Elecrticity bills are set to surge by as much as 20 per cent from July, as power companies apply for their second round of rises within a year.

It come as Sydney is hit with rolling blackouts and demand is pushed to new peaks after a week of extreme weather conditions.

Today, the heatwave will push the temperature to above 40 degrees in the city's west, prompting fears of more blackouts as ageing power equipment wilts under the pressure of exceptionally high demand and the hot weather.

Parts of Enfield, Croydon Park, Belfield and south Strathfield were expected to lose power again yesterday while two EnergyAustralia main cables were being repaired.

But 22 mobile generators were in use and peak afternoon use was slightly lower than it had been.

re than 12,000 residences and businesses have lost power in the past five days.

The average household bill will rise by $130 a year under applications to the Independent Pricing and Regulatory Tribunal yesterday. The rise is about twice that already approved.

NSW power bill rises to double, more power cuts