The duck curve challenges the Swedish power system


Solar energy still accounts for only a small share of Sweden’s electricity production — but it’s already beginning to impact power system dynamics, according to Fredrik Karlsson from Polar Capacity. As the sun sets, the need for balancing power rises sharply, a phenomenon illustrated by the so-called duck curve. He points out that Sweden will soon face situations where almost no other generation is needed during midday, while the demand for ramp-up capacity may increase by as much as 10,000 MW when the sun goes down.

In Sweden, we have ambitious goals to increase the use of renewable energy, and both solar and wind power have become increasingly common, now accounting for a significant part of our electricity production. More wind and solar in the system means more emissions-free production, helping to electrify transportation and industrial processes without harming the climate. Solar and wind also produce electricity with virtually no variable costs. Like hydropower, they require no fuels, meaning that production costs are low or even negligible. This benefits both the climate and consumers’ wallets.

The Swedish power system has long been known for its reliability and sustainability. With a large share of hydropower and a well-functioning grid infrastructure, we have enjoyed a stable energy system. But this stability is now threatened by a phenomenon spreading around the world — the duck curve. This challenge, arising from the increasing share of solar and wind power, requires active measures to protect the grid from stress and, in the long term, to avoid major costs.

What is the duck curve?

The duck curve describes the shape of electricity demand on a graph during sunny days, resembling the outline of a duck. During the day, when solar energy production peaks, the need for conventional generation (such as hydropower or nuclear) decreases. While this might sound positive from a sustainability perspective, it creates challenges for the power system.

As the duck curve becomes more pronounced, we increasingly face situations where demand for other energy sources fluctuates dramatically throughout the day. This is not primarily an energy issue, but a capacity challenge.

This spring, Sweden may have experienced the duck curve for the first time during a few sunny early summer days in May. The biggest concern with the duck curve is the rapid drop in solar production later in the day. As evening approaches and solar generation declines, the need for fast-response balancing power surges. This leads to extreme price swings — from just a few öre per kWh during midday to 20–30 times higher prices in the evening — along with significant technical challenges.

Solar energy’s surprising impact on Sweden’s electricity system

It might seem surprising that solar power can have such a major impact in a country where it accounts for only around 1.5% of annual electricity production (approximately 2 TWh in 2022). Traditionally, the energy system has been analyzed in terms of energy needs — TWh per year. From that perspective, solar energy will take a long time to become a major player in Sweden. But in terms of capacity, solar will soon play an important role — during certain hours, on certain days.

Between 2016 and 2022, Sweden’s installed solar capacity grew 17-fold, from 140 MW to nearly 2,400 MW. If installations continue at a pace of around 50% annual growth, Sweden could have 8,000 MW of grid-connected solar capacity within four to five years. That would cover 75–80% of the capacity needed on a typical summer day — and even more during sunny weekend hours in summer.

A growing challenge for the grid

This means the system will soon have to handle scenarios where almost no other production is needed at midday, while other energy sources and storage systems need to ramp up by as much as 10,000 MW in just a few hours when the sun sets. Until now, capacity challenges have largely been analyzed based on the hour of the year with the highest demand — typically a cold winter morning. But managing a rapid ramp-up of 10,000 MW is an equally significant capacity challenge, whether it’s on a summer evening or a winter morning.

Infrastructure and flexible generation need to be available and functional — even if profitability is lower due to fewer annual operating hours. In a society where more and more sectors are electrified, total electricity consumption may double. Just as we may need very little additional generation beyond solar power during certain summer hours, we may also face winter hours with insufficient capacity when demand is high, and it’s dark and windless.

Traditional, centralized solutions will not be able to handle these extremes. Large-scale investments in transmission grids, nuclear power, or combined-cycle plants become increasingly expensive as their utilization rate declines.

Local solutions and flexibility are key

When electricity often becomes cheap or even free, the challenge is to prevent system costs from skyrocketing. If the last 1,000–5,000 MW of capacity are only needed for 800 hours per year instead of 8,000, capital costs per operating hour become ten times higher. Relying on traditional tools like large centralized power plants and costly grid investments would be an economic disaster — both for companies and society as a whole.

Instead, capacity challenges must increasingly be addressed locally. This applies both when the sun shines and the wind blows — and when demand peaks on dark winter days. The focus must shift toward the lowest possible investment costs per MW, combined with high availability and flexibility. This represents an entirely new way of viewing the system, but it’s an inevitable consequence of moving from an energy-based focus to a capacity-based one.

Fredrik Karlsson
Head of Strategy & Public Affairs, Polar Energy
fredrik@polarenergy.se