The ISA is an inter-governmental organisation that aims to help sunshine-rich countries around the world harness solar energy at an affordable cost.
Launched on November 30, 2015, on the sidelines of the Paris Climate Change conference, the alliance currently has a goal of enabling production of 1 trillion watts of solar power by 2030.
So far 61 countries have signed up and another 32 have ratified the treaty. Heads of states of 24 countries including Australia and Sri Lanka are attending the two-day founding summit.
French President Emmanuel Macron, delivering the inaugural address, called on private industry and international financial institutions to become agents of change by pitching in.
Macron said France's international development agency was committing an additional $700mn to solar energy by 2022, taking its total investment in financing solar projects in ISA member countries to $1bn.
Indian Prime Minister Modi flagged the need for investment in better and more accessible solar technology, concessionary finance for solar projects and easing of regulatory mechanisms.
Both leaders also mentioned a need to invest in skills development in the sector.
"Financing together, sharing expertise, innovative technology - along with total transparency at the top," was what the ISA was all about, Macron said.
French President Emmanuel Macron meets with members of the "solar mamas" -- a group of rural solar engineers -- in New Delhi on Sunday.
Modi -- who has committed to reducing India's sizeable carbon footprint through a massive scale-up in renewable energy -- said it was vital that nations were not priced out.
"We have to make sure that a better and cost-effective solar technology is available to all," Modi told the gathering of investors and world leaders from about 20 mainly African nations.
"We will have to increase solar in our energy mix."
India, the world's third-largest polluter, is undergoing spectacular growth in its solar sector and is on track to become one of the world's largest clean energy markets.
It pledged at the Paris climate summit in 2015 to source at least 40% of its energy from renewables by 2030, mainly via solar.
The energy-hungry giant of 1.25 billion people is banking on solar to electrify homes for hundreds of millions of its poorest citizens without adding to its considerable emissions output.
Macron and Modi hope the alliance will spur $1tn in new solar spending by 2030 in 121 countries lacking investment in the sector.
These countries have "the paradox of being the sunniest in the world while enjoying the least solar energy," said Ségolène Royal, a former French minister in India as a special envoy for the alliance.
Macron told world leaders in Paris in December that "we are losing the battle" against climate change and urged faster action to combat global warming.
The French leader called on private sector attendees in New Delhi to engage more actively because "solar investments are becoming more profitable".
He and Modi will open a new 100 megawatt solar plant near the holy Indian city of Varanasi on Monday. The French leader will also visit the Taj Mahal in Agra later on Sunday.
