Since he was a child, Santipada Gon Chaudhuri had sought ways to help India's rural poor, so when the electrical engineer was invited to visit a co-worker's home in the Himalayan village of Herma in the early 1980s, he saw his chance.
"I was appalled to see how local communities were living in darkness after sunset," remembered Chaudhuri, 71, who then worked for the government in the northeastern state of Tripura.
"Some used kerosene lamps, but even kerosene was not always easy to get. Since I had both the skill and position to try and provide power to them, it made me act," he said.
The villages of Tripura are located on tough, hilly terrain, where Chaudhuri realised it would be hard to put up power lines.
"But they had solar energy in abundance," he told the Thomson Reuters Foundation in an interview.
In 1983, he used government funding to install solar panels for 70 homes, as well as running a community television and water pump - the first time anyone in the hamlet had seen electric light.
That small project sparked a career dedicated to bringing energy to people in impoverished, remote communities, a mission that earned Chaudhuri the moniker of India's "Solar Man".
Today, more than 100 homes and businesses in Herma are lit by an updated solar energy system, allowing villagers to be more productive while reducing their use of expensive, polluting fuels like kerosene.
"Life in the village would come to a complete standstill after sunset. But with light in our homes now, our children are studying until night," said villager Sumoti Riyang, 33.
"Shops and business establishments remain open in the evening. We can work more. All this is generating more income for us," she said.
In his Kolkata office, adorned with awards he has won since his first project nearly 40 years ago, Chaudhuri said he gets "great satisfaction" from seeing how solar power has changed lives in Herma, connecting residents to the modern world.
CAREER OF FIRSTS
Herma was the first tribal village in the country to gain access to solar power, and by 1989 Chaudhuri had led the installation of solar technology in nearly 40 villages across India's northeastern states.
Four years later, he developed India's first centralised solar power station with a distribution network on Sagar Island in the Sundarbans, home to one of the world's largest mangrove forests, supplying 100 households through power lines.
The project was considered a breakthrough at a time when solar technology "was largely confined to laboratories and prototypes", said Samrat Sengupta of the New Delhi-based Centre for Science and Environment (CSE), a nonprofit think-tank.
By 2000, more than 400,000 people in villages around the Sundarbans national park were using solar power, through a mix of mini-grids and domestic solar-power systems.
At the time, the area had the highest per-capita consumption of solar power in the world, Chaudhuri noted.
The project earned him an Ashden Award, known as the "Green Oscars", and the Euro Solar Award from Germany.
In 2006, it also inspired India's then-President APJ Abdul Kalam to invite Chaudhuri to design a captive solar unit for the presidential palace.
"Chaudhuri's work is a classic example of empowerment of indigenous communities through solar power," said Arun Tripathi, director general of the National Institute of Solar Energy, an autonomous body under the renewable energy ministry.
In 2009, Chaudhuri installed the country's first grid-connected solar plant in West Bengal's Jamuria village, a 2-megawatt (MW) project serving 5,000 families.
This was lauded as an "environmental breakthrough" because, until then, solar power had been limited to remote areas without access to electricity, said CSE's Sengupta.
Jamuria was the first location to use solar to replace coal power in the grid, bringing clean energy into the mainstream, he said, noting it cut the amount of coal burned locally by 2,000 kg (4,400 pounds) per hour and decreased carbon emissions.
FLOATING SOLAR
Sengupta and others said Chaudhuri's work helped pave the way for India's National Solar Mission, launched in 2010.
The initiative, on which Chaudhuri consulted, had an initial target of producing 20 gigawatts (GW) of solar power by 2022.
Having already nearly doubled that ahead of time, India has set a new goal of 100GW.
But as its solar power expansion has gained pace, a growing population and increasing urbanisation have made finding enough land for big projects more difficult.
In response, Chaudhuri came up with India's first floating solar power station.
In 2014, after joining the nonprofit NB Institute for Rural Technology, which he now heads, he led construction of an experimental 10-kilowatt government-funded floating solar panel on a lake in Kolkata's New Town.
"Designing the floating structure of the panel and anchoring it in the water body were major challenges," he said.
That project grew into a national programme that now generates more than 1,700MW of solar power from floating panels in various coastal states around the country.
Despite its progress, India's solar push has some limitations including high capital costs, scarcity of land and the need for sunny weather, said Partha S Bhattacharyya, former chairman of Coal India Limited, the world's largest coal producer which is also investing in solar energy projects.
"Thermal (coal) power is reliable and consistent, due to greater grid stability," he added.
Chaudhuri and his team are currently experimenting with solar-powered pumps that push water up to a higher storage reservoir that can then generate hydro-electricity using micro turbines, supplying villages when needed.
"The very concept of solar power has changed from simply providing lights to controlling carbon emissions," Chaudhuri said. "It is time that we seriously think about how to leave behind a more livable world for future generations."