In late June, a heat wave in France left nearly 70,000 households without power after a transformer failed under high temperatures. For about 24 hours, as the mercury hovered near 40 degrees Celsius (104 Fahrenheit), residents in parts of Brittany sweltered in deadly heat without fans or air conditioning.

Then as the United States baked in extreme heat in the run up to its 250th birthday celebrations, federal authorities issued blackout warnings. Grid operators were allowed to order large energy users, such as data centers, to switch to backup generators to maintain power to homes and emergency services like hospitals.

Power grids worldwide are struggling under intense heat waves and other extreme weather events — and in the US, outages are already happening. Heat-season power outages in the country, some linked to hot temperatures, rose about 60% over the past decade compared with the 2000s, according to data from the nonprofit Climate Central. Having no access to cooling in heat waves is especially dangerous for children, the elderly and those with chronic illness.

That trend is expected to continue as human-induced climate change drives longer, more frequent and intense heat waves, making it crucial for energy networks to adapt.

"As it gets hotter, things stop working quite so well," said Iain Staffell, associate professor of sustainable energy at Imperial College London. That means one "should expect faults to be more common at these very high temperatures."

"I think we do need to adapt the power system to cope with the changing weather," added Staffell, referring to Europe, which is warming much faster than the rest of the world.

Why are heat waves a problem for power networks?

When heat waves hit, electricity demand surges as people turn to ACs, fans and other cooling devices for relief. That strains the power grid, especially later in the day when heat lingers but solar power starts to fade.

At the same time, the infrastructure that keeps electricity flowing — from power plants to transformers — is vulnerable to heat stress. Transmission lines moving power across the country expand as temperatures rise, for instance. This causes them to sag, sometimes enough to brush against trees or other obstacles, increasing the risk of short circuiting or outages.

"The grid operator has to reduce the amount of power that flows down these wires in the very hottest temperatures to make sure everything remains safe," said Staffell.

Heat and sagging lines were contributing factors in 2003 when around 50 million people across the northeastern US and parts of Canada were left without power during high temperatures. It was the largest blackout in North American history. While grid operators have since introduced safeguards to prevent a repeat outage of that scale, experts say extreme weather still poses a threat.

Thermal power plants are not immune either. In extreme heat, cooling in coal, gas and nuclear plants becomes less effective, meaning operators sometimes have to reduce output.

"Efficiency of coal and gas as well as nuclear power stations falls by roughly 1% for every degree it gets hotter," Staffell said — so they're 10% less effective at 35 C than at 25 C.

During Europe's heat wave in June, several nuclear facilities in France and Switzerland had to reduce output or temporarily shut down because the rivers they use for cooling had become too warm. Regulations require plants to limit the temperature of discharged water to protect ecosystems and aquatic life.

Renewable energy sources are affected as well. Low water levels can hamper electricity production from hydropower, for instance. And solar panels become slightly less efficient in high temperatures, while wind speeds often drop during heat waves, reducing their output.

These factors, taken together, can contribute to grid instability, with operators needing to source energy from elsewhere if the mismatch between demand and supply becomes too tight.

If cheap solar or wind isn't available during these critical crunch times, grid operators often have to rely on more expensive backup sources to fill the gap. And those tend to be planet-heating fossil fuels.

Even when blackouts don't occur, heat can drive up wholesale electricity prices, which are sometimes passed on to consumers depending on the market and the contract. During the recent European heat wave, power prices spiked across countries, including in France and Germany, especially during evening peak hours.

Can we heat-proof energy systems?

There are some smaller fixes "that can have a really big impact," said Staffell.

That includes upgrading grid components to withstand heat and installing better cooling systems for powerlines, transformers and other equipment. That could be as simple as "bolting on additional fans for putting a shade over the top of them," added Staffell.

Nuclear power plants, for example, could be built with "hybrid cooling systems, so they are not so reliant on the rivers," said the sustainable energy expert.

While the European electricity grid is one of the most resilient in the world, the infrastructure is aging, says Alexander Roth, an energy and climate policy fellow at Brussels-based economics think tank Bruegel.

At the same time, countries are transitioning to renewable energy systems largely based on electricity that will power everything from electric cars and data centers to heat pumps and air conditioning.

As Europe decarbonizes its economy, electricity is expected to account for about half of total energy use by 2040, up from about 20% today. That means demand will keep rising, even without factoring in hotter summers.

"And the current system is not fit for that," said Roth.

Meeting that challenge will require modernizing the grid, boosting the flow of power between countries, and creating more flexibility in the system. Battery storage, for example, can help reduce stress on electricity networks, especially during heat waves.

"These batteries could, for instance, store cheap generation of solar PV around midday and then actually feed it out in the evening hours ... because the wind is gone and the sun is gone but there's still high demand," said Roth.

Demand-side measures could help too. Dynamic pricing — whereby electricity costs vary throughout the day — can incentivize consumers to use power during off-peak hours, easing pressure on the grid, added Staffell.

What's the hold-up?

In Europe, a significant obstacle is the huge backlog in projects, including batteries and an estimated 1700 gigawatts worth of renewables, waiting in a queue for a grid connection. Legislation is under discussion in Europe to improve the grid, with massive investment required. But it'll likely take some time to build the required infrastructure, say analysts.

In the US, the Department of Energy has also announced a major investment to expand and modernize grid infrastructure. The country has seen an uptick in power outages over the past decade, as its ailing grid has struggled to cope with more frequent extreme weather events such as hurricanes and winter storms.

At the same time, the grid is facing "historic stress" from the growth in electric vehicles, renewable energy and energy-intensive data centers, according to JP Morgan Chase. The bank noted that smart grids and advanced grid tech were vital for energy security.

Meanwhile, in Europe, Roth believes there is a huge opportunity to create a better-functioning, more flexible grid that "is smarter, where we use more electricity, and also, that in the end, benefits the people."

Edited by: Jennifer Collins