When discussing solar energy systems, the first question I often get is “Are you off the grid?” To which I reply, “No. I wouldn’t want to be; our solar array produces more …
When discussing solar energy systems, the first question I often get is “Are you off the grid?” To which I reply, “No. I wouldn’t want to be; our solar array produces more energy than we use, so we’re ‘net-positive’.”
There’s often confusion about how solar energy works. This month, I’ll try to clear up some of that and explain the whys behind my answer.
But let me clarify, this discussion is limited to small-scale solar photovoltaic (PV) systems that you would typically find on a house roof, or on a small business. Community solar (popular in New York but not approved in Pennsylvania yet) and utility-scale solar are somewhat different, especially in economic and regulatory aspects.
The photovoltaic effect was first discovered in 1839 by a 19-year-old French physicist, Edmond Becquerel, when sunlight shining on his invention produced an electric current. Becquerel went on to discover other electrical effects and photographic processes, but his solar cell never “saw much light,” as it was impractical outside of a chemistry lab.
In 1905, Albert Einstein finally explained the physics of how and why PV works, for which he was awarded the 1921 Nobel Prize.
Only since 1954, when Bell Telephone Laboratories publicly revealed the first practical solar cell using a silicon semiconductor, have we had solar electricity without fluids or moving parts.
Solar PV panels are generally a series of silicon solar cells, soldered and laminated under tempered glass, then bound within a frame. The electrical generation capacity (measured in watts) of a solar panel varies with its exposed surface area, light intensity, temperature and quality of the semiconductor material. So modern PV panels benefit hugely from manufacturing advances driven by the electronics industry’s need for greater purity for its silicon chips.
PV panels produce direct current (DC) electricity. But most of the electricity we use, and that a utility supplies to homes and businesses, is alternating current (AC). Therefore all “grid-connected” PV systems will also have one or more inverters to convert DC to AC; but in that process—and in the wiring and other components—some power is lost as waste heat.
This, plus the fact that sunshine intensity hitting a solar cell depends on its orientation and shading, on the weather and by the season, means the solar panel capacity cannot just be added together to determine a PV system’s AC power output, measured in kilowatts; nor determine its electricity production, which is measured in kilowatt-hours (kWhs). The National Renewable Energy Labs has a comprehensive web-based modeling tool, PVWatts, that forecasts PV system production using local historical weather data, a system’s location and design parameters.
When a solar PV array is not connected to a building’s electrical system, it’s at the same time considered “off-grid,” e.g. walkway solar lights and phone and battery chargers. But when any type of energy generation is connected to a building’s electrical system while it is at the same time interconnected to an electric utility, then it’s grid-connected and is required to abide by the rules of that utility (usually as approved by state regulators). Construction of a solar array in most municipalities also requires a building permit, so it must comply with building codes (fire safety, electrical, structural etc.), and may also be subject to zoning regulations or other rules, as in a historic district or homeowner association.
Solar energy displaces other (usually dirtier) fuels, reducing local pollution and global climate change. The direct benefit to a solar array owner is the energy it produces from free sunshine, reducing or eliminating energy purchases from other sources. Like all electricity generators, solar PV will power the nearest connected load, whether that is house lighting, fans, a refrigerator or a heating system.
But sunshine is highly variable, so off-grid solar systems must also include expensive batteries to store electricity for use at night, and hopefully also during tomorrow’s storm. And once the batteries are full, any electricity produced goes to waste, and batteries have slow drainage losses.
Additionally, all solar systems have much lower production during winter. So off-grid homeowners must be frugal with their electricity use, or significantly oversize their solar array and batteries to meet critical winter needs.
On the other hand, grid-connected solar arrays can operate without batteries; sending any unused electricity out to the utility grid and drawing from the grid at night or whenever needed.
Most utilities implement a policy (required under PA and New York state laws) called net-metering, where any electricity exported to the grid effectively rolls the utility meter backward, meaning that the array owner gains a credit equal to that same number of kWhs. In turn, it sells those exported kWhs to others.
On my utility bill, PPL calls these credits a “renewable energy bank” (other utilities may use different labels), but this bank has units of kWhs, not dollars. Our home solar array produces lots of excess electricity from March through November, and PPL “saves” that in our bank until winter, when we use a lot of electricity for heating. Effectively, PPL’s grid is a nearly free and loss-less battery!
We don’t need to buy gasoline for our all-electric Bolt, nor any kWhs from PPL. We do pay PPL approximately $15 every month for being connected to their grid.
Each May, PPL zeros our bank balance and sends us a check at their Price-To-Compare for those exported kWhs (rules differ in New York and New Jersey, and for non-investor-owned utilities in PA). In our case, it doesn’t make up for the year of monthly fees, but it comes close.
Our grid-connected solar array also has batteries, sized to get us through a moderate grid outage, not to live off-grid all the time. That would be a big lifestyle change. No, thank you.
Learn more at www.energy.gov/eere/solar/homeowners-guide-going-solar.
Jack Barnett is a retired electrical engineer, and is now a volunteer solar energy and sustainable living advocate on the board of SEEDS of Northeastern PA (www.SeedsGroup.net). He is also the co-founder of the Clean Energy Cooperative (www.CleanEnergy.Coop).
Have questions about solar energy? Send them to email@example.com and Jack will attempt to answer your questions in future Sustainability articles.
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