Only in Texas - negative electricity prices

Slate Magazine reported an interesting, if somewhat bewildering, turn of events in the American state of Texas the other day, when circumstances conspired to make the local price of electricity in Texas negative.
We are perhaps used to thinking of Texas as an exclusively oil-and-gas state, home to rednecks, tycoons and Tea Party-voting ranchers. But Texas is also home to over 10,000 wind turbines, and it produces 9% of its electricity from wind power, over twice the US average. So, on a windy night, when there is little electricity demand requiring baseload generation, wind can actually become the dominant source of electricity there.
OK, but negative prices?
This unusual occurrence is the result of four factors specific to Texas' electricity system. One, it is an electricity island, not connected to neighbouring states, so that it can neither directly import power from other states, nor, by the same token, can it export surplus electricity production elsewhere. Secondly, the aforementioned relatively large wind generation capacity in the state. Thirdly, the Texas electricity grid has a unique system of purchasing its power through online real-time auctions from providers, whereby, every five minutes, it buys the cheapest electricity being offered in order to fill the demand from its consumers. And finally, wind operators receive a $23/megawatt-hour federal tax credit, whatever the price of electricity sold, so it is always in their interests to produce and sell as much power as they can.
Given these factors, on the particular night in question, the spot price of Texas' electricity fell from $17.40/megawatt-hour earlier in the evening down to zero by 1:45am, and then into negative prices for the rest of the night until about 8:15am, as it was cost effective for wind turbine operators to pay the state's electricity system to take power off their hands. At 5:45am, it reached its lowest cost at -$8.52/megawatt-hour. Oh, for a cheap and reliable battery storage system!
As the Slate article remarks: "Only in Texas, folks".