The duck curve is the shape of electricity demand that grid-scale power stations must supply across a day once a lot of rooftop solar is installed. Demand dips deeply in the middle of the day, when panels on homes and businesses cover much of the load, then climbs steeply in the late afternoon and evening as the sun sets and people get home. On a chart, the midday sag resembles a duck's belly and the evening climb its neck. That shape is changing when electricity is cheap and when it is expensive, and household tariffs are gradually following.

Underlying demand versus operational demand

Two measures of demand explain how the curve forms:

• Underlying demand is everything homes and businesses actually use, including power supplied by their own rooftop panels.
• Operational demand is what the grid must deliver from large-scale generation and storage. Rooftop solar used behind the meter, and rooftop solar exported to neighbours, both reduce it.

Underlying demand follows a familiar daily rhythm: a morning rise, a steady day and an evening peak. Operational demand is roughly underlying demand minus rooftop solar output. Each year of new installations carves the midday belly deeper, while the evening peak, which arrives after solar output has faded, stays largely where it was.

A day on the duck curve

The shape is most extreme on mild, sunny days in spring and autumn, when heating and cooling loads are small. On hot summer evenings, air-conditioning can lift the neck of the duck even higher.

The steepness of the neck matters as much as the depth of the belly. Within a few hours, solar output falls while household demand rises, so the grid must bring a large amount of supply online quickly. Resources that can start fast and change output rapidly become more valuable, while slower plant has to be scheduled well in advance. Storage suits this pattern particularly well, because it can switch from charging to discharging almost instantly.

Why midday wholesale prices can go negative

In the wholesale market, generators offer electricity at various prices and the cheapest offers are dispatched first. At midday, solar and wind farms can offer power at very low prices, because sunshine and wind cost nothing to use. Some generators will even accept negative prices to keep running, for two main reasons:

• Large coal-fired plants are slow and costly to shut down and restart, so running through a few hours at a loss can cost less than switching off.
• Large renewable generators can earn certificate income for each megawatt-hour they produce, so they can keep generating while wholesale prices are slightly below zero.

Rooftop solar does not respond to wholesale prices at all; it simply generates whenever the sun shines. Together, these effects push midday prices down, sometimes below zero.

Evenings are the opposite. Solar output has gone, demand is at its highest, and the market relies on resources that can ramp up quickly, such as gas turbines, hydro and batteries. Those resources tend to set higher prices, and the steepness of the ramp itself can trigger price spikes.

How the duck curve reaches household tariffs

Retail tariffs change more slowly than wholesale prices, but the direction is clear:

1. Peak windows sit later in the day. Time-of-use peaks increasingly cover the late afternoon and evening rather than the middle of the day. Their structure is explained in understanding time-of-use tariffs.
2. Midday is becoming the cheap period. Some network tariffs and retail plans now offer lower prices during solar hours to encourage daytime use.
3. Exported midday solar is worth less. NSW feed-in tariffs are now commonly a few cents per kWh, far below the 30-45 c/kWh commonly paid for grid electricity. Understanding feed-in tariffs covers why.
4. Controlled loads are moving. Electric hot-water systems that traditionally heated overnight are increasingly scheduled to heat in the middle of the day instead.

How storage and flexible demand flatten the curve

The duck curve is not a shortage of energy; it is a problem of timing. The solutions move energy, or demand, from the belly of the curve to its neck:

• Home batteries charge from midday solar and discharge through the evening peak, reducing both the household's evening imports and the grid's ramp.
• Grid-scale batteries and pumped hydro store energy when prices are low and release it when they are high.
• Virtual power plants coordinate many home batteries to respond to price spikes and grid events, as described in what is a virtual power plant.
• Load shifting moves flexible demand such as EV charging, pool pumps, dishwashers and hot water into solar hours.

What it means for solar households

The duck curve explains a quiet shift in how rooftop systems are valued. When feed-in tariffs were higher, the headline question was how much a system would export. Today, more useful questions are:

• How much of the solar output can the household use directly while it is being generated?
• Which appliances can be scheduled into the middle of the day without inconvenience?
• How much energy would need to be stored to cover the evening peak, and is a battery worthwhile for that pattern?
• Does the current retail plan reward midday use and penalise evening imports, or the reverse?

Answering those questions with real interval data, rather than averages, is what turns the duck curve from a grid-level concept into a practical guide for a home.

Next steps

Two digital reports on the energy market apply these ideas to a real household: the Retail Electricity Tariff Comparison ($79 per property) compares market offers against a home's usage and solar export profile, and the Battery Feasibility Study (from $199 per property) analyses usage to size a battery and estimate payback. Prices are indicative and confirmed after a site assessment. To explore solar and storage for your own evening peak, request a free assessment from Blue Energy Solar.

Frequently asked questions

Is the duck curve an Australian problem or only an overseas one?

The name was popularised overseas, but the pattern is clearly visible in Australia. Several Australian states have high levels of rooftop solar per household, and on mild, sunny days their grid demand shows the same deep midday dip and steep evening climb. NSW shows this shape too, and it tends to deepen as more rooftop systems are installed, which is why tariffs and network planning are adapting to it.

Will cheap midday wholesale prices automatically lower my bill?

Not automatically. Wholesale energy is only one part of a retail price, which also covers network charges, environmental scheme costs, retail costs and the daily supply charge. Most households pay the rates set by their plan, whatever the wholesale price at that moment. The benefit reaches households mainly through plans with cheaper midday rates and through using more solar and stored energy at home.

Do electric vehicles make the duck curve worse?

That depends on when they charge. Plugging in on arrival home at 6pm adds load right at the steepest part of the evening ramp. Charging in the middle of the day, from rooftop solar or at a workplace, helps fill the midday dip instead, and overnight charging uses a quieter period. Smart chargers that follow solar output or a schedule make the helpful pattern easy to maintain.