It should be noted that most energy is purchased in the day ahead market, which I can't recall seeing negative - although maybe in Texas (ERCOT).
The provider will need to buy back the energy in at the realtime price if they are not generating. But I am not sure about the market rules for negative prices.
Look at the North Zone of New York, it spent a good chunk of Sunday negative: http://mis.nyiso.com/public/csv/realtime/20160508realtime_zo...
Use the LMP column(Locational Marginal Price)
It should be noted that most energy is purchased in the day ahead market, which I can't recall seeing negative - although maybe in Texas (ERCOT).
The provider will need to buy back the energy in at the realtime price if they are not generating. But I am not sure about the market rules for negative prices.