Net Zero Continued from page 68


phone bill. When the sun goes away, you buy electricity back from the grid. The grid, not batteries, is the storage medium. Most utilities in the U.S. have net metering pro- grams that account for your energy production and con- sumption on an annual basis. This is one of the keys in making net zero energy homes practical. BS: What factors do you take into consideration when determining whether or not a net zero energy


home is practical? DK: There are several factors to consider. One is the


local climate, which determines the heating and cooling loads as well as the solar production. Adequate roof space and site orientation, ideally unshaded and south facing, are also very important. Local incentives for installing solar electric systems play a key role, and the grid rate of elec- tricity is also a strong determinant. For example, in most of California we have a mild cli-


Table 1


solar incentives and an inexpensive electric rate, which means that a net zero energy home is not as practical. In other words, mild, sunny states with good solar incentives and high grid electric costs benefit the most from net zero energy. While it is nice to have all four variables, having at least two of the four can make building a net zero ener- gy home a sensible economic decision. These statistics support Knight’s Theory, which is that homeowners would rather produce their own electricity from solar than buy from the local utility company, if the after tax price is the same or less. BS: What is the best way to determine the rate of grid-provided electricity versus the cost of solar-pro-


vided electricity? DK: Table 1 represents the comparison between the


costs of solar provided electricity to the grid. I call this my SPER chart, which stands for Solar Provided Electric Rate. SPER is calculated by dividing the annual after- incentive cost by the kilowatt-hours produced. SPER is basically your fixed electric rate for the life of the system, typically about 30 years, the same amount of time as your mortgage. To calculate the average after tax equivalent cost of electrical power, I made the conservative assumption that the homeowner was in the 25% tax bracket and added any applica- ble state taxes. For example, in Illinois the average retail electric rate is $0.113. Since mortgages are tax deductible and an electric bill is not, we added the cost of the 25% federal taxes and the 3% Illinois state taxes to determine the after tax equivalent rate of $0.16/kwh BS:What overall conclusions can


be made from the SPER chart? DK: In the last year, solar has become cost effective in almost every major city in the U.S. The simple way to use this chart is to compare column 9, the SPER, to column 11, which is the after tax equivalent rate of your electricity. In 31 of the 40 cities shown, the SPER is lower than the after tax grid elec- tric rate. Homeowners understand that their mortgage is tax deductible but that their utility bill is not. That is why we compare the SPER to the after tax equivalent electric rate. If you believe that grid prices will increase, another thing to note on this chart is column 9, the SPER rate, compared to column 12, which


mate, thus low heating and cooling systems, good solar incentives and very high grid electric rates. That combina- tion makes it very practical to do a net zero energy home. By contrast, Omaha, Nebraska, has a harsh climate, no


Plumbing Engineer


calculates the average price per kwh if the grid rate increases 5% a year. As you can see, in all 40 cities sur- veyed, the solar electric rate is significantly less than the


Continued on page 72 January 2011/Page 71


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