Tokyo Hopes Rooftop Solar Mandate Will Help It Get Through Hot Summers
Like California, the Japanese capital wants the sun to power new homes, but some call policy ineffective and too reliant on China
4 min
TOKYO—Heat waves this summer are pushing Tokyo’s power grid near the limit. The city of 14 million has mandated installing solar panels on new single-family homes to get some breathing room—even if it has to buy most of the panels from China.
The policy in one of the world’s largest metropolises is a test case for whether solar power makes sense on urban rooftops. The idea has long drawn attention as a way to fight global warming but has advanced relatively slowly worldwide, apart from a rooftop solar mandate in California pushed by Democratic Govs. Jerry Brown and Gavin Newsom.
“We need to prepare to protect not only our national security, but also the energy security of individual households,” Tokyo Gov. Yuriko Koike, the architect of the mandate, said in an interview.
Koike has had panels on her own house for a decade. As a government minister, she created a “Cool Biz” plan that led Japanese salarymen to ditch suits and ties in summertime so they could keep the thermostat higher.
In the Land of the Rising Sun, Koike said, “we unfortunately aren’t rich in oil and gas, but wouldn’t it be fitting if we could instead harness the sun?”
Proponents say more rooftop solar would help on hot summer days when electricity demand peaks, and make the city resilient if an earthquake or typhoon knocked out the power grid.
The downsides: More solar also means higher electricity bills, including for lower-income apartment dwellers, because homeowners with rooftop panels can sell their excess power to the grid at an above-market rate. Even with the mandate, solar power generated in Tokyo is projected to supply only 4% of the city’s electricity in 2030.
“We cannot have a sufficient amount of power generation just by having solar panels on limited rooftops,” said Tatsuya Terazawa, head of the government-affiliated Institute of Energy Economics, Japan.
Still, Koike said it is important to move toward carbon-neutral electricity and help Japan buy less Russian natural gas.
The city predicts that by 2050, half of Tokyo’s current structures will be replaced. “By targeting newly built houses, we can decide the future 50 years from now,” Koike said.
Tokyo’s mandate, released last December, will initially apply only to about 50 of the largest home builders in the city, which it says account for around three-fifths of new homes. Starting in April 2025, those builders will be given a quota that will in effect require them to put panels on most new homes, while allowing them leeway to forgo solar in cases where the roof is too small or lacks sunlight exposure.
“We want companies to make sure that wherever you can put them, you put as many as you can,” said city environmental official Toshifumi Fukuyasu.
Japan relies on imported natural gas and coal for most of its electricity, and Russia supplies about one-tenth of its natural gas. The power company serving the capital, Tokyo Electric Power, or Tepco, isn’t operating any nuclear plants currently after the meltdowns at its Fukushima Daiichi plant in 2011.
In March 2022, the city came dangerously close to running short of electricity on a cold day, and it faced another crunch months later during a heat wave.
Tepco has managed to keep the lights on this summer, despite weeks of sweltering weather in which the thermometer consistently topped 95 degrees. Still, it says it worries about electricity running short, and the city encourages people to keep their homes at 82 degrees.
Tokyo University professor Yumiko Iwafune, who studies energy demand, said that even if rooftop solar panels don’t generate much electricity overall, they still can help households become self-sufficient on summer days, lessening demand on Tepco.
Tokyo has drawn comparisons between its policy and California’s rooftop solar mandate, which took effect in 2020 and similarly applies to new low-rise residential buildings. The California Energy Commission estimates that 150,000 new homes covered by the law have been constructed with solar panels.
Newsom, the governor, said in May 2022 that rooftop solar is “an industry that is essential to our state’s future.” California expanded its mandate last year to encompass certain new commercial buildings.
But the state’s mandate is contentious even among advocates of renewable energy, some of whom say it would be cheaper to build large solar and wind installations.
James Bushnell, an economics professor at the University of California, Davis, said rooftop solar mandates tend to raise electricity bills for people who don’t have solar because grid operators often pay higher prices for home-generated electricity. “A large ‘benefit’ of solar is in fact a shift of costs onto other, often lower-income non-home owning customers,” Bushnell wrote in an email.
Another problem in Japan is its lack of domestically produced solar panels, although it was once a leader in the technology.
According to the International Energy Agency, China produces around 85% of the globe’s solar cells and 95% of key components needed to make them.
“We’ve realised we can’t win against China,” said a spokesman for the Japan Photovoltaic Energy Association, an industry group representing companies in the solar-energy business.
Unlike the U.S., Japan still permits the import of solar panels with materials sourced from China’s Xinjiang region, where the U.S. has alleged the Beijing government has carried out genocide against the Uyghur ethnic group. Beijing denies that.
“Most people aren’t very aware about the human-rights violations associated with solar panels. They think of it as a good thing because of the environment,” said Taishi Sugiyama, a research director at the Canon Institute for Global Studies, a Tokyo-based think tank. He said Tokyo should scrap its solar mandate.
Fukuyasu, the Tokyo city official, said he agreed Japan needed more suppliers of solar panels. “But in Tokyo, we need solar to create a zero-emission society, and we can’t postpone that,” he said.
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From making unpopular decisions to learning when to listen, leaders from five businesses reveal what nobody prepared them for — and how running a company has challenged their expectations.
Technology and semiconductor shares fell sharply as investors questioned the debt financing the AI infrastructure boom and confronted higher rates and Chinese competition.
Elon Musk is exploring a potential merger between Tesla and SpaceX, a deal that could reshape the future of AI, robotics, autonomous vehicles and space technology. With a combined valuation potentially reaching $3.5 trillion, the move could bring Musk’s vision of an interconnected technology ecosystem closer to reality, although a deal remains uncertain.
5 min
Elon Musk has engineered ties between Tesla TSLA 3.41%increase; up pointing triangle and SpaceX SPCX 7.79%increase; up pointing triangle that are hard to break, and now he wants to fuse them together permanently.
Musk has run both Tesla and SpaceX concurrently over the past two decades, using a schedule that dictated which days he spent focused on each business.
The companies have often shared resources and personnel, signaling their close ties. Last year, SpaceX looked into creating a spacesuit for Tesla’s humanoid robot Optimus, a person familiar with the matter said. Tesla last week showed off its new Roadster, a vehicle featuring SpaceX rocket thrusters that it hopes to soon bring to market.
And in recent months, Musk has repositioned both companies by expanding beyond their core operations toward a shared mission centered on artificial intelligence.
The world’s richest man on Tuesday signaled his interest in tying up SpaceX and Tesla, marking what would be the biggest corporate merger of all time. Because he serves as chief executive of SpaceX and Tesla, Musk said committees of independent directors at both companies are evaluating a possible deal, adding that a transaction might not come to fruition.
The committees will likely evaluate, with lawyers and bankers, how each company would fare in a potential deal and how a deal might best be structured. They then present their conclusions to their respective boards, who typically vote on whether to proceed.
“It’s an odd Kabuki dance where two groups of people are talking and I can’t be in the conversation,” Musk told SpaceX investors on Tuesday in response to an analyst’s question about a potential deal.
For Musk, the merger supports his vision of a world full of autonomous cars, humanoid robots and weekly rocket launches, as well as the data centers and satellites required to keep the whole thing running.
Some investors and analysts said they expect a transaction would be structured as an all-stock deal, with SpaceX potentially paying a modest premium for Tesla shares.
SpaceX, which has a market value of about $2 trillion, said it is seeing sharp growth in revenue across its businesses, which include operating data centers, launching rockets and operating satellite networks such as the internet service Starlink. Tesla is valued at about $1.5 trillion and is expanding its operations beyond its automotive and energy business. Shares in both companies traded higher following the news Tuesday.
Musk’s substantial ownership stakes in both companies, along with voting control at SpaceX, has enabled him to set the deal in motion. Analysts and investors said the deal will probably face regulatory scrutiny internationally, though some see Musk’s close relationship with the Trump administration as an asset to getting approval.
Blurred lines
The companies haven’t said which directors are on the special committees evaluating a potential deal. Several SpaceX and Tesla board members have either personal or business ties to Musk. And like Musk, venture capitalist Ira Ehrenpreis sits on both boards.
In a filing this week, Tesla outlined millions of dollars in business agreements with SpaceX and its subsidiaries last year, including $143 million in car sales to SpaceX, $3.3 million in advertising that it bought on the social-media platform X and $430 million in revenue from the sale of Megapack batteries for SpaceX’s data centers.
The companies also split the cost of resources including the use of Musk’s SpaceX jet and the bill for his personal security, which he runs through a private company.
Employees at both companies routinely move between companies to work on special projects, and some executives have dual roles, including Charles Kuehmann, vice president of materials engineering at both SpaceX and Tesla, and Ashok Elluswamy, the vice president of AI software at Tesla who runs a joint venture with SpaceX called Digital Optimus.
Over the years, Musk has launched new startups within his empire, including brain-implant company Neuralink, underground-tunnel developer the Boring Company and xAI, the AI startup that merged with SpaceX in February.
It is that AI business that Musk thinks can serve a market worth tens of trillions of dollars, justifying the need for a merger.
The business case goes like this: Tesla is developing robotaxis and robots, which it calls “physical AI.” SpaceX is working on the back end of physical AI: the data-center capacity and internet connection required for those robotaxis and robots to operate continuously.
“I don’t see what the value of Tesla is if it’s not merged with SpaceX,” said investment adviser Ross Gerber, a former Tesla bull turned critic invested in both companies. “If they don’t merge, the brains of everything that runs Tesla is owned by another company.”
On top of it all, both companies face constraints on the supply chains they need to make this world a reality, particularly when it comes to chips. To address this, Musk earlier this year announced a joint venture between Tesla, SpaceX and Intel to design and manufacture chips in Texas. “The most epic chip-building exercise by far,” he has said.
Musk at the wheel
For Musk, a merger would also solve two problems: demands on his attention and his lack of control at Tesla.
Tesla for years was Musk’s only public company, a status that came with greater scrutiny. There, Musk reports to a public board, as well as individual investors who make up around 30% of Tesla’s voting power, according to FactSet.
The question of Musk’s attention became a particular focus for investors and the board in recent years after he acquired Twitter in 2022 and launched xAI in 2023, and again in 2025 as he spent time in Washington, D.C., running the Department of Government Efficiency in the early months of President Trump’s second term.
As Tesla’s car business suffered in 2025, Musk threatened to take his AI ambitions outside the company if he wasn’t granted 25% voting control. Last November, Tesla shareholders approved a new pay package for Musk, which created a path for him to vastly expand his stake.
As of Sept. 30, Musk owned a roughly 20% stake in Tesla, according to the company’s proxy filing.
The SpaceX IPO gave Musk a different path. When SpaceX went public in June, the deal’s structure enabled him to have more than 80% voting power at the company—so much that he can’t be fired and decisions can’t be made without his support.
If SpaceX acquires Tesla, he will likely stay in control.
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