Stock in the new company, which bundles RWE's power grid, renewables and sales arms, along with 40,000 of its 60,000 workers, touched €37.30 shortly after trading opened before falling back.
Shares remained just above the €36 ($40) starting price at €36.07 around 10.45am.
The flotation values Innogy at around €20 billion, instantly making it the most valuable German energy provider by market capitalisation.
It is the largest IPO in the European Union since 2011, and the largest in Germany since microchip manufacturer Infineon floated in March 2000.
Only around a quarter of Innogy shares were offered on Friday, bringing in around €5.0 billion.
RWE has followed in the footsteps of German competitor EON, which floated its own subsidiary Uniper, a bundle of all its fossil fuel operations, in mid-September.
The moves come as traditional energy firms in Germany are battling to adapt to a new environment of low wholesale electricity prices and competition from government-subsidized renewables.
"The successful flotation of Innogy indeed solves [RWE's] tense financial situation, but with an aftertaste of in part disposing of the valuable 'family silver'," analysts at DZ bank wrote.
"The risks in the remaining activities of RWE shouldn't be overlooked," they went on, pointing to likely continuing low electricity prices that will weigh on generators for years to come and uncertainty over the costs of nuclear waste disposal.
RWE stock had fallen 4.3 percent to €13.84 by 10.20am on Friday, making it the worst performer on the DAX index of leading German shares.