energy use
performance data
Although sustainability comprises much more than energy, there is no doubt that when most people think about green buildings they are interested in how energy efficient a building is. We have now lived in the Ross Street house for a little more than one year and have some real data. The series of charts make comparisons to the Rem/Rate calculation that was done as a part of the certification process for the house.
hers rating
Ross Street received a HERS (Home Energy Rating System) rating of 42. This is based on a scale of 100 with a house designed to the current WI energy code receiving a rating of 100. An Energy Star house needs a rating of 80 or less. This means that an Energy Star house would use less 20% less energy than a code house. Our rating of 42 means that we should use about 42% of the energy of a house built to the 2006 energy code. At the end of the first year, we used a little more than half of our predicted energy based on the HERS rating. This equates to a HERS rating of approximately 23.
HERS Rating: Wisconsin Energy Code Home vs. Minimum Energy Star Home vs. Ross Street House Prediction
energy use
During the first year we used about 39% of the electricity than was projected in calculating the HERS rating. We believe this is a result of careful selection of efficient appliances, systems and the use of mostly LED and CFL lighting. Additionally, the large number of windows on the south face of Ross Street provide a terrific quality of light during the day and eliminate the need for artificial lights on most days. We also have an LED television and induction cooktop. Cooktops are not Energy Star rated, but an induction cooktop is 40-50% more efficient than conventional gas or electric cooktops. We are very careful about turning off power users when they aren’t in use.
Energy Use: 2006 IECC Code Home vs. Ross Street House
Energy Use Compared to REM/Rate
Energy Use compared to PV Watts
hvac load
In the last 12 months it was only necessary to run the air conditioning for 2 days. This was during a period when the temperature was about 95F and we had about 100 people in our house for an event. The house stays comfortable most of the time during warm weather simply by opening the windows when the outside temperature is less than the inside temperature and closing them when it isn’t. Good cross ventilation is a real plus.
natural gas usage
Natural gas use is strongly weather dependent. In Madison we normally have about 7500 heating degree days based on a 65F balance point. The last 12 months was warmer than normal and we only had 6832 heating degree days. Based on this, one would expect to use about 9% less fuel to heat your house. We used 483 Therms which is 41% less than predicted by the HERS calculation. We think that a lot of this is due to the passive solar affect of sunlight through the south facing windows. We have noticed that on cold days the furnace doesn’t operate when the sun is shining. Overall it cost us a little more than $500 to heat our house and water for the last year.
solar power generation
Solar power generation has been less than the PV watts prediction of 3045 kWh by about 16%. Winter and lots of snow don’t help. We have fixed panels and it takes some time for the snow to melt clear of the PV panels (If we had to do it over we would raise the panels more in relation to the garage roof). Also, sunshine is pretty variable so we will have to see what really happens over a longer period of time. We generated about 58% of our annual electrical use and purchased green power from Madison Gas and Electric for 50% of our use. Madison Gas and Electric pays a premium for solar power and it cost us a little over $100 for all of our electrical requirements over the last year.
In addition to saving money, the use of less energy in a home helps the environment by also limiting green house gases. According to the Energy Center of Wisconsin, an average single family home in WI creates about 36,000 lb. of CO2 each year with vehicles adding another 26,800 lb. per year. We have calculated that our housecauses a little less than 9,000 lb. of CO2 to be released into the environment.
Actual vs. Predicted Solar Power Generation, 2010