The ZEMA graph shows day-ahead peak electricity prices for December 2015, as reported by ICE. The red line represents CAISO SP15, the green line represents PJM APS Zone, the black line represents ERCOT North 345KV, and the blue line represents NYISO Zone A. Units are in USD/MWh.
The noticeable electricity price increase starting December 24th in SP15 cannot be explained by seasonal temperature changes since it remained steadily warm in Southern California. It is likely that the holiday season boosted the demand driving prices up.
An unusual price decrease in Zone A of NIYSO, even during the holidays despite slightly colder weather in New York also supported by climbing natural gas prices, was likely supported by ample generation and healthy transmission capacity.
Data Sources: ICE