Fortum, which paid 6.5 billion euros ($7.9 billion) for a 75% stake in Uniper, also outlined deal synergies for the first time, expecting annual benefits of over 50 million euros by end-2023 and about 100 million in 2025.
These are "fairly low", JP Morgan said in a note to clients.
Shares in Fortum fell as much as 3.9%, while Uniper's stock was down as much as 1.9%.
As part of the strategy update, Fortum said that the goal for the combined group was to be carbon neutral in its European generation by 2035 and in all its operations by 2050.
State-controlled Fortum has agreed to keep Uniper separately listed until the end of 2021 at least. It had sought control of the German energy firm since 2017 and overcame a final ownership hurdle in March after years of resistance.
The remaining 25% stake in Uniper is currently worth 2.6 billion euros.
"Building on the strengths of both companies, our future growth will be driven by CO2-free power generation and clean gas," Markus Rauramo, Fortum's CEO since June, said.
Uniper is already planning to be carbon neutral in its European generation by 2035. The target does not include Russia, where the firm operates more than 11 gigawatts of coal- and gas-fired capacity via its subsidiary Unipro.
"With the working teams ... now in place we will be able to better align the already identified business potentials," Uniper CEO Andreas Schierenbeck said.
Uniper's labour leader Harald Seegatz, an outspoken critic of the Fortum takeover in the past, said the atmosphere had improved markedly since Rauramo took over from former CEO Pekka Lundmark, adding workers were fully involved in discussions.
Fortum also said it will initiate a strategic review of its consumer business, which is responsible for electricity and gas retail businesses in the Nordics, Poland and Spain. The business made a core profit of 141 million euros in 2019, it said.
The company targets a dividend of 1.12 euros per share for 2020 and plans to increase it over time. Investments for the whole group, including Uniper, are estimated to be about 1.4 billion euros in 2021.
($1 = 0.8248 euros)
(Additional reporting by Tom Kaeckenhoff in Duesseldorf; Editing by Edmund Blair, David Evans and Emelia Sithole-Matarise)
By Anne Kauranen and Christoph Steitz