Its Renewable Power Generation Costs in 2019 report shows more than half of the renewable capacity added in 2019 achieved lower power costs than the cheapest new coal plants.
The report is based on analysis from 17,000 renewable power generation projects and data from 10,700 auctions and power-purchase agreements.
The report said on average, new solar pv and onshore wind power cost less than keeping existing coal plants in operation and auction results show this trend accelerating.
It estimates up to 1,200 gigawatts of existing coal capacity could cost more to operate than the cost of new utility-scale solar pv, and replacing the costliest 500GW of coal with solar and onshore wind could save governments up to US$23 billion every year as well as reduce emissions by around 1.8 gigatonnes of CO2.
"We have reached an important turning point in the energy transition," IRENA director general Francesco La Camera said.
"The case for new and much of the existing coal power generation, is both environmentally and economically unjustifiable."
Since 2010, utility-scale solar pv power has shown the sharpest cost decline at 82%, followed by concentrating solar power at 47%, onshore wind at 39% and offshore wind at 29%.
Electricity costs from utility-scale solar pv fell 13% in 2019, reaching a global average of US6.8 cents per kilowatt-hour.
Onshore and offshore wind both declined about 9%, reaching 5.3c/kWh and 11.5/kWh, respectively.
For the first time the report also looked into investment value in relation to falling generation costs.
It found in 2019, twice as much renewable power generation capacity was commissioned than in 2010 but required only 18% more investment.
The rapidly reducing costs have yet to convince China however, who according to Wood Mackenzie committed to new coal-fired generation at its Two Sessions summit last week, despite also pursuing its coal-to-gas switching work in some provinces.
"We see 120-130 GW of new capacity approvals over the next five years, with capacity peaking around 2025 at around 1,200 GW. But greater environmental scrutiny will be applied and tightening emissions regulations," Wood Mackenzie analyst Frank Yu said.
However the China's state grid is also planning for higher capacity renewables additions than previously expected - 29GW of wind and 39GW of solar, with WoodMac noting there was little supply chain risk given most wind and solar parts are manufactured in the country.
The report noted the level of government-backed subsidies for renewables has been widely overestimated, estimating supply-side support for renewables in 2017 was around US$166 billion globally.
This is compared to US$220 billion for petroleum products, US$82 billion for electricity-based gas, and US$17 billion for thermal coal.