Blackout fears are growing for 20 million Americans as extreme heat pushes the power grid toward its limits.

The US Department of Energy (DOE) on Sunday authorized the operator serving parts of 17 states to activate idle power sources in an effort to keep the lights on.

The move allows backup plants that are normally switched off to generate extra electricity if surging demand threatens to overwhelm the grid. 

The emergency order covers Arizona, Arkansas, Colorado, Iowa, Kansas, Louisiana, Minnesota, Missouri and Montana. It also includes Nebraska, New Mexico, North Dakota, Oklahoma, South Dakota, Texas, Utah and Wyoming.

The move gives Southwest Power Pool (SPP), which oversees the central US grid, permission to deploy backup generators if electricity supplies become strained.

It is the final step before a Level 3 Energy Emergency Alert, which could trigger rolling blackouts to prevent a wider collapse.

The order will remain in effect through August 3 to 'mitigate the risk of blackouts,' according to the DOE. 

It comes as 40 million Americans face extreme heat warnings, with soaring temperatures and oppressive humidity driving up demand for air conditioning.

Former DOE official Kerry Duggan said extreme heat is increasingly becoming an energy challenge rather than a simple weather event.

'As we experience more frequent and longer-lasting heat waves, our ability to keep homes, businesses and critical facilities cool depends on having a reliable and resilient electric grid,' Duggan said.

National Weather Service's Monday alerts across much of that region forecast temperatures or heat indexes above 100F, with readings reaching 113F to 115F in parts of Louisiana. 

Texas, Arkansas, Oklahoma, Kansas, Colorado and Nebraska were also facing dangerous conditions approaching 110F. 

Meteorologists have blamed the brutal conditions on another heat dome, which traps hot air over a region and causes temperatures to climb for days. 

US Secretary of Energy Chris Wright said: 'The Trump Administration is tapping into an abundant supply of unused backup generation to maintain affordable, reliable, and secure power for hardworking American families and businesses.

'The previous administration's energy subtraction policies weakened the grid, leaving Americans more vulnerable during emergency events. 

'Thanks to President Trump's leadership, we are reversing those failures and using every available tool to ensure Americans have continued access to affordable, reliable, and secure energy to power and cool their homes.' 

DOE estimates that more than 35 gigawatts (GW) of unused backup generation remain available nationwide. 

The ⁠risk of rotating blackouts escalated in SPP in the past week. 

On Friday, a sudden loss of electricity imported from neighboring grids left the utility company with razor-thin reserves for possible power plant outages and falling renewable generation in its territory spanning parts of Arizona, Colorado, Montana, Nebraska, South Dakota, Wyoming and Utah. 

SPP ​avoided outages that day ​by appealing for voluntary ⁠conservation of electricity to maintain reserves. 

But the company expected the western territory's highest demand of the week on Sunday evening as temperatures rose above 90F.