In Europe, builders interested in energy efficiency are gravitating to the Passivhaus standard. Meanwhile, American researchers — and a few American builders — have developed a fascination with the idea of the net-zero-energy house. The U.S. Department of Energy has established as a goal that new buildings in the U.S. will be built to a net-zero-energy standard by 2030.
Passivhaus buildings and net-zero-energy buildings have a lot in common. Both types of buildings aim to reduce the amount of energy used for space heating or cooling by designing envelopes with a low rate of air leakage, thick insulation, and high-performance windows.
While Passivhaus designers are content with achieving a very low energy budget, net-zero-energy home designers add frosting on the cake by including a rooftop photovoltaic (PV) array sized to produce enough site-generated electricity to balance the home’s annual energy use.
While both approaches have merit, both approaches are also open to criticism.
The best aspect of the net-zero-energy approach is the fact that, in order to balance energy loads with energy production, designers are forced to evaluate the cost-effectiveness of each energy-efficiency measure and compare it to the cost of a PV array. The aim is to find the least-cost path to building optimization.
Here’s the way such analysis works. Say you are building in Syracuse, N.Y. Designers know that a 1-kW PV array — that is, an array that now costs about $7,000 to install — will generate 1,123 kWh per year in Syracuse. In other words, each $1,000 you invest in PV will reduce your energy expenditures by 160 kWh per year. [Update: in September 2012, the cost of a 1-kW PV system has dropped to $3,500. That means that a $1,000 investment in PV will generate twice as many kWh per year — about 320 kWh — as the calculations shown in this article.]