On an hourly basis, spot prices for Spain hit zero in the early hours of December 21 and 22 for the first time since January 1, 2003.
"Demand today is like it is on a Sunday, there's lots of wind and we haven't seen this much hydro for a couple of years, so those three factors have come together," a dealer said.
Spain's wind parks, the world's third-most productive, were meeting 28.1 percent of all demand at midday, while hydroelectricity accounted for 13.7 percent of the generating mix after recent heavy rainfall.
Traders expected spot prices to remain weak even after demand picks up with people returning to work in the New year, because additional supplies of cheap nuclear power are expected.
The Asco I plants is due to finish maintenance in January and the Almaraz I plant is awaiting approval for expanding its output of thermal power.
Together, both plants produce 2,000 megawatts, or about 7 percent of average demand for power.
Spain's six other nuclear plants were generating a total of 5,343 MW, equivalent to 16.3 percent of demand, according to data from the CSN nuclear regulator and national grid operator REE.
Forwards were little changed in light volume, with dealers reporting little trading in calendar-year 2010 — the benchmark for most of 2009 — because trading in the contract is about to expire.
Cal-2011 traded up 0.09 to 43.90 euros/MWh on the Omip exchange, which accounts for about 30 percent of trade.
Spanish power stations were meanwhile emitting 4,918 tonnes per hour of carbon dioxide, an unusually low amount for a weekday.