ASHRAE Journal - June 2021 - 52

TECHNICAL FEATURE
Discussion
The return on investment (ROI) as of 2020 will be 14
years assuming the home will be on average net zero
in the coming years‡ and that the electricity cost for
this home is, according to the utility bills, $0.17/kWh
(Table 2). For the current national average electricity cost
of $0.13/kWh,6 the ROI, including government incentives,
would be approximately 20 years, but it may vary
depending on local costs.7 While these ROIs are not
particularly attractive-although not prohibitive either-
there are ways one could reduce overall investment at
Phase 1 by focusing more heavily on load reduction. A
leak-tight and well-insulated home will consequently
reduce HVAC use, thus reduce overall energy demand,
so fewer PV panels could be required.
In the home featured in this case study, the construction
was inefficient from the start with respect to
infiltration and insulation from the perspective that
construction just met code in 1997. The structure was not
" tightened " nor insulated beyond that code, and these
infiltration and insulation issues were not tackled in
the beginning, which corresponded to greater energy
demands and equally greater investments.
Triple-pane windows may reduce the U-factor by 30%
to 50% compared to double-pane windows,8 and highergrade
insulation material for external walls and attic may
add at least incremental insulation. For homeowners,
however, these modifications will add value to the home
so that despite the seemingly slow cash return, a substantial
fraction of the investment will be converted into asset.
In the long term, the return could be greater than 100%
if the home is consistently NPE, since in the long run the
energy generation credits could surpass the investments.
Sustainability Options: Efficient Homes and Electric Vehicles
The cost of a sustainable residence and personal
transportation can be in fact very similar. State-ofthe-art
full electric vehicles (EVs) consume on average
0.25 kWhe/mile (0.16 kWhe/km),9 while internal
combustion engine vehicles (ICEV) consume on average
1.34 kWh/mile (0.83 kWh/km), assuming 25 mpg
(11 km/L)10 and gasoline's lower heating value in the
range of 112 kBtu/gallon to 116 kBtu/gallon (31 MJ/L to
32 MJ/L).11 The average mileage per year in the U.S.
is approximately 13,500 miles (21 726 km),12 which
TABLE 2 Average energy cost for the home by year.
YEAR
Ee COST
($/KWH)
2009
2010
2011
2012
2013
2014
Average
0.16
0.17
0.17
0.19
0.18
0.17
0.17
Et COST
($/KWH)
0.07
0.06
0.07
0.07
0.06
0.11
0.07
TOTAL ENERGY
COST ($/KWH)
0.09
0.08
0.09
0.09
0.08
0.13
0.09
corresponds to 3,375 kWh and 18,000 kWh consumption
for EV and ICEV, respectively. The average cost for
a lower-end EV model currently on the market varies
between $30,000 to $40,000. Therefore, the cost
of replacing an ICEV with an EV is on the order of $2/
kWh, disregarding any tax benefits and subsidies for
EVs. The GSHP in the home of the present study reduced
the energy consumption approximately 8,500 kWh at
an approximate cost of $1.90/kWh; the PV panels offset
9,500 kWh from the grid at an approximate cost of
$2.20/kWh. In summary, the home investments toward
an NZE or NPE are of the same order of magnitude as
investing in an EV.
If one is determined to reduce their carbon footprint,
switching to EVs and NZE/NPE retrofit are virtually
equally reachable options. The greater the up-front
investments in load reduction (insulation/infiltration),
the lower the total investment per kWh as discussed
above, and potentially less than an EV.
Conclusions
This article presented a case study for a real-life application
of retrofitting a home to achieve NPE. A timeline
of five years included implementing the following:
energy demand reduction by using technologies such as
GSHP, primary energy shift with PV panels, and elimination
of fossil fuel demand with a heat pump water
heater and heat pump dryer. The modifications made
between 2014 and 2019 resulted in an overall NPE and
100% renewable home in 2019 with an approximate
investment of $41,000.
This home is partially occupied throughout the year,
but when it is occupied, the number of occupants is
high. Furthermore, the thermostat is set to maintain the
‡The 12-month rolling average as of 4/14/20 was +2,926 kWhEe surplus, with three high-power generating summer months to come.
That would complete 12 full months since adding the heat pump dryer, so NZE is a conservative assumption.
52
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