We looked at the numbers to see just how far solar power can go.
An Accessory Dwelling Unit (ADU), guest house, or backyard cottage, even when built to the minimum standards of the current building code, makes efficient use of materials by maximizing the use of space. At New Avenue, we are always interested in taking a few extra steps in sustainability, and one way to do that is to invest in solar photovoltaic panels to create a net zero-energy home (Net Zero-Energy Habitat for Humanity Home. poptech.org).
We’ve struggled with the fact that a number of “green,” “sustainable,” “net zero-energy” homes are energy-inefficient to begin with, and then the owners or designers just cover them in panels when they really could have done better. Often, they focus on offsetting the electric load of the home and ignore the energy used from natural gas and other sources.
A bit of background
Any well-planned solar array will be connected to the power grid so that when the solar panels are producing more power than the home needs, the electric company will buy that power, and when it’s dark and the panels are not producing energy, the home can use the power supplied from the electric company. Our goal is to design a net zero-energy solar array that will cut the owner’s energy bill to zero or even make the owner money from the extra energy produced. Therefore, the electrical grid will serve as our battery and storage system (Grid tied PV system. flgoesgreen.com).
Crunching the numbers
A solar array doesn’t just help your checkbook on a monthly basis; it also has a positive environmental impact. Adding solar power will add to your initial investment, but New Avenue is happy to work with you to determine how long it will take for the solar array to pay for itself. For example, one Kyocera KD250GX-LFB2 solar panel costs around $375 and will produce about 370 kWh of electricity over a year’s time in the San Francisco Bay Area, saving the homeowner about $84 per year in electricity and paying for itself in less than 4.5 years.
Predicting the performance of a photovoltaic (PV) array comes down to determining how many hours of sun a location gets during a year, how much power the panel is rated to produce during test conditions, and how much the power is expected to decline in real-world conditions.
In planning for a net zero-energy home, we must make a few assumptions based on the following data points:
- The U.S. Energy Information Administration states that the average home in California uses 567 kWh of energy each month.
- The average heated home square footage in California is 1,180 sq. ft.
- The Bay Area has approximately 5.4 hours at “1 sun” equivalent of sun each day. (Solar Insolation Map of USA. National Renewable Energy Laboratory)
In designing a PV system for an average 500 sq. ft. ADU in Berkeley, CA, we estimate that the home would use 2,883 kWh of energy a year. Making the ADU a net zero-energy home would require approximately 1,950 W of installed capacity. Eight Kyocera KD250GX-LFB2 panels would produce 103% of the energy required.
(National Association of Certified Home Inspectors)
Using information from the above graphic, and assuming these statistics represent traditional technology, we can make further improvements to the energy efficiency of the ADU:
- Using a heat pump for space heating brings the total energy load down to 1,960 kWh per year.
- Include a heat-pump water heater and an efficient air-conditioning unit, and the energy load drops to 1,470 kWh/year.
With these improvements, the home will only require about 1,000 W of installed capacity, or four solar panels—a savings of 50 percent. Using New Avenue’s top-of-the-line insulation, precision construction, and efficient lighting will reduce the energy load even further.
The bottom line: solar panels are effective, but energy efficiency delivers more bang for the buck.
It is less expensive and lower impact to make the home more energy efficient than it is to add solar panels to compensate for less-than-optimal efficiency. Efficiency and solar are a very cost-effective combination, even better than solar power alone.
A German building process called Passive House (Passivhaus) is an excellent way to radically increase home efficiency. See our post on Passive House and what this building system can do for accessory dwelling units.
For detailed budgets of our projects and example floor plans or to use the free New Avenue project management system, you can sign up for free here: Get Started