How electricity is used at the device, household, and system scales — and how it differs from solar's generation profile
The duck now looks like a canyon. The canyon is getting deeper.
This module covers the following four lessons:
In this lesson, you will learn how electricity demand varies at the smallest scale — individual devices and appliances — and understand why some tasks are more 'solar-friendly' than others.
We begin with something familiar: your phone. Charging a mobile phone requires a small amount of energy, roughly 10–15 WhWatt hours, the equivalent to one watt of power for a full hour. to fully charge a modern smartphone. This is a flexible, low-power task that can easily be done during the day when solar power is available. If you plug your phone in at 11 am and unplug it at 1 pm, you have used solar energy at the time it is generated. No storage is needed, no grid interaction required.
This is the simplest case: a small, flexible load that naturally fits within the solar generation window.
Unfortunately, most electricity demand is not this convenient. For example, a fridge compressor cycles on and off around the clock — it sometimes works within the solar generation window, but not completely. You could fill the fridge with bottles of water to increase its thermal mass, so it keeps cool overnight, but that wont leave much room for food.
Phone charging is 'solar-friendly' — small, flexible, and easy to align with daytime generation.
A fridge, however, will not operate overnight if it is only powered by solar energy. It is a useful mental model for understanding how solar works best when demand and generation overlap.
In this lesson, you will explore how electricity demand varies across a typical day, and how this pattern creates a fundamental mismatch with solar generation.
Household electricity demand is more complex than that of a single device. A typical home uses electricity throughout the day, driven by a range of devices. The pattern is far from uniform because the devices switch on and off at different times.
In the morning, there is a spike as people wake up: lights go on, kettles boil, showers run (if electric), and breakfast appliances are used. During the middle of the day, when many people are at work or school, household demand drops. Then in the evening, demand rises again as people return home, cook dinner, watch television, run washing machines, and turn on heating or air conditioning.
This creates the classic 'twin peak' demand pattern at the household level: a morning peak around 7–9 am and a larger evening peak around 5–8 pm. The evening peak is typically the highest point of household demand in the day.
We can see this pattern in the chart below by adding individual devices to create the household demand. Click each appliance to add it to the 24-hour timeline and watch the demand "curve" emerge.
Now compare this with the solar generation profile from Module 2. Solar output peaks around midday, precisely when household demand is at its lowest. The evening peak, when demand is highest, occurs just as solar output is declining toward zero. This is the behavioural mismatch: solar generates when people are out, and demand peaks when the sun is setting.
Three regions stand out where the two curves separate: a morning deficit on the left, a midday excess in the centre, and an evening deficit on the right. Together, they are the household behavioural mismatch made visible.
At the household level, electricity demand peaks in the morning and evening, while solar generation peaks at midday. This means a household with rooftop solar will overproduce electricity during the day (when no one is home) and underproduce in the evening (when everyone returns). Without storage or grid export, much of the solar energy goes unused.
In this lesson, you will explore how grid-level electricity demand varies across the day and understand the 'duck curve' — a critical concept for high-solar grids.
At the level of the entire electricity grid, demand follows a broadly similar pattern to individual households. It is aggregated up from all household demand plus commercial and industrial demand. Because there are so many users, the total is usually a lot smoother. The grid demand curveSometimes called the 'load curve'. shows total electricity consumption across a region or country over the course of a day.
Grid demand typically shows a morning ramp (as businesses open and industry starts up), sustained high demand through the working day, and an evening peak as residential demand overlaps with commercial activity. The exact shape varies by country, climate, and season, but the fundamental pattern is consistent: demand is low at night, rises in the morning, stays elevated through the day, peaks in the early evening, and declines overnight.
In grids with high solar penetration, the demand curve — as seen by conventional generators — takes on a distinctive shape known as the 'duck curve', so called because the residual-load line traces the silhouette of a duck. During midday, solar meets so much demand that the residual load (what's left after subtracting solar generation) drops sharply. Then, as the sun sets, conventional generators must ramp up to meet the evening peak. This steep ramp puts stress on the grid and on conventional power stations that are not designed for such rapid changes. The duck curve is a direct consequence of the timing mismatch and becomes more pronounced as more solar is added to the system.
Use the slider to increase or decrease solar penetration and watch the residual demand curve morph into the "duck shape".
At 25% solar penetration, a duck curve has clearly emerged: the midday belly deepens and the evening ramp steepens. Move the slider higher and the shape becomes more pronounced.
When we overlay the solar generation curve onto the grid demand curve, the timing mismatch becomes starkly visible. Solar produces most of its energy during the midday hours, but high grid demand has a much longer duration and the period of maximum solar output does not coincide with the period of maximum demand.
The household-level mismatch we met in Lesson 2 now repeats across millions of homes and businesses. What one household handles by drawing from the grid in the evening becomes a system-wide problem when everyone needs the same thing at the same time — and the grid, unlike a single home, has no grid of its own to draw from. It must actively balance generation and demand in real time.
In this lesson, you will explore two practical strategies for addressing the timing mismatch and understand how they work together.
Faced with the timing mismatch between solar generation and electricity demand, there are two intuitive responses.
Before either of those, it is worth naming the 'do nothing' baseline: keep running conventional fossil-fuel plants to follow the duck curve up and down. It works, but it is the expensive option. Those plants were never designed to ramp down sharply at midday and back up steeply in the evening, so making them chase the curve is inefficient, hard on the equipment, and costly. Both responses below exist to reduce how much of that we have to do.
The first response is to shift demand to match generation — the 'solar-friendly' logic we met with phone charging in Lesson 1, applied more broadly. At the household level, this might mean running the washing machine at lunchtime rather than in the evening, or setting the dishwasher to run during the afternoon. At the system level, it means scheduling industrial processes, electric vehicle charging, and other flexible loads into solar peak hours.
This approach is known as demand-side response or demand flexibility. It can reduce the timing mismatch, but it has limits. Many domestic activities (cooking dinner, evening lighting) cannot easily be shifted to midday, and commercial and industrial demand is even more stubborn.
In the interactive below, explore a new 24-hour demand shape. Loads in navy are fixed; loads in yellow are flexible and can be shifted to match solar generation. See how close you can get to a solar generation curve.
Prefer a shortcut? Press the All button below the graph to load every appliance, then the yellow Shift flexible loads to midday button to see the maximum effect in one step.
The second response is to store the excess solar energy generated during the day and release it when it is needed — typically in the evening. This is the role of battery storage. At the household level, a home battery can charge from rooftop solar during the day and discharge in the evening, reducing or eliminating the need to draw from the grid after sunset. At the grid level, large-scale battery electric storage systemsOften shortened to BESS. can absorb midday solar surplus and inject it back into the grid during the evening peak and throughout the night.
Battery storage is the subject of the next module. It is arguably the most important complementary technology to solar because it directly addresses the timing mismatch that is solar's central challenge.
Demand flexibility: Shift when you use electricity to align with when solar generates it. Effective for some loads, limited for others.
Energy storage (batteries): Store solar energy during the day and release it when needed. Directly addresses the timing mismatch at all scales.
A third tool, grid interconnectionHigh-voltage links between separate grid systems, often High Voltage Direct Current (HVDC) subsea cables — HVDC loses less over long distances than ordinary AC, which is why it is the choice for interconnectors. Sometimes called 'interconnectors'., allows countries and regions to share electricity, smoothing out local mismatches due to differences in the generation and demand profiles. Interconnectivity between grid systems is growing rapidly at the moment.
All three tools work together in a well-designed energy system, but only energy storage is able to provide a full solution at any scale.
It is important to note that this timing mismatch and the need to balance supply and demand are already present in all energy systems, at every scale. Solar doesn't create this issue, but it is critically important because of the uncontrollable and inflexible way that solar power is generated.
Six questions to test what we've covered. Don't worry if a few trip you up; you can take it again whenever you want.
Electricity demand varies across scales. At the device level, some small loads like phone charging are naturally solar-friendly. At the household level, demand peaks in the morning and evening, creating a fundamental mismatch with solar generation, which peaks at midday. This 'behavioural mismatch' appears again at the grid level, where it creates the 'duck curve', a supply and demand mismatch that becomes more pronounced as solar penetration increases.
Two intuitive responses exist: demand flexibility (shifting when we use energy to align with solar generation) and energy storage (batteries that absorb midday surplus and release it during evening peaks). Both are essential, and both must work together with grid interconnection to create a resilient, solar-dominant energy system.
Next up: In Module 4, we will explore battery storage systems — how they work, how they complement solar, and their critical role in solving the timing mismatch.
You now understand electricity demand patterns at all scales and the central challenge that solar energy must overcome: the timing mismatch between when the sun generates power and when people need it.
