Net Zero

August 20, 2021: Is it really possible to get through a Wisconsin winter on solar power alone? We had a 10-day cold snap this past February and it got down to minus 17 degrees at night. My air-source heat pump (mini-split) was sized to take it, but it was losing efficiency and consuming lots of electricity.

When I set out to build this house (and afford it), I knew my design had to be small and smart. My goal was an all-electric, fossil-fuel free, net-zero energy home that was super-insulated and super-airtight. I knew I couldn’t spend freely on uber-expensive windows or cutting-edge high performance materials. I’d have to rely on mostly standard construction methods and off-the-shelf products, paired with extra attention to detail. I knew from others who’ve come before me that the cost of extra insulation and the extra effort could be partially offset by replacing a standard furnace and separate air conditioner with a mini-split. Of course, I’d chose energy-efficient appliances and go with all LED lighting. Even so, was I risking burst pipes, weekend service call bills, or a spartan lifestyle just to prove a point? What was this all going to cost? What’s a reasonable payback? How many solar panels would I need? And would the solar panels be ugly and make my house look like a middle school science project?

My first step was to find out how much energy my home would use. I looked at a number of energy modeling programs before choosing REM/Design. This user-friendly software is recognized by Energy Star and comes with a free 14-day download. I knew my foundation, wall, and ceiling R-values from my plan, and I looked up the manufacturer specifications for windows, heating/cooling equipment, appliances, and lighting. Here’s what I found out:

Heating:                                                 5246
Cooling:                                                    527
Hot Water:                                             3040
Lights & Appliances:                            4601
TOTAL ELECTRICAL DEMAND:        13,414 kWh/year

So what size PV system would I need? After consulting with Mike Dearing of Driftless Solar, we decided on a 7.32 kW system with a year one estimate of:

TOTAL OUTPUT:                                10,664 kWh/year

This would fall short of my net-zero goal by about 20%, but seemed reasonable. For one, I was pretty confident that my personal energy use will fall under whatever “typical” use patterns may have been assumed in the energy modeling program.  If wrong, or if a future families use pattern was more demanding, more panels could be added.  The other reason why this size system made sense was cost.  Mike advised me that a 7kW system is the largest he can economically install in a residential application given current electrical codes and existing technology. Several other solar installers I contacted advise sizing a system to 2/3 of demand where “monthly true-up” is in place (more on this below).

I’ve now lived in the house one full year and can make the following report:

TOTAL OUTPUT:                                  8,839 kWh/year (what my PV panels produced)

TOTAL USED:                                       5,788 kWh/year (what I produced + what I bought from the utility)

TOTAL SOLD TO UTILITY:                   3,051 kWh/year (what I produced and didn’t use)

It’s true, my lifestyle is pretty quiet and some people may think my home a bit chilly in the winter. I do prefer open windows to AC in the summer, but not when it’s ghastly hot. Of course, I’m pretty darn proud that I produced 53% more energy than I consumed!

But how did I make out money-wise? Every utility company has a different rate structure. Alliant Energy serves my Village, and their standard residential rate is $0.12 kWh. My surplus in July won’t help me in December—this utility calculates my bill by what’s called a “monthly true-up”. Each month, I can buy or sell as much or as little electricity as I like but it never carries over to the next month. When I use more than I produce in December, they charge me $0.12 kWh. When I produce more than I use in July, they buy from me at $0.03 kWh.

I produced a surplus every month of the year except for December, January, and February, when bills averaged $75/month. I “made” $148 in the good months. But that money, plus a bit more went to pay Alliant’s “customer charge” of $0.50/day. Otherwise known as a “meter charge”, it’s what every customer pays to simply be hooked to the grid.

ELECTRICITY CREDIT:                      $148

ELECTRICITY DEBIT:                        $225

CUSTOMER & MISC CHARGE:        $187

TOTAL ELECTRICAL BILL:                $264/year OR $22/month

How much did my PV system cost?

Installed cost of 7.32 kW system   $20,345
Focus on Energy incentive             ($2,000)
Federal Tax Credit (30% in 2019)  ($6,103) can be rolled over multiple years if your income tax due is less
MY COST                                            $12,241

How do I know if I’ve made a good financial decision? I’m all about the environment and doing my part to invest in a fossil-free future for my grandchildren (and everyone’s grandchildren), but I’d like to know (and you probably do too): what’s my payback?

According to the author of “When You’re Financing a Green Home, Payback is Irrelevant”, the best way to evaluate an energy-efficiency improvement is to calculate monthly savings, assuming the cost is rolled into your mortgage. I chose a 25 year amortization because the system comes with a 25 year warranty. Here’s how it works for my system:

HOME WITH NO PV

Loan=0

Electricity used=5788 kWh @0.12=$58/month

True Monthly Cost=$58/month

HOME WITH PV

Loan for $12,241 with 25 year amortization @3%=$58

Electricity debit minus credit=$6.42/month

True Monthly Cost=$64/month

EXTRA COST FOR 25 YEARS=$6/month

So, I’m a wee wee bit disappointed. This system is over-sized for a one-person household. I’m using less than half of the electricity than the energy model predicted. I could have gotten by with a smaller system and optimized my utilities rate structure. On the other hand, I could capitalize on my investment by getting an electric vehicle. I have a hedge should utility prices spike. Perhaps a larger family is in my future. Surely the system will last longer than 25 years, when I’ll finally be money ahead. And, I’ve helped the Green Revolution along by being a (somewhat) early adopter.

What size system would better match my useage and provide a neutral or positive payback? If I had heeded the 2/3 rule (mentioned above), a 4.6 kW system would have exactly matched my needs. My electricity debit minus credit would be $16.80/month. Assuming the cost of a 4.6 kW system to be approximately 3/4 of the cost of my existing 7.32, the cost of running a “home with no PV” matches the cost of running a “home with PV” for the first 25 years.

Besides reducing the size of my system, how else could I have optimized my investment? My design has inherent inefficiencies. The 3:12 pitch is very low and snow pack is a problem. There were days and once this winter, a 10-day stretch when production came to a complete stop. It’s possible to clear panels with a snow rake, but mine are too high for that. Panels mounted on a rack on the ground could be maintained, but costs considerably more than the mounting hardware installed on the standing seams of my metal roof.

Even steeper roofs can hold snow for long periods. I played around with PV Watts to find out what the optimal roof pitch is at my latitude. To make it easy, I just looked at annual production (not winter production). The optimal pitch is 8.5:12—-similar to what you might see on a Cape Cod or Arts & Crafts style house. When I dialed back to my more contemporary 3:12 pitch and increased the system size to match the annual production of the 8.5:12 pitch, I found I would need to add one extra solar panel. The panels from Driftless Solar are $510 each, installed. I think it’s safe to say that a steeper roof pitch or the addition of an angled roof rack would cost significantly more than one extra solar panel.  In this case, system inefficiency is best made up by simply adding more panels.

Here’s a drawing of how my panels look. There are 24 panels, each 39” x 66”. I placed them high on the roof to clear any shading from neighboring trees to the south. This placement is less noticeable from the ground. Code requires at least 36” of clearance around the panels for emergency access.

solar FOR BLOG

I’m pleased with how my panels look and pleased with my production. There haven’t been any problems. Please let me know if you have any questions about what I’ve explained here or if I can help you with your project.  Shine on!

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