Feb. 6 (Bloomberg) -- Windmills owned by Iberdrola SA and Acciona SA, the world’s biggest operators, may generate a record amount of electricity in Spain next week that will slash local power prices, calculations from Bloomberg weather data show. An Atlantic Ocean storm is forecast to blow winds averaging 17.6 miles an hour across the peninsula. Lighter breezes, at 13.6 miles an hour, were enough to set the previous record output three weeks earlier. The extra supply then cut power prices 11 percent. “This will push the spot market lower next week,” Manuel Palomo, a Citigroup Global Markets analyst in Madrid, said in an interview. Palomo covers Spanish generators Iberdrola, Endesa SA and Acciona, which own or run wind farms in five continents. Spain and Germany, the world’s biggest wind-energy markets after the U.S., have changed the dynamics for wholesale power trading by forcing sellers to read weather reports. Because their fuel is free, wind turbines undercut traditional generators that burn coal, natural gas and oil. The wind-speed measurements are calculated from a composite of forecasts for Castile-La Mancha and other regions, weighted by Bloomberg in proportion to the number of turbines in each zone. Wind has become a bigger factor in Germany and Spain because they both subsidize rates for the renewable energy and give producers preference to sell in wholesale markets. Utilities that acquire power, from Essen-based RWE AG in Germany to Union Fenosa SA in Madrid, must buy any available wind and solar power before tapping fossil-fuel plants. Carbon Versus Wind The two governments’ financial incentives spawned an investment boom that’s helping wean both nations from carbon-based combustion, a source of greenhouse gases that warm the planet. In Germany, swings in wind speed can move power prices as much as 10 percent. “Wind power can produce 15 to 20 gigawatts sometimes, which is the equivalent of turning several nuclear power plants on and off at one time,” Chris Panton, senior energy trader in Prague at KIH Energy Trading GmbH, said today by telephone. After adding 11 percent more wind-power capacity last year, Spain’s one-day production record was broken on Jan. 23. On that day winds swelled across the nation, propelling turbines to generate 221 gigawatt-hours, or 27 percent of national demand for homes and businesses. The Atlantic storm next week is forecast to blow winds across the plains of Spain at 29 percent faster than in the previous record-output week of Jan. 19-23. Crude Effect CIMD SA, a Madrid-based broker, offered yesterday to buy power for working days next week at no more than 37 euros a megawatt-hour, one of its brokers said. That’s 11 percent below this week’s average price. A megawatt-hour supplies about 1,500 Spanish homes for 60 minutes. London-based broker ICAP Plc said power for 24-hour delivery on Feb. 9 fell 4.3 percent today to close at 37.80 euros a megawatt-hour, a 15-month low. Daily prices have already fallen more than 40 percent in the last six months, reflecting the plunge in crude oil this year. Rates utilities pay for natural gas, Spain’s second-largest fuel for power plants, are linked to oil in supply contracts. While gusty days can lower prices for generators, they prefer them to still days, when their turbines make no money at all. Wind power in Spain is a relatively high-margin business. Generators get paid a premium for wind energy, money on top of the fixed price set by the wholesale “pool” market that’s paid to all kinds of plants -- from coal and oil to gas or nuclear. The premium is as much as twice the rate of conventional power traded on the pool exchange. Castile-La Mancha Nearly two-thirds of wind generation is concentrated in three parts of Spain. They are Galicia, in the northwest, and the central regions of Castile-La Mancha and Castile & Leon, according to data from local trade group Asociacion Empresarial Eolica. Iberdrola and Acciona, with wind farms in four continents, are the biggest wind-energy companies, followed FPL Group, of Juno Beach, Florida, according to the Brussels-based Global Wind Energy Council. The regional wind data is provided to Bloomberg by San Francisco-based Custom Weather.