What is the duck curve?

Here’s a super quick crash course on the electric grid: When you plug in a kettle or turn on your air conditioner, you add a little bit of load to the electric grid. This causes the voltage on the grid to drop by a tiny amount.

If enough people do that at the same time, the operators of the grid respond by asking power plants to generate more power. This also works the other way: if power plants are cranking out more electricity than customers are using, power plants are asked to dial their power output down, or even shut off completely.

Unless something has gone wrong, you’ll never notice any of this. It happens every minute of the day, every day of the year.

As you know, renewable electricity is great because it doesn’t rely on burning natural resources. While solar energy is sometimes called unreliable, that’s not really true. Solar output can fluctuate, but the daily general trend is that solar output is highest around noon, and tails off in the evening.

If you have a lot of solar panels on the grid, as is the case in California and Hawaii, the graph of electricity demand on the grid can have a distinctive shape known as the duck curve. It’s characterized by a peak in the morning (when people are waking up, making breakfast, and getting ready for school and work), a dip in the middle of the day when solar panels hit their maximum output, and a bigger peak in the early evening when residential demand spikes again and solar power output is simultaneously dropping off.

The shape of this curve, if you kind of squint and use your imagination, looks like a duck in profile. If you’ve heard of the duck curve, you might also know that it’s causing the operators of electric grids some concern.

Read on to learn more about why, and what role solar homeowners can play in solving the problem.

How does the duck curve happen?

If you were to draw a graph of demand on the electric grid, it has a predictable shape. Electricity usage is low overnight, rises throughout the day to an evening peak, and then falls again as people go to bed.

Unless it’s a cloudy day, a graph of solar power generation also has a predictable shape. It’s a symmetrical curve with its highest peak around noon.

One thing about residential solar power that’s different from other types of power plants is that the output can’t be curtailed. The utility companies don’t have any control over solar panels owned by homeowners, so when it’s sunny, any excess power that home solar panels generate gets dumped onto the grid.

This means that if there’s a lot of solar power being added to the grid and not enough demand, other power plants must be throttled back. Usually, it’s natural gas power plants that are asked to dial back because they can respond quite quickly.

That’s where the duck curve happens. Wikipedia has a helpful graph to illustrate this.

The blue line is the total load on the grid - that is, the amount of electricity being consumed at that point in time. The bottom grey line is a graph of solar power generation on a sunny day.

The orange line is the power generated by non-solar and wind sources. As you can see, it takes on the characteristic duck shape because of power plants that throttle down their output due to the availability of solar power in the middle of the day and ramps up when the sun goes down in the evening.

This mean that the duck curve isn’t a graph of solar power output, but the output of non-solar power plants in response to solar generation.

What’s the problem with the duck curve?

Thermal power plants (the kind that use fuel) can adjust their power output, but they can’t always do it very quickly. Hydropower or a modern natural gas-fired power plant can increase or decrease its power output in minutes, but changes take hours for a coal plant, or days for a nuclear plant.

If a local grid has a sharp duck curve in the early evening, grid operators need to respond quickly to ramp up power output or risk an outage. This can be a problem if there isn’t enough fast responding spare capacity available, or electricity that can be imported from neighboring grids.

This is why grid operators are starting to be concerned about the duck curve. They may not always have enough fast-responding spare capacity to provide the quick evening ramp-up that the duck curve demands. In Hawaii, there is so much residential solar connected to the grid that homeowners are discouraged from connecting new solar arrays to the grid unless they also have batteries.

What’s so great about batteries?

While the most advanced natural gas-fired power plants can adjust their output in minutes, lithium batteries can respond in milliseconds.

It might seem like science fiction to have a battery big enough to power a town, but that’s exactly what’s happening around the world. One of the first large scale examples is the Hornsdale Power Reserve in southern Australia. Built by Tesla to help make Australia’s grid more stable, it’s so good that smoothing out the electrical supply that it saved the Australian grid $79 million in just its first year of operation - and it did this with carbon-free power.

These so-called grid scale batteries are being planned and deployed around the world, including Nevada, California, and Hawaii.

How is this possible? The same market forces that made the price of solar the cheapest source of electricity in many places in the world is now bringing down the cost of batteries so much that utilites can buy them by the megawatt and still save money compared to traditional power plants.

What does the duck curve mean for solar homeowners?

If you’re a solar homeowner, the duck curve doesn’t have any direct impact on your home. However, it does make it harder for grid operators to balance the grid, and this can eventually affect you. You might experience more blackouts, or your cost of grid electricity may go up.

As a solar homeowner, is there anything you can do to help solve the duck curve problem?

One thing is to be aware of when you use electricity. If you have a time-of-use plan with your utility - and many solar homeowners are required to - then you already know that electricity costs more during peak hours. Even if you don’t have a TOU plan, you can help the grid by curtailing your peak-hour usage.

This is particularly easy if you are at work or school during the day, and much of your power consumption is due to air conditioning. If you have a smart thermostat, precool your home by a few extra degrees during the middle of the day when solar generation is high, and let the building “coast” on the colder temperature until later in the evening when peak rates end. If you time it right, you won’t experience a change in comfort but will benefit from lower peak-hour bills.

The good thing about smart thermostats is that you can often get a discount from your utility company. Sometimes you can get even get them for free.

You can also program the running time of other appliances such as your dishwasher, but these usually use much less electricity than a central air conditioner does.

Get a battery for your home

The other major thing you can do is get a solar battery, such as the Tesla Powerwall or Enphase Encharge. These will allow you to store your excess solar electricity instead of sending it into the grid. When peak hours roll around and solar generation tails off, you can switch your house to using the battery and avoid putting any load on the grid.

Unless you set up a DIY battery storage system with lead-acid batteries, any home battery you get these days will be a “smart” battery with programmable settings that can automatically help you avoid peak-hour usage and flatten the duck curve.

For example, the Tesla Powerwall has three modes: self-consumption mode to avoid using grid electricity as much as possible; time-of-use mode which is aware of utility peak hours and will minimize your peak-hour charges; and backup-only mode that only activates during a blackout.

Other battery products, such as those from Enphase and LG, have similar options. To learn more about solar batteries, read our guide.