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  • Lorianna Kastrop

Installing a Solar Power System for New Residential Construction

By D. Michael Kastrop, A.I.A., Principal Architect and Lorianna Kastrop, VP/CFO, The Kastrop Group, Architects

Adding solar energy systems to new and existing buildings is an important step in reducing our reliance on fossil fuels (coal and natural gas) and less environmentally-friendly energy sources such as nuclear power plants. Building codes are changing as more communities require solar power to be used in construction to reduce loads on an aging and inadequate power grid infrastructure. There are tax incentives available to encourage homeowners to add solar power to their residences. Residential solar power is big business these days. Installers are very familiar with and competent at retrofitting existing homes with solar panels and systems. They can use the past year of bills from your existing power company to determine the amount of energy you typically use. Unfortunately, that data is not available when you are building a new home that is primarily going to be solar-powered.

When we started building a 699 S.F. Accessory Dwelling Unit in our backyard, the solar companies bidding on the project were very erratic in determining system size and battery capacity. That meant that their bids were VERY different in cost and energy production. It seemed to us that they didn’t know how to use the architectural plans that they received to determine the projected power use.

The following is information on lessons learned from this first-hand experience. We hope that these will help you avoid costly problems with your new home or ADU construction.

Start with determining your goals. A common target would be to have the solar system provide 50% to 75% of your electrical needs in a new residential building. If you want to go to 100% and be able to be off the grid, the cost increases quite a bit. Also, decide if you want to pair your solar panels with a home battery. That adds cost, but it allows your home to have power during outages. Depending on weather, and if you reduce load by not using appliances that are non-essential, you could stay powered by your home battery while waiting for the power company to re-establish service. That could be a real benefit in an emergency. Determine what appliances and systems are critical and how long they will be needed if the power is off. For example, you will probably want your refrigerator, overhead lights, the microwave, and the HVAC system. In our casita (ADU), we will not use the washer/dryer and the electric range during a power outage, as they will be big drains on the battery. You may be required to have a separate electrical meter from the main house. This is usually required if the ADU will have a separate address or suite number.

Ask your architect to design your ADU with the maximum south facing roof area possible. Think about where the home battery will be installed so that it is in an inconspicuous place. Consider existing landscaping. Will trees need to be trimmed or removed to allow the sun to hit the roof? It is also important that your ADU is very energy efficient. Specifying additional insulation, LED lights, and good quality windows are a few of the details that your architect can include for energy efficiency.

The solar companies are almost all geared toward selling and installing in the residential market as a retrofit and it is difficult to get them to shift gears to do new construction. The first thing they will ask you for is your electrical bill for the past 12 months. After talking to several companies and getting basically nowhere, we ended up finding EnergyPal. They are basically a broker that represents several solar companies. Marcus Joo at EnergyPal worked with us to get the right fit for our project.

We received bids from three solar companies. These bids do not reflect an expected 26% solar tax credit when we file our federal tax return for 2021. They also do not include the Self-Generation Incentive Program (SGIP) rebate available from your California energy provider; for us that is Pacific Gas & Electric (PG&E). The SGIP rebate is approximately 15-20% of the solar home battery cost (not the panels). We must apply for that rebate ourselves. So, the following bids are what gets paid up-front to the solar company before any tax credits or rebates that might bring down the net price of the system.

Luminalt Solar Energy Solutions bid $17,315 for a Tesla Powerwall battery storage system and installation. On top of that they bid $26,765 for 11 Sunpower photovoltaic solar panels and inverters, with a power rating of 4.51 kW system. That’s $44,080 for the total system. Their bid showed that as being enough energy to power our main house of 2,000 S.F., not our little casita.

The bid we received from Sunrun was more affordable, but the system was even bigger. It had 16 LG panels, and 1 inverter, and 1 LG battery for a 5.68 kW system. The price was $29,867.

The last bid was from Sunpower. The price was $24,380 for a much smaller system of 2.68 kW. It has 8 panels, 1 inverter and the SunVault storage system (battery).

We chose Sunpower not just based on price, but on the more closely matched power production based on the expected usage. In addition, Sunpower has been a solar power manufacturer since 1983 and is based in San Jose, CA, which avoided some of the major supply chain problems experienced during the pandemic in getting panels from other parts of the country or overseas. Their warranty is 25 years and includes replacement/repair and labor. They make their own SunVault battery that has a greater capacity than the Tesla Power Wall and can handle more of the home circuits. We designed our roof to be able to fit up to 12 solar panels, but the design only required 8 panels and a single battery ….and yes, we still had to provide a copy of our main house’s utility bill to get the process started, even though our main house was not being powered by their system!

Solar panels usually produce between 250 and 400 watts of power. Sunpower’s panels rate 380-400 kw per year. There are variables based on your roof orientation, shading and hours of sun (time of the year). Our system for our ADU is designed to provide 3,157 kW per year with our estimated usage of 3,684. This covers 86% of the projected use.

Batteries: We looked at two batteries. The Tesla Power Wall limits you to four 20A circuits and then you buy another Power Wall if you need to expand. It is our understanding that the Power Wall requires recharging when it is down to 30% capacity, and we wanted one that will drain down farther in an emergency outage. (We live in earthquake country, after all.) The SunPower SunVault system has a single battery that handles all eight of our ADU’s 20A circuits in power outages. We are still learning the system, but we expect to be able to set our power preferences to apply the solar power and battery to engage during peak billing times so that we will be “off-the-grid” when power is most expensive.

The solar battery installed at our ADU.

Timing: Plan ahead and order as soon as you can in the construction process. The solar company will do their own drawings and apply for their own building permit, separate from the permit for the ADU construction. This takes a long time! They also will set up their own inspections with the Town/City Building Department. We signed the contract with the solar company on January 21st and they are finishing up the final installation steps on July 27th.

Construction: Try to get the solar company installer to work with the ADU electrician subcontractor to make the installation smoother. (Remember, they always think retrofit, so explanations by the electrician may help.)

Permitting: We found out that the Fire Department now requires an emergency shut-off for the solar system in the event of a fire so that emergency personnel can turn off all the power. The solar company was not aware of that. It also requires special signage at the home battery. This signage is to alert first responders that there is a solar system and battery that will still have an electrical current even with the main switch shut off. This emergency shut off switch requirement does change from area to area, but it is a great idea to have one installed.

As you can see, the most important lesson from this experience is that if you are doing new construction you need to do some homework with your architect and your solar energy supplier to right-size the system for your actual projected energy use. (And try to stop them from looking at the PG&E bills from a different house!!!)

Thanks for reading, and as always, we are Designing for Your Reality.

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