Comparison of projected energy costs between this house and a hypothetical identical house built to the Energy Star standard
Posted from: MD, USA
During Earth Week we were asked to do presentations at NIH and DOE on this house. In preparation for that I revisited our cost numbers and our energy numbers for the house and put together a comparison of this Passive House to a hypothetical identical house built to the Energy Star standard.
Here are the assumptions I made:
- The Passive House components of this house increased the cost by 8% over building it to the Energy Star standard. This is based upon actual construction cost figures from our subcontractors.
- Both houses are financed at 20% down in a 30 year jumbo mortgage at an interest rate of 6.37%.
- Downpayment savings of the less expensive hypothetical Energy Star house are invested at 3% per annum, and the annual income applied to the mortgage payment.
- Projected Passive House energy numbers are derived using Passive House energy modeling software, which has proven a very accurate predictor of actual energy use in Passive Houses around the world. To convert that to predicted energy costs we used current Maryland energy residential energy rates. While we are confident that these numbers are broadly reliable for our house, actual energy numbers may vary depending upon user habits.
- To get the comparative predicted energy costs of the hypothetical Energy Star house, we used a factor based upon the comparative HERS ratings of Energy Star and Passive Houses. That factor is assumed to be 85/30 in this case.
- We then put together the following graph showing annual cost of ownership numbers (mortgage cost + energy costs) of both houses. We looked at two scenarios: one where energy rates increase at a 1% annual rate and a second where they increase at a 5% annual rate. We think both of these rates of increase are conservative given the volatility of today’s energy market.