House Construction Details
Net Energy Use
Energy Use Details
Costs and Payback for Net-Zero
Infrared Images of REL
Energy Efficient Design
Comparison of PV Systems
R-Value of Cellular Shades
Energy Codes for Windows
Solar PV Raw Data
Costs to Modify a New House Design
for Net-Zero Source Energy
requires an extra investment to achieve a net-zero source energy house,
but of course, utility bills are very low. How much extra cost?
For this grid-connected, demonstrated
net-zero energy house, the following extra costs that totaled $22080
were incurred beyond the cost of the standard production new house.
All of these costs were for professionally installed systems, and
included all material, labor, and permit costs. (The only
exception was the window shades that were installed by the owner.)
These costs were for additions beyond the minimum code
requirements of IECC (International Energy Conservation Code) 2006, which are significant in this climate zone 6 environment. The solar systems were top-of-the-line quality.
|Item||Initial Cost - |
|Cost Savings, |
|Years to Pay Back|
|3.15 kW Solar PV System||$5451||$585*||7|
|Solar thermal hot water||$4293||$84||26|
|High solar gain double-pane windows**||$265||$67||4|
|Add additional window (40"x60") on south side to increase solar gain||$410||$13||20|
|Remove outside door in MBR from standard plans||-$405||$29||0|
|R-38 to R-60 in attic***||$1120||$21||28|
|2" of XPS (extruded polystyrene) on outside of walls||$5000||$40||42|
|R-14 to R-19 on crawl space walls||$390||$6.50||29|
|1" XPS under crawl space, 2" XPS inside footings||$1489||?||?|
|Lasco air boxes around electrical outlets||$214||?||?|
|Insulated cellular shades with side seals||$1875||$22||35|
|Heat recovery ventilator||$1978||$96||15|
The cost savings for the solar PV system are computed as if the total
electricity generated were used by the house, which would be true for
most families. In fact, for this house, only a little more than
50% of the generated energy is used by the house, with the rest sold
back to the utility at the wholesale rate. The retail cost of
electricity is about $0.10 per kWh, while the wholesale cost is about
$0.03 per kWh.
** The extra cost for the high solar
gain windows that were used on the S, E, and W sides of the house was
computed for the incremental cost for high solar gain coatings but
applied to the same type of standard windows used by the builder.
There was some concern about the high solar gain windows meeting
the maximum U-value code requirement, plus the owner wanted wood rather
than vinyl windows. The triple-pane, wood-clad windows actually used in
the house were more expensive (about $6500) than the value shown.
*** Raised heel trusses that were used in this house
and these allow full-height insulation all the way to the edges of the
ceiling and over the side walls, and these trusses make the code
standard R-38 instead of R-49 for the attic insulation.
Therefore, R-38 was used as the baseline insulation depth.
The heights of the heel trusses and soffit baffles were extended
to allow full insulation height for the added insulation.
The following caveats and comments should be noted.
modifications were made to the house design before construction
that did not incur extra charges but were important to the solar
- changing the roof at the back of
the house from a gable roof to a hip roof so that an overhang could be
added to shade the high solar gain windows in the summer time.
In addition, this hip roof provided an ideal orientation and location
for the solar thermal system. Overhang calculations were performed at a
modest price ($10) using a shareware program at: http://susdesign.com/overhang/index.php
the garage roof from a hip roof to a gable roof with one side facing
south-south-east so that there was sufficient area to mount the solar
- The cost of the 3.15 kW solar PV system
including installation and permit fees before rebates from the federal
government and the local utility was $18337.
- A larger PV system
would likely be required to achieve net-zero source energy for the
house if it were inhabited by more than one person.
- The cost of
the solar thermal system including installation and permit fees before
the tax rebate from the federal government was $6133.
- The solar thermal hot water system is not well instrumented, so the cost savings for that system are an estimate.
year cost savings were computed based on a natural gas cost of $0.772
per CCF ($0.705 per therm), and an electrical cost of $0.0997 per kWh.
The yearly inflation of energy costs was assumed to be 4.7% per
year, based on the actual cost of natural gas in Colorado over the
period from 1989 to 2010.
- The total cost for window coverings was used for these calculations, but some type of window covering is often required
for privacy and cosmetic purposes, independent of any energy
conservation goals. The cost savings shown do not include savings
due to shading during the summer since the house does not have
air-conditioning, but the window coverings are key to reducing unwanted
solar gain during the summer, and keeping the house cool without
- The payoff periods would be
shorter for lower amounts of insulation. Each additional cm or
inch of insulation costs the same, but the benefits decrease
asymptotically toward zero.
of the energy savings and payoff periods are specific to this location
with its characteristic solar insolation, weather, etc. For
example, in parts of the world with lower solar insolation, the solar
PV system would have a longer payback time.