* Cool weather, less green power support prompt
* Curve posts gains after previous day's losses
* EEX stats, economic news reflect bearish picture
FRANKFURT, June 5 (Reuters) - European prompt power prices firmed on Tuesday on an expected halving of wind and solar power output in Germany on Wednesday.
"It's a kind of yo-yo effect on the day-ahead market, yesterday the trend was the opposite for today," one trader said.
Wednesday delivery baseload gained 3.10 euros in Germany to 47.90 euros a megawatt hour. in France, the same position gained 2.40 euros to 48.90.
Weather data showed wind power will likely drop to 2.5 gigawatts (GW) on Wednesday and solar to 5 GW, compared with 5 GW and 10 GW of materialised capacity usage on Tuesday.
This will result from low pressure fronts curbing sunshine, accompanied by cool temperatures and rain, which will lift demand.
In France, EDF's 900 MW Gravelines 1 nuclear reactor stopped for an unplanned outage late on June 4, power grid RTE showed on its website, providing no reason or restart date.
The effect of plant outages elsewhere on supply were limited.
Power curve prices rebounded from their new lows on Monday without big volumes or driving factors, as UK fuels market remained closed.
With Britain still shut for the Queen's Diamond Jubilee holiday, there was no input from OTC gas and coal markets. Exchange traded EU carbon rights edged a little higher.
German Cal '13 baseload power was 10 cents higher at 48.65 euros and the equivalent French contract was 20 cents higher at 49.70 euros. Both contracts are at their lowest levels in 18-1/2 months.
Oil prices slid towards $98 a barrel as demand expectations were hit by another round of poor data from the euro zone and comments by the International Energy Agency (IEA) that oil prices were still a threat to the global economy.
Elsewhere, the European Energy Exchange said it traded 68.6 terawatt hours of German and French power futures in May, down from 85.9 TWh in May 2011.
Analysts say that recession and regulation have curbed operators' interest in forward markets. Power curve prices have fallen by 20 percent in the past 12 months.
May spot power trading by the Epex spot market in Paris amounted to 29.2 TWh compared with 25.0 TWh in May 2011, the exchange reported. This includes day ahead auctions for Germany/Austria, France and Switzerland and an intraday market.
In macroecnomic news, German industrial orders slumped more than expected in April, posting their steepest fall since November 2011, adding to signs of a slowdown even in Europe's biggest economy that has so far held up.
In utility news, E.ON said it could face a low double-digit million euro hit from "irregularities" of one energy trader. (Reporting by Vera Eckert,; Editing by Alison Birrane)