Solar Case Study: Single Family House

In honor of Earth Day today we hope to provide you with an informative post regarding Solar in Residential use.  Happy Earth Day Y’all!

solar panels 1

Solar Case Study: Single Family House

by John Vesel

Austin TX, 6.75kW Photovoltaic Array installed January 2015

Electric Utility: Austin Energy

 In January 2016 we completed our 12th billing cycle as a solar power generating household. The figures are in, but first a rundown of the situation:

1518sf Single Family Home

3 bed / 2 bath; 2 car garage

Constructed 2004

Located in Southeast Austin

Oriented with East-West axis*

Subsidized & Financed

*Crucially, our house is constructed with a long hipped roof with the main ridgeline running almost exactly East-West. This means our largest roof surface is facing South. There are no trees that interfere and the two-story house to our South does not shade any portion of the roof. This existing situation allowed for the easiest possible setup. This was confirmed by the installer when they measured the roof surface and calculated the sun angles. The most helpful terms to know are that kW = Kilowatt which is a measure of energy (1000 watts); kWh = Kilowatt hour which is a measure of power, or, energy over time.

Our solar installer recommended a few different options for solar panel modules. We chose a 270W base panel. During our research, we found that solar panels typically lose about .5% of their production capability a year, but there are still productive panels that were made during the early years of photovoltaic power. We also chose to install a micro-inverter with each module. This is a more expensive option than connecting a single inverter to the entire system, but we felt it was more advantageous. A single inverter system is like an old strand of Christmas lights – if one light goes out, they all go out. With micro-inverters, we could insure that if one panel was shaded or malfunctioning for some reason, then only that panel would be affected instead of the entire system. Inverters are required to convert the DC power generated by the photovoltaic process into the AC power used by the power grid. We had to connect to the grid to receive the Austin Energy solar subsidy.

solar panel 1

 

We chose a 25 panel 6.75kW system capable of producing about 9,000kWh annually. They cover about 443sf of roof area and are built to withstand the elements. Since there are no moving parts, there isn’t a whole lot that can go wrong, outside of a cloudy day, and both the panels and inverters are warrantied for 25 years.

Our system is barely noticeable from the street, only just peaking above the roof. We did inform our HOA, and they were all for our addition. According to our installer, once one house installs, the rest of the neighborhood tends to popcorn as the benefits become known.

The installation process only took a few days. Stanchions were lapped beneath the existing asphalt tile shingles and locked to the existing trusses. These connect to tracks which hold the panels. A single encased electrical line runs down the other side of the roof to the electric meter. We now have two meters from Austin Energy: one tabulates our production; one tabulates consumption. Readings occur as usual with the consumption figure less the production figure.

Enphase-1

The system is linked to an app that I love to check each night. The above graph displays each day’s production for the past year, the brighter the blue, the greater the production. Our highest production value for a single day was 43.7kWh and our worst was under 1kWh. July was a great solar production month this year, hardly any clouds! Below is an 18 day period broken down by hour. Production is updated every 15 minutes. You can definitely see how sun angle increases production at midday before tapering off, or when a bank of clouds moves through.

Enphase-2

 

The app does a variety of things, but my favorite is a comparison of your production to more relatable terms. For instance, on our best day, our system produced enough energy to power and light the Eiffel Tower for 13 minutes. In our first year, we offset the same amount of CO2 as 172 newly planted trees!

We had only been in our house for a couple of months when we began the install process, so the system was sized based on average consumption for similar houses in our neighborhood. Austin Energy deducts our generation from our consumption at each reading with any excess generation becoming a credit. This means that the system can build up a buffer account credit during high-generation summer months which then evens out during less productive winter months. We have yet to pay beyond our account credit.

Our system cost, including installation, was a little over $1,000 per panel. Both the City of Austin and the Federal Government currently provide subsidies to help private owners become solar generators. Both subsidies were for 30% of our retail cost and then the remainder was financed. It is important to note that the Federal subsidy takes into account local aid before calculating the rebate. Our installer coordinated everything although our actual loan is via third party that specializes in such financing with typical rates at about 3% for solar loans. The Austin Energy subsidy was paid directly to the installer and the Federal subsidy was included in our tax return as a Renewable Energy credit. There is a per-year cap to that amount so we will have some carryover to this year’s return.

Since the Federal subsidy came well after our system became operation, we ended up with two parallel loans. One loan was more of an IOU intended to be paid by the Federal return amount. We had 1 year, interest free, to pay that loan in full which we did. The second loan was for the remainder of the system retail price. Typical solar loans are for 12 years, but 7 year and 30 year options were also available. We chose the 7 year loan option at just under 3% interest. Our house does not have gas, and the impact of this payment is like having a large electric bill every month. Aside from being able to look forward to the end date, now only 6 years out, we also have a set payment that we can always plan on rather than a variable use-based payment.

7 Year Financing Cash Flow-1

As you can see, we project to break even on the system at about 10 years. Going solar also fixes the rate at which Austin Energy compares our production to our use, so if rates go up, so does our rate of savings. Lastly, while the hard value added to the home has already boosted our investment value, the solar addition is exempt from property tax calculations.

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About clarkrichardson

Clark Richardson Architects practice design with the goal of creating contemporary architectural experiences through form, sustainability and detail. We specialize in green building, sustainable residential and commercial design and have architectural studios in Santa Fe and Austin and a design studio New Orleans.
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