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  • Writer's pictureJacopo Montalenti

Residential solar panels - Bay Area. Unveil the secret power of the sun in five steps!

San Francisco and the bay area are aiming towards zero net energy. Independence is the key to unlock the true value of your home.


1. The first step is to establish the electrical consumption of your home. The U.S. Energy Information Administration (EIA) set the average usage in California between 550 and 600 kWh/month, according to 2016 data. For the Bay Area, California Public Utilities Commission (CPUC) data from 2016 gives a range of 375–425 kWh/month. Consequentially we can calculate an average daily consumption of, let’s say, around 15 kWh/day (450 kWh/ 30 days). This figure is slightly lower (12–13 kWh/day) for new green homes and slightly higher (17–19 kWh/day) for older homes.


2. Unfortunately, the sun doesn’t shine with the same intensity everywhere! This factor is called “average solar insolation” (ASI) and is measured in kWh/m2/day. In San Francisco & the vicinity, it ranges between 4.0 and 5.5. (data from the National Renewable Energy Laboratory, average over an 8-year range 2005-2012). Therefore, each day, the sun projects' quantity of energy onto 10 ft2 of the Earth (1 m2 = 10.7 ft2) is about 4.75 kWh.


3. Today’s solar panel technology can capture only a fraction of that total energy. That fraction is called “efficiency.” Solar panel nominal efficiency ranges between 15% and 21% in optimal conditions, and they usually have a surface of about 17–18 square ft. An 18 square ft panel will thus receive an ASI of 9–10 kWh per day (4.75 over 10sf), and you can expect to capture at best (i.e., with the most expensive panels) on average 1.8 kWh/day in optimal conditions (20% nominal efficiency).



4. Optimal condition & nominal efficiency refers to the angle at which the panel faces the sun. The ideal situation would be for it to tilt and rotate following the sun's orbit as it changes from morning to night (as well as in winter and summer), but we know that this is not what will happen on our roof! We usually have to compromise and install the panels in a fixed position. The best-fixed position in the bay area will be facing south, with a tilt angle between 35 and 40 degrees. (tilt angle is the angle a panel has over the ground level, think about the degree of a sloped roof). Unfortunately, this will result in only slightly more than 2/3 of your nominal efficiency; in fact, it will lover the efficiency of an expensive 20% panel to a realistic 14%. (And down to 8-10% for a cheaper 15% efficiency panel)


5. Finally, we can divide our daily consumption of energy, 15 kWh, by the real kWh produced daily by our single panel, 1.2 kWh (14% of 9.5 kWh ASI over 17sf area). This calculation will result in a recommended array of at least 10 high-tech panels per new, green home and 14 to 16 for existing homes in the Bay Area. This will cover average consumption, but you will need more panels for total independence from the grid. Unless you plan to store energy in a battery, the peak of energy production and energy usage will not align. (I will do an article about store energy and home battery in the future).

I feel compelled to add that it’s true that the solar panel is most efficient when facing south if your goal is to produce energy and store the energy in a home battery. But there are reasons to point them south-west (or west) with a higher tilt (45-60 degrees). The main reason is that during summer afternoons, people tend to turn on air conditioning and appliances when they come home from work/school (afternoon), and a south-west facing panel will produce the most energy right at the peak of energy usage.



This advantage is not so significant in the temperate climate of the Bay Area, where there is not such an intense need for air conditioning, and hottest days don't always coincide with the best summer sun angle.

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