The term refers to the excess market of energy that has not been contracted for - as distinct from "mercado contrato." ("mercado ocasional ... [e]ste... término, conocido también como energía en stock, cubre el margen que no abastece la energía contratada") [http://burica.wordpress.com/2008/07/10/el-mercado-ocasional-...] Thus, "spot market" would fit given that "[t]he spot market or cash market is a commodities or securities market in which goods are sold for cash and delivered immediately. . . The spot energy market allows producers of surplus energy to instantly locate available buyers for this energy, negotiate prices within milliseconds and deliver actual energy to the customer just a few minutes later" [http://en.wikipedia.org/wiki/Spot_market]