Is solar viable in cloudy climates?

In many of America’s cloudiest cities, one thing is clear: their residents are producing tons of solar power.

Cloudiness and solar energy may seem incompatible, but the truth is that solar panels can provide tremendous value even in cloudy climates. The most famous international example of this is Germany. While getting about as much sun each year as Alaska does, Germany has become a global leader on solar. In 2015, its more than 1.5 million solar installations generated about 7.5 percent of Germany’s net electricity consumption – one of the highest shares of any country.

Similarly, the United Kingdom – in spite of its reputation for dreary weather – recently achieved an impressive solar milestone: a six-month period wherein solar panels produced more electricity than the country’s coal-fired power plants.

As the U.S. continues its own path toward becoming a solar powerhouse (it currently ranks 4th globally for cumulative installed capacity), more and more of that solar power is coming from parts of the country you also might not expect. The growth in solar isn’t just happening in famously sunny cities like Los Angeles and Phoenix, but also in areas with a cloudier reputation – from Boston to Burlington to upstate New York.

How is that possible, you ask? On an overcast or cloudy day, what happens to solar power production? And in regions like the Northeast, what results can solar customers expect over the course of a year? Let’s shed some light on the subject.


A glorious cloudless sky – bursting with uninterrupted sunlight every hour of the day – is a dream scenario for a solar panel. But it’s often unrealistic. And by no means is it necessary to produce solar power.

Here are some key reasons that solar panels can thrive even in cloudy conditions.SolarCity customers in the Albany, NY area have produced enough electricity to power the Empire State Building for a full year

-Even with a cloudy or overcast sky, some usable sunlight is getting through.

There are a few important types of sunlight that your solar panels can convert to electricity.

The most straightforward and most powerful fuel for a solar panel is “direct sunlight,” which arrives in a direct line from the sun. But solar panels also use “diffuse sunlight,” which is sunlight that has collided with other things in the sky – such as clouds, haze, and dust – and as a result has been broadly scattered before reaching your roof. There’s also “reflected sunlight,” which is sunlight that has bounced off buildings or the ground before hitting your solar panel.

Although diffuse sunlight is less intense than the direct kind, it does get converted to electricity – just less electricity than what would result from pure direct sunlight. For example, on an overcast day (when most of the sunlight hitting your rooftop is diffuse), a solar panel might produce only 20 percent of its maximum capacity. This isn’t ideal, but not insignificant either – and important to the overall economics of a solar power system.

-What really matters is solar panels’ total annual output, which can be large in spite of extended cloudy periods.

A cloudy month or even a cloudy season does not eliminate the value of solar panels. Rather, a solar rooftop’s success is based on how much it reduces your overall use of costly grid electricity. And it makes the most sense to evaluate that on an annual basis, rather than daily or monthly, since even the cloudiest climates get several clear days over the course of a year. As we’ll see, many cloudy areas in the U.S. get more than enough sunlight (direct, diffuse, and reflected) to produce more than a thousand dollars worth of electricity each year, while also preventing thousands of pounds of carbon pollution.

Moreover, the technology has arrived to use solar-derived electricity even when the sky is cloudy, overcast, or even pitch black. By using a battery that stores excess power during sunny periods, you can tap into that stored energy during times when your solar panels aren’t able to provide enough power.

-In some cases, a cloudier climate can actually be correlated with more solar power production.

Here’s an interesting fact: a solar panel in famously foggy San Francisco can produce slightly more energy per year than a solar panel in sunnier and hotter Sacramento. A key reason is that San Francisco has relatively cooler temperatures, while still getting a healthy amount of sunlight over the course of the year. That’s good news for solar power, because solar panels actually operate more efficiently in cooler conditions due to the nature of semiconductor materials. So although Sacramento gets more sun overall, its higher temperatures limit solar panel output. (Hat tip to for inspiring this example.)A typical solar rooftop can produce more than $1,500 in San Francisco

SF and Sacramento are both excellent places for solar panels, but it’s noteworthy that the cooler and foggier location can be slightly more conducive to solar power.

Running the numbers

Now that you know why a cloudy climate need not be a solar killer, let’s look at some specific examples in the northeast United States.

According to data from the National Oceanic and Atmospheric Administration, the cities in the table below experience cloudy or partly cloudy conditions on more than 260 days per year – that’s an average of more than 7 out of every 10 days. Yet the typical solar rooftop in each of the cities still receives enough total sunlight to produce well over $1,000 worth of electricity each year! In all these cities, that amounts to more than 80 percent of an average home’s annual electricity use.

Solar rooftops in cloudy cities:
How much energy can they produce? 
City Cloudy + partly cloudy days/yr  Estimated yearly $ value of electricity from
6-kW rooftop system 
Burlington, VT 307  $1,281
Albany, NY 296  $1,359
Hartford, CT 283  $1,494
Concord, NH 275  $1,413
Newark, NJ 272  $1,147
Boston, MA 267  $1,506
Providence, RI 267   $1,518
Sources: Comparative Climactic Data (NOAA, 2015) and PVWatts Calculator (NREL), with standard assumptions & average 2015 state-level electricity prices from Energy Information Administration.
SolarCity Logo

The economic and environmental benefits of solar panels in the Northeast have not been lost on its residents. Rooftop solar has spread at an impressive rate among homes and businesses throughout the region. For example, the map below shows the growth in SolarCity customers around Albany, New York – a city whose 296 days of clouds don’t stop the typical solar rooftop from producing more than $1,300 worth of electricity per year.

solarcity-installation-map-albany-nyThe story is similar in the Hartford, Connecticut area, which gets 283 days of clouds annually, yet still captures enough total sunlight for a typical solar rooftop to produce more than $1,400 worth of electricity each year.

Growth of SolarCity installations in Hartford, CT area

Likewise, sunlight is fearlessly cutting through the fog in San Francisco, as we highlighted above and also in our recent analysis of solar’s bipartisan popularity. The typical solar rooftop in SF can produce more than $1,500 worth of electricity per year.

The impact of SolarCity customers in cloudy cities is already sky-high, and much more potential remains

In each of the cloudy cities we’ve mentioned, SolarCity customers are already generating huge amounts of energy and doing the right thing for the environment.

For example, our customers in the Albany, New York area – which sees cloudy conditions more than 8 out of 10 days, on average – have already produced enough rooftop electricity to power the Empire State Building for a full year.

Just down the road in Massachusetts (whose capital city of Boston gets 267 days of clouds of year), SolarCity customers have produced enough electricity to power one of the state’s most prestigious universities – the Massachusetts Institute of Technology – for a full year. You can read our calculation notes for details.

But, of course, much more potential remains. As more people learn that solar panels can be effective even in cloudy climates, get ready to see more homes and businesses installing them.

Whether you live in a cloudy city or a sunny city, get in touch with us to find out how much electricity your rooftop can produce. There’s a good chance you’ll be pleasantly surprised.


Methodology and Calculation Notes

Cloudiness and Energy Value Table: Days of cloudiness is reported by NOAA’s Comparative Climactic Data. “Cloudy” denotes 8/10 to 10/10 average sky cover. “Partly cloudy” denotes 4/10 to 7/10 average sky cover. Dollar value of electricity uses annual kWh production based on results from NREL’s PV Watts solar model (with default assumptions for a 6-kilowatt system), using the closest TMY2 weather file for each city. To compute total dollar value, we multiply annual kWh production by average state-level residential electricity prices (rounded downwards to be conservative) for 2015 from Energy Information Administration. The same approach is used to estimate the annual dollar value of electricity for a rooftop system in San Francisco.

Empire State Building comparison: In 2011, the annual electricity consumption of the Empire State Building was reported to be 55 million kilowatt-hours. Since then, the building’s ambitious energy efficiency efforts suggest that its current consumption may be less than that. Solar power systems deployed by SolarCity in the Albany, NY area — which is broadly defined as the upstate region of New York that is north of Newburgh, south of Glens Falls, and west of Utica – have cumulatively produced more than 58 million kWh through June 30, 2016.

MIT comparison: In 2015, MIT reported its annual electricity consumption to be 108.8 million kilowatt-hours. Solar power systems deployed by SolarCity in Massachusetts have cumulatively produced more than 132 million kWh through June 30, 2016. Incidentally, MIT itself has demonstrated a strong, admirable commitment to solar power in recent years.


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