(Reuters) – California power companies urged customers to conserve energy through Thursday to avoid more rotating outages as the brutal heat wave blanketing the state pushes the demand forecast for Wednesday over the prior day’s three-year high.

The California ISO, which operates the grid for most of the state, told utilities to prepare for possible rotating outages this week but never issued the order on Sunday, Monday or Tuesday because the weather moderated and consumers did their part to reduce the strain on the grid by using less power.

On Friday and Saturday, the ISO ordered utilities to start outages that left over 400,000 homes and businesses sweltering for about an hour each as air conditioning demand outstripped available generation resources.

The last time the ISO imposed rotating outages was in 2001 when several energy companies manipulated the power market to cause prices to spike and electric supplies to run artificially short.

California power companies with outages over the weekend included units of PG&E Corp, Edison International (NYSE:) and Sempra Energy (NYSE:).

Meteorologists at AccuWeather forecast high temperatures would reach the 90s and 100s Fahrenheit (35-41 Celsius) in some of the biggest cities in California this week. That is over 10 degrees F higher than normal.

The ISO forecast demand would reach 47,396 megawatts (MW) on Wednesday, which is just over Tuesday’s three-year high of 47,067 MW but still well below the grid’s all-time high of 50,270 MW in 2006.

After hitting record highs for Tuesday, power prices for Wednesday averaged just $201 per megawatt hour (MWh) at the Palo Verde hub in Arizona and $200 in SP-15 in Southern California, according to data on Refinitiv Eikon going back to 2010.

The Intercontinental Exchange (NYSE:), however, said prices for Wednesday averaged what would be record highs of $1,640/MWh at Palo Verde and $511 at SP-15.