Arboriculture & Urban Forestry 36(3): March 2010 MODEL AND ANALYSIS Equation 1 is a conceptual representation of the estimating model linking residential energy consumption to hedonic character- istics of the house, characteristics/behaviors of the occupants, climatic conditions of the neighborhood, and the extent and density of shade cast on the house at different times of the day. [1] Daily Electricity Consumption = f (hedonic charac- teristics, occupant characteristics, climatic conditions, percent shade, shade density) Empirical analysis covered the period from May 1–September 30, which includes the months with a positive difference between mean outdoor temperature and desired indoor temperature. Equa- tion 2 is the specific model estimated to identify the impact of tree shade and density on daily electricity consumption dur- ing the summer season, controlling for other relevant variables. [2] DECijk = α0 + α1 Freezerij α4 Occupantsij + α5 HouseAgeij α7 Laundryij + α8 Floorsij + α11 Tempdiffij + α12 Humidij + α14 ShadeDensityijk + εijk where, DEC = daily electricity consumption (kilowatt hours) by an individual house in a given service period Freezer = whether the house has one or more additional freezer units Cook = whether the household uses any amount of electricity for cooking Air = whether the air conditioning system is window unit (1), or central-air (0) Occupants = number of family members in residence HouseAge = age of the house in years Space = living area of the house in square feet Laundry = average number of laundry loads run per week in the house Floors = whether the house has multiple floors Pool = whether the house has a swimming pool WaterHeat = whether the house has one or more water heaters that use electricity Tempdiff = the average daytime outside temperature minus the daytime thermostat setting for a given service period Humid = average relative humidity for a given service period PercentShade = the extent of the roof area covered by tree shade, in decile percentages ShadeDensity = the intensity of tree shade cast on the dwelling, assigned one of four categories: no shade, light, moderate, or heavy εijk i j k = model error term, assumed to be normally distributed = sample households (i = 1 to 160) = service period or electricity billing cycle for each i (j = 1 to 5; e.g., May, June, July, August, and September) = shade monitoring times in a day per month (k = 1 to 3; 1 for late a.m., 2 for early p.m., and 3 for late p.m.) + α2 Cookij + α9 Poolij + α6 Spaceij + α3 Airij + + + α13 PercentShadeijk + α10 WaterHeatij RESULTS Table 2 reports Ordinary Least Squares regression results for Equa- tion 2, in which family size is treated first as a single variable, then with the number of males and females identified, and finally with categorical variables that control for the age distribution of family members. Taken together, the results reveal relationships that are consistent with expectations and are consistent across model spec- ifications. The overall explanatory power of these relatively parsi- monious models is quite high, with almost all explanatory variables having a statistically significant impact on daily electricity usage. As reported in Table 1b, the mean electricity usage across the sample was 58.50 kWh/day. Each additional family member above Table 2. Regression results (family size and composition). Dependent variable = kWh/day. Explanatory Variables Model Intercept Family size # Females # Males 12 or under 13–24 25–60 Over 60 Living area House age # floors Elec. Cooking Elec. H2 Window AC Laundry loads/wk Second Freezer Swimming pool Average humidity Percent shade Adj. r2 : : F-statistic: Nw O heat 0.013z (0.001) 0.103y (0.046) 1.974x (1.066) -1.172 (1.285) 4.547z (0.979) 4.115 (2.675) 1.076z (0.169) 2.475z (0.959) 21.010z (1.761) 0.133 (0.127) Daytime temp. diff. (mean) 2.233z (0.085) -0.159z (0.029) 0.5932 170.3 1510 z significant at 0.01 level y significant at 0.05 level x significant at 0.10 level w The total number of observations for five summer months during two years was 1,510. A number of participants, out of 160, reported only one year worth of electricity data because they occupied the house only during the study year. Due to differences in monthly observations between the two years, the total number of observations is 1,510 not 1,600. 0.013z (0.001) 0.108y (0.046) 2.231y (1.063) -1.421 (1.283) 4.569z (0.976) 4.375x (2.660) 1.085z (0.168) 2.807z (0.959) 20.84z (1.757) 0.138 (0.126) 2.224z (0.085) -0.163z (0.029) 0.5962 160.1 1510 -10.450 (9.622) 3.390z (0.474) 5.250z (0.786) 2.394z (0.607) 4.014z (0.616) 0.640 (1.081) 1.716 (1.153) 0.847 (1.196) 0.013z (0.001) 0.145z (0.047) 2.099y (1.063) -1.025 (1.299) 4.007z (0.999) 5.437y (2.727) 1.142z (0.169) 2.945z (0.980) 21.179z (1.763) 0.150 (0.126) 2.208z (0.085) -0.164z (0.029) 0.5964 140.4 1510 Model 2 -12.520 (9.608) Model 3 -9.491 (9.747) 77 ©2010 International Society of Arboriculture
March 2010
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