* Offers 22 eur/shr vs 23.33 eur closing price
* E.ON can tender 46.65 pct at that price in early 2018
* Failure to do so triggers compensation payment (Adds further details, context, share prices, CEO)
FRANKFURT/HELSINKI, Sept 26 (Reuters) - Finnish power utility Fortum will launch an 8.05 billion-euro ($9.5 billion) takeover bid for Uniper, the power stations operator and energy trading business part-owned by German utility E.ON, it said on Tuesday.
It will offer 22 euros per share in cash, the primary aim being to acquire for 3.76 billion euros E.ON's remaining 46.65 percent stake in Uniper, which was demerged from E.ON last year.
Uniper, with total sales of around 67 billion euros in 2016, operates roughly 40 gigawatts of power plants in Europe and Russia. It has hydroelectric, coal- and gas-fired plants as well as stakes in gas pipelines, liquefied natural gas terminals and nuclear plants across Europe, plus energy trading operations.
But sources close to the matter have said that Fortum is mainly interested in Uniper's hydropower plants and interests in Swedish nuclear power stations and has already been working with a partner that might take on Uniper's coal-fired plants.
Fortum and E.ON said last week they were in advanced talks over a deal, and the full offer is unconditional on a minimum acceptance level.
But because E.ON cannot sell its own stake before 2018, as this would trigger a hefty tax payment, E.ON has the right to tender its own Uniper stake for the same price early next year.
If it opts not to, Fortum can sell to E.ON any Uniper shares by then already acquired in connection with the offer and, on top, would get compensation from E.ON that amounts to at least 20 percent of the equity value of its Uniper stake, or 752 million euros.
Under German takeover rules, Fortum has to make a bid for the whole of Uniper as E.ON's stake is above a critical 30 percent hurdle.
Uniper said in response to the news that its management board would analyse and assess the takeover offer.
"Our focus is on acquiring the E.ON stake, although we do believe our offer will also be attractive to other Uniper shareholders," Fortum Chief Executive Pekka Lundmark said in a statement.
"I am confident that our investment will strengthen the position of both companies to advance Europe's energy transition and believe that all stakeholders will greatly benefit."
Fortum said its bid of 22 euros a share offered a 36 percent premium to the price that prevailed before speculation on a potential transaction surfaced in May.
Earlier on Tuesday, Uniper shares closed 1.2 percent higher at 23.33 euros.
State-controlled Fortum is focused on carbon-free power generation, mainly in the Nordic region and Russia, and has been looking for a deal since selling its power distribution grids for 9.3 billion euros in 2014 and 2015.
Fortum also sought to placate Uniper management's opposition to a deal after the German group's chief executive told Reuters on Monday he viewed the approach as hostile, saw no strategic fit and voiced doubts about how Fortum would fund a deal.
"We intend to be a long-term investor in Uniper and we take our responsibility to all stakeholders very seriously," Fortum CEO Lundmark said. "It is absolutely clear that we will honour all existing commitments of Uniper towards its employees in full." ($1 = 0.8477 euros) (Reporting by Ludwig Burger and Christoph Steitz in Frankfurt, Jussi Rosendahl in Helsinki; Editing by Jane Merriman, Greg Mahlich)