Solar-rich South Australia has set a new record low for electricity demand from the grid but the renewables-dominated state still had the highest average prices in the December quarter as it struggled to meet soaring demand as summer weather took hold.

Wholesale power prices in all regions of the National Electricity Market were between 10 and 20 per cent lower in the December quarter than the year-earlier period, falling below $90 a megawatt-hour for the first time since late 2017, according to an Australian Energy Regulator report to be released on Monday.