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The Solar Homeowner’s Guide To The Duck Curve


We expect to have electricity available for us 24/7, so the utility company needs to be able to meet consumer demand all the time. However, power consumption shifts based on what people are doing, so it increases and decreases throughout the day.

Therefore, there is a balancing act between supply and demand. Grid operators must ramp up electricity output during the day and evening and then dial back production at night. Renewable energy sources can help supplement the grid, but don’t entirely cover the swings in demand throughout the day.

In order to visualize and understand how solar energy output impacts the demand curve, researchers developed the duck curve. We’ll explain what the duck curve is and how it might impact you.

What Is The Duck Curve Problem?

The duck curve is a chart that highlights electricity demand that rises and falls during peak and off-peak hours, as well as available solar energy throughout the day. California Independent System Operator (CAISO) published their findings about electricity demand in 2013. The report showed a chart that coincidentally is shaped like a duck – and thus, the duck curve got its name.

Duck Curve Chart: CAISO 2013 published findings about electricity demand.

Source: CAISO, accessed via the National Renewable Energy Laboratory.

Solar panels typically produce a lot of energy in the middle of the day, represented in part by the “belly” of the duck when demand is lowest. The head of the duck represents when electricity demand on the power grid rises in the late afternoon and evening hours. Solar power production tapers off when the sun sets, so as demand rises in the evening, grid operators must rely more heavily on other energy sources during these peak hours. The net load is the difference between the total demand on the electric grid and wind and solar energy production.

The duck curve exists most prominently in states with a lot of installed solar energy capacity from solar farms and rooftop solar, like California and Hawaii. It highlights the need to plan for the solar energy supply in areas with a lot of capacity because it impacts when specific power plants are cycled on and off to meet demand. Unfortunately in these locations, overproduction can be an issue when solar production is highest during the day. This can cause some of this solar electricity to be wasted if there is not ample energy storage capacity.

Why Is The Duck Curve Important?

Grid operators must balance electricity production with energy demand and use the load curve to anticipate power consumption. They often use peaker power plants to ramp up electricity generation to meet peak power demand. These “peakers” as they’re sometimes called, are specific power plants that are used primarily when demand is at an all-time high. Unfortunately, these facilities typically use fossil fuels such as natural gas. Thus, these power plants produce greenhouse gas emissions and pollute the environment.

Ideally, the power grid would use as much clean energy as possible. Renewable energy can supplement the grid during peak demand but at the current capacity, wind and solar do not cover the majority of demand.

One major hurdle to a more sustainable power grid is a lack of storage to harness excess, unused solar energy. This problem is called overgeneration and happens when more solar power is generated than the grid needs at the time. It is most common currently in areas with large installed photovoltaic capacity or at certain times of the year.

Rates are often higher in the late afternoon and early evening, especially in the summer, and lowest in the middle of the night and during the winter. Many utility companies use demand response to determine peak pricing, in an effort to encourage customers to limit power use during peak demand. For example, in some areas, customers are charged a higher cost per kilowatt-hour when demand is highest to encourage energy curtailment. As a result, residents will sometimes delay consuming power, such as charging an EV in the middle of the night instead of in the evening.

Batteries, Solar And Other Solutions To The Duck Curve

In addition to higher electricity rates during peak demand, there are other solutions for the duck curve. For example, solar power systems with energy storage allow homes to use electricity in the batteries during peak hours, which reduces the strain on the grid and the need to use peaker power plants. This also aims to reduce waste by capturing any excess power generated during times of high solar production.

Solar homeowners with batteries can save even more on their power bills in areas with time-of-use rates because they can draw power from the grid when prices are lowest and use either renewable energy or electricity from the batteries during peak hours. This helps flatten the duck curve by reducing the spike in demand in the evening, which is the head of the duck.

Likewise, utility-scale battery energy storage systems can help meet energy demand on a large scale. Many experts say that large-scale energy storage is critical for the energy transition because it enables more electricity generation from renewable sources. In addition, as storage technology advances, the cost of batteries is falling, encouraging greater clean energy use.

How Much Does Energy Storage For Homes Cost?

Residential battery energy storage is an added cost for a solar panel system. Although the battery bank can reduce your electricity bills, especially if you live in an area with time-of-use rates, storage is more commonly installed for greater energy independence and resilience.

The cost of a lithium-ion solar storage battery with a 10 to 14 kWh capacity starts at about $8,000, plus the installation cost according to EnergySage. The total cost of installing a battery when you go solar is typically between about $10,000 and $18,000, including the labor. Price depends on the battery technology used and how many batteries are installed, so work with your installer to determine how much your setup would be.

However, one battery probably can’t power all the loads in your home. Therefore, solar technicians usually install a critical load panel, which is like a second electrical panel. Vital loads from the home are connected to this second panel, like your fridge and hot water heater.

The battery then powers these critical loads and not everything in your house. Although the panel isn’t particularly expensive, it is time-consuming to perform this electrical work, increasing the labor costs associated with your solar system.

However, for eligible taxpayers, there is a federal solar tax credit available for 30% of the cost of installing a solar system; this benefit can apply to battery storage, too.

Also, some state and local governments or utility companies offer incentives for batteries, such as the Self-Generation Incentive Program (SGIP) in California. Please refer to the DSIRE website for information about battery incentives in your area.

What if you already have solar panels and want to add battery storage to it? It’s certainly possible and growing in popularity, but keep in mind you may need to replace your existing inverter, depending on the battery technology.

Some of the most common home storage batteries are the Tesla Powerwall, Generac PWRcell, sonnen eco, LG Chem RESU, Enphase IQ, and Panasonic Evervolt. Solar batteries typically include a manufacturer's warranty for around 10 years, which is shorter than most solar panel warranties. Often, solar batteries have a shorter lifespan than solar panels, so you might need to replace the battery after 10 or 15 years.

The Bottom Line: Solar Homes With Batteries Can Help The Duck Curve

Grid operators must ensure that we have reliable power around the clock. As more and more solar power systems are brought online, they need to balance demand with renewable energy production. In areas with a lot of solar energy production, the overall demand curve looks a lot like a duck, especially during certain times of the year.

The head of the duck represents when energy demand peaks during the late afternoon and evening hours, yet solar electricity production is tapering off. Energy storage systems can help even out this curve, reducing the need for peaker power plants and the overproduction of solar electricity.

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