What is the Duck Curve?

Learn about the duck curve and how solar can help balance hourly energy loads.

In 2013, the California Independent System Operator published a chart that is now commonplace in conversations about large-scale deployment of solar photovoltaic (PV) power. The duck curve—named after its resemblance to a duck—shows the difference in electricity demand and the amount of available solar energy throughout the day. When the sun is shining, solar floods the market and then drops off as electricity demand peaks in the evening. The duck curve is a snapshot of a 24-hour period in California during springtime—when this effect is most extreme because it’s sunny but temperatures remain cool, so demand for electricity is low since people aren’t using electricity for air conditioning or heating.

The duck curve represents a transition point for solar energy. It was, perhaps, the first major acknowledgement by a system operator that solar energy is no longer a niche technology and that utilities need to plan for increasing amounts of solar energy. This is especially true for places that already have high solar adoption, such as California, where one day this past March, solar contributed nearly 40% of electricity generation in the state for the first time ever.  

Utility Challenges

California Independent System Operator

High solar adoption creates a challenge for utilities to balance supply and demand on the grid. This is due to the increased need for electricity generators to quickly ramp up energy production when the sun sets and the contribution from PV falls. Another challenge with high solar adoption is the potential for PV to produce more energy than can be used at one time, called over-generation. This leads system operators to curtail PV generation, reducing its economic and environmental benefits. While curtailment does not have a major impact on the benefits of PV when it occurs occasionally throughout the year, it could have a potentially significant impact at greater PV penetration levels.

While the mainstream awareness of these challenges is relatively recent, the U.S. Department of Energy’s Solar Energy Technologies Office (SETO) has been at the forefront of examining strategies for years. Most of the projects funded under SETO’s systems integration subprogram are performing work to help grid operators manage the challenges of the duck curve.

Duck Curve Solutions

Using Storage to Improve Grid Resiliency

Learn about SETO's project with Austin Energy.

Solar coupled with storage technologies could alleviate, and possibly eliminate, the risk of over-generation. Curtailment isn’t necessary when excess energy can be stored for use during peak electricity demand. SETO launched several projects in 2016 that pair researchers with utilities to examine how storage could make it easier for utilities to rely on solar energy to meet customer needs around the clock. This research will enable even more solar energy to be integrated into the grid, while tackling the obstacles utilities face when incorporating solar.

In 2012, SETO also launched a research program that helped utilities, grid operators, and solar power plant owners to better predict when, where, and how much solar power will be produced. More accurate solar power predictions, known as forecasts, allow utilities and electric system operators to better understand generation patterns and maximize solar resources. One key success came from IBM, whose machine-learning technology enabled prediction accuracy to be improved by 30%. However, as the amount of solar energy generation connected to our electric grid continues to grow at a rapid rate, further improvements in predictive accuracy will be needed.

Bringing it Back to the Duck Curve

There are many potential solutions to the duck curve. The lessons learned from SETO’s projects will be critical to improving the flexibility of the grid and addressing over-generation risks as solar grows throughout the country. According to the Energy Information Administration, the installed amount of PV is expected to triple by 2030—potentially migrating the duck curve outside of California. New and improved technologies will allow PV to provide on-demand capacity and fulfill a greater fraction of total electricity demand.

Learn more about our work to improve grid integration.

Generating your own electricity with solar photovoltaic (PV) panels works anywhere in the U.S. year-round. There’s plenty of sunshine for PV, even in winter, although at a slightly lower production level. Germany leads the world in solar power output and it doesn’t have a sunny climate. Its solar radiation is about the same as Alaska’s. So, cloudy days will come and go, but on average, it’s not going to affect the return on investment of solar panels significantly.

#1 Solar’s never been cheaper
Over the past ten years, PV panel prices have dropped more than 50 percent. Concurrently, solar leases and power purchase agreements have made getting solar easier—with little or no upfront costs. As for money, there are many new options for long-term, low-interest loans for clean energy and many states, utilities and others offer a variety of tax credits, rebates and incentives.

#2 Solar saves on utility bills
Solar gives immediate utility bill savings, but long-term savings are becoming less predictable. In Hawaii, California and other states, utilities are changing to time-of-use rate programs for homes with solar that charge more for electricity consumed during peak use periods (evenings) and less during off-peak periods (midday). Still, having solar will mean paying less than not having it, but maximizing savings may require adding energy storage to shift your electricity load.

#3 The federal investment tax credit for solar is still on
Congress renewed the 30 percent credit with no reduction until the end of 2019. On a $20,000 system that’s a $6,000 savings. Increasingly, states and local utilities are offering additional tax incentives and direct rebates.

#4 Net energy metering is now available in most states
Net energy metering means utilities compensate you for excess solar generation. You get credit to use utility-supplied power when your system isn’t producing enough solar power, like at night or on very overcast days, or your utility may offer cash for your contributions to the grid. Again, future utility rate structures, such as time-of-use billing, may reduce the value of credits.

#5 A good return on investment
You’re likely to produce 75-90 percent of the electricity you need and save $100-$200 per month, which means your system could pay for itself in 7-10 years. Because PV systems operate effectively for 20-30 years or longer, it adds up to secure long-term savings and increased home value.

#6 Solar increases your home’s value
Depending on where you live, solar PV panels can add up to $15,000 to the value of your home, according to a study by the U.S. Department of Energy’s Lawrence Berkley Laboratory.

#7 It’s good for the environment
A typical residential system will eliminate around 4 tons of atmospheric carbon emissions each year. That’s equal to about the annual amount of greenhouse gas emissions from one gas-powered car.

Bottom line on solar
Although the long-term value proposition for solar PV can shift with future changes in utility billing structures and rates, there really aren’t any substantial reasons not to go solar now.

Ready to get serious about solar?
Learn about installation costs, available incentives and more at CSE’s Solar Energy Center.

Nationwide, solar photovoltaic (PV) systems are being installed in record numbers, with the total solar capacity approaching 50,000 megawatts of power. According to the Solar Energy Industries Association, that’s enough solar output to run more than 9 million homes.

Even with this solar proliferation, and that the cost of PV panels has fallen by more than 55 percent in the past five years, lower-income earners–especially those who reside in multifamily rental housing–have largely been left out of the clean energy boom.

Low-income renters often are disproportionately affected by energy costs, paying up to four times the proportion of their income on energy relative to their higher-income peers. And, since they don’t own their homes, they can’t buy into solar the way homeowners can, with favorable lease agreements, large government incentives and special loan offerings.

Thankfully, a variety of programs and resources for on-site solar installations are providing avenues for increased solar access for apartment residents and offering potentially greater profits for multifamily property owners. In California, multifamily residences account for 30 percent of all households.


Bringing solar to low-to-moderate income tenants in apartments presents a variety of challenges. Foremost is that most properties have a separate electricity meter for each residence, plus one for powering the facility’s common areas and exterior lighting. It’s impractical to connect each meter to its own solar system.

However, in many states a special utility billing arrangement, called virtual net metering, allows energy generation credits from a single solar system to be effectively shared by accounts with separate meters. Both property owners and their tenants receive credit for a percentage of the PV output and realize direct utility bill savings.

CSE estimates there are over 250,000 multitenant residential and commercial properties in California that could take advantage of solar virtual net metering, yet only a few hundred are doing so now.


For tenants, virtual net metering provides long-term utility savings and greater electricity rate stability. Typically, tenants receive a proportionate share of monthly solar energy in the form of kilowatt-hour credits on their bill, ideally enough to more than cover any increase in rent that the property owner may apply to recoup the investment.

For the property owner, solar on multifamily buildings can offer significant savings in operational costs. Multifamily property owners typically spend about 10 percent of annual operating costs on energy, which can be cancelled out by on-site generation. With solar, once the initial investment is recovered, typically over 5-10 years, property owners have essentially free energy for 10-15 years or more.

In addition to virtual net metering savings, property owners can take advantage of various solar loan programs, low-cost leases, property assessed clean energy (PACE) programs and other financial mechanisms to pay for system installation, as well as the potential to collect a 30 percent federal investment tax credit and accelerated depreciation allowances that can greatly reduce costs. Even nonprofit multifamily housing owners, who may be ineligible for federal tax benefits, can monetize the tax credit and depreciation though a solar lease or power purchase agreement with a third party that can claim the deductions and may offer a discounted price.


CSE is engaged on several fronts to break down barriers to solar for multifamily housing. Through a $712,000 award from the U.S. Department of Energy SunShot Initiative, CSE is leading a project to expand use of virtual net metering. We have developed a set of toolkits for tenants and multitenant property owners that guide owners and residents through the solar process.

At our Solar Energy Center’s Multifamily webpage, you can click through a series of fact sheets that provide basic information about solar, system costs and incentives, financing and connecting with a solar contractor. To make contacting a solar contractor easier, we’ve linked up with the online solar marketplace EnergySage where you can enter basic information about your property and get competing quotes for a PV system from prescreened installers in your area anywhere in the U.S. CSE also teamed up with the Interstate Renewable Energy Council and Spark Northwest in creating guidelines for local governments and housing providers in the Seattle, Wash., area outlining ways to enable greater solar access for renters and multifamily residents and low-to-moderate communities.

Supporting policies and programs for PV installations at multifamily housing can provide equal solar energy access for all consumers, increase employment in renewable energy, decrease greenhouse gas emissions and help build a more resilient electrical grid.

The greening of California’s electric grid gets a lot of attention – we’re committed to 50 percent renewable energy by 2030, with some state legislators pushing to increase that commitment to 100 percent by 2045. This bold leadership on reducing fossil fuel use and climate action planning from the world’s sixth largest economy is encouraging.

However, electricity is only one piece of the puzzle. According to the California Air Resources Board, natural gas consumption in residences and commercial buildings accounts for nearly 10 percent of California’s total greenhouse gas (GHG) emissions. The lion’s share of this natural gas use is for space and water heating. If we want to achieve our ambitious GHG reduction goals, we need to find a better way to heat our water and indoor areas.

Recognizing this, Governor Jerry Brown signed AB-797 (Irwin) into law on October 4, extending the California Solar Initiative Thermal Program for an additional two years, through the end of 2019. The program provides rebates for qualifying solar water heating systems for single-family and multifamily homes, commercial and industrial properties and commercial swimming pools.

An oldie, but goodie

Solar water heating is not new. The first patent for a solar water heating system in the U.S. was filed in 1890, and in 1979, President Jimmy Carter put solar water heating on the White House. It’s a tried and true technology, which uses heat from the sun to warm water, rather than create electricity. Over the past four decades, its adoption has shifted with the rising and falling of natural gas prices, but for homeowners and business owners who installed solar water heating, their utility costs savings have continued.

The state’s solar thermal market is growing, especially in the multifamily housing sector, according to the California Solar Energy Industries Association. Almost every business uses hot water, whether it’s for hand sinks and showers or high-volume commercial dishwashers, heavy-duty laundries or industrial processes, and solar water heating helps achieve profitable and sustainable operations.

Rebates make the difference

With natural gas so cheap today, the economics of installing a solar water heating system often don’t pencil out – that’s where the rebates come in.

Combined with the federal investment tax credit of 30 percent for systems in service by the end of 2019, CSI Thermal rebates can reduce the out-of-pocket expenses for a single-family homeowner to under $2,000. A residential system, which will typically last for 25 years, can pay for itself through reduced natural gas bills after around 10 years. For larger multifamily and commercial systems, customers who make use of the federal tax credit and CSI rebates of up to $800,000 per installation, are recovering their costs in as short as six years.

By encouraging market activity, these rebates also promote competition among manufacturers and installers, driving both innovation and cost reductions.

Higher rebates for low income

AB-797 also calls for a 50 percent carve-out of the program’s total budget for low-income and disadvantaged communities, in addition to higher rebate levels. This helps ensure these communities, many of which have disproportionately borne the brunt of air and water pollution, are able to benefit from clean energy resources and lower utility bills for water heating.

Learn how you can save

CSE administers the CSI Thermal Program for the California Public Utilities Commission in the San Diego Gas & Electric service area. Visit our CSI-Thermal website to learn more about solar water heating and available rebates.




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