Historically, wholesale power prices for Northern and Southern California tracked closely with one another, indicating minimal market differences between the two areas. However, after the shutdown of SONGS in early 2012, the relatively inexpensive nuclear generation produced by SONGS had to be replaced with power from more expensive sources. Consequently, since April 2012 Southern California power prices have persistently exceeded Northern California prices, with the spread averaging $4.15/MWh, or 12% of the Northern California price.