cumulative (1+1=2). If you have two trees planted in the same area, then simply input two (2) trees in the input form under the "Number of Trees" question and the program will calculate the total benefits for two trees automatically.

11. What if I plant trees that are not in a standard 5-gallon container?
The methodology is based on the "standard" nursery raised trees which are typically sold in 5-gallon containers, and which are usually 1 inch in diameter at the tree base (1 foot above the ground). However, SMUD Shade Tree program has experienced that 5-gallon container trees will grow quickly and catch up with the larger 15-gallon container trees within the next couple years and thus the methodology applies for both 5- and 15-gallon container trees. Since the difference in age between the trees planted in 5- and 15-gallon containers is approximately three years, the other option is to add three (3) more years to the tree age input field. Also, this methodology assumes that the standard (5 gallon) trees are age "0" when planted.