The best summary I have seen on levels of energy independence is found
in the introduction to Johnson and Gibson's "Toward a Zero Energy Home - a
Complete Guide to Energy Self-Sufficiency at Home". The
book is a good read for anyone interested in sustainability--it is full of
success stories for both new construction and remodeling. They differentiate
between net zero energy homes, off-the-grid homes and carbon neutral homes.
Net Zero Energy Homes
These homes produce as much energy as they consume. They are tied to the utility grid with
net-metering, have photovoltaic arrays or wind turbines and store any excess production on the grid rather than in batteries. The utility typically pays a fair price for
the energy it gets when the house is producing more than it is using. However, some utilities drop the price to a pittance once net-metering reaches zero.
Off-the-Grid Homes
These homes live on an energy budget. When production exceeds usage, the excess
goes into on-site batteries that have a limited storage capacity. If the sun isn't shining or the wind isn't
blowing and the batteries are running low, there's a problem. The solution is lifestyle adaptations that
ration or manage energy usage. Toasting
bread for breakfast and taking a hot shower simultaneously might not be possible.
Carbon Neutral Homes
A carbon neutral home carries zero energy to another level. It produces enough energy, not just to
handle its daily needs, but to cover the costs of building the home in the
first place. This means covering the
embodied energy in extracting, manufacturing and transporting the building
materials to the home site (which, according to Johnson and Gibson, is 8% of
the home's energy use). Only the most
principled and ethical home owners go there (which is hard to do anyway if the utility
refuses to pay a fair price after net-metering reaches zero).
Passive House Movement
According to the authors, the Passive House Movement began in Germany in 1990 based upon passive solar research in the '70's by the U S Dept of Energy, spread rapidly across Europe and finally made its way back to this country in '03. The first net zero energy Passive House was built in Urbana, IL where the Passive House Institute U.S. is located. According to the Institute, the standards for Passive Houses are as follows:
With exception of the first standard listed above (we did a DIY design without software), our home will exceed the Passive Home standards......so I guess it's safe to use the term "net zero" energy, or at least "near zero" energy to describe it once it's finished and the photovoltaics have been added. And we will take great pride in achieving net-zero at a fraction of the cost of typical new construction -- green or otherwise.
- Use of the Passive House Institute software to model the house
- Super-insulate
- Eliminate thermal bridges
- Make the house airtight
- Use heat-recovery ventilation
- Optimize passive solar design
- Use high-performance windows and doors
- Use internal heat gain (people, appliances, electronic equipment)
- Zero out energy needs with renewable energy (my addendum, based on the authors' discussion following the list in the text)
With exception of the first standard listed above (we did a DIY design without software), our home will exceed the Passive Home standards......so I guess it's safe to use the term "net zero" energy, or at least "near zero" energy to describe it once it's finished and the photovoltaics have been added. And we will take great pride in achieving net-zero at a fraction of the cost of typical new construction -- green or otherwise.
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