As South Australians endure a severe heatwave with temperatures soaring into the low to mid 40s, residents have been warned they could face power cuts as the state struggles to meet power supply demands.

On Wednesday night, after the mercury tipped 42 degrees Celsius in Adelaide, 90,000 homes and businesses had their power shut off for 45 minutes, in a process called load shedding, because there was not enough supply to meet demand.

What is load shedding?

Load shedding occurs when the Australian Energy Market Operator (AEMO) directs power companies within Australia to start switching off customers' power supply because the power system is at risk.

The system always has to remain in balance between supply and demand — if there is no extra supply available, the authorities have no choice but to reduce demand by shutting customers off.

If they don't, the entire system can fail, causing a statewide blackout like what happened in South Australia in September 2016.

Why did they order it?

On Wednesday night, the AEMO predicted there was more demand than available supply.

The major Heywood Interconnector was importing power from Victoria close to its limit.

Wholesale power prices were at their maximum ($14,000 per megawatt hour) and AEMO called for more power stations to offer electricity into the market but none did.

Why is SA running out of power?

With the interconnector to Victoria included, South Australia has more than enough power generation to cover demand even on the most extreme days.

South Australia has one of the highest mixes of wind and solar energy in the world. ( AAP: Angela Harper )

But a large and growing proportion of that generation is from intermittent renewable sources like wind and solar.

When the wind isn't blowing, the market operator can't direct wind farms to dispatch power.

The Heywood Interconnector was recently expanded to allow greater power to flow in or out of the state. But it still has a limit of about 600 megawatts.

The market for electricity is much tighter in South Australia since the closure of the state's sole coal-fired power plant in Port Augusta, Northern, last year.

AEMO has forecast potential lack of power reserves across this summer, and every summer, until additional supply is added to the mix.

Why are renewables being blamed by the Federal Government?

SA has one of the highest mixes of wind and solar energy in the world.

It can be argued that the advent of high levels of renewable energy have helped bring down wholesale prices in South Australia.

Subsidised via renewable energy certificates, wind farms can produce power at a much lower market cost than traditional coal and gas generators.

But in turn, this flood of cheap renewable energy has helped hasten the demise of thermal generators like Northern.

With fewer traditional coal and gas generators in the market, the state has less capacity to generate energy on demand. This is a problem when demand is high and the wind isn't blowing.

Map The suburbs and areas affected by load shedding on Wednesday in South Australia.

Is this just an issue for SA?

No. The AEMO is warning of potential load shedding in New South Wales on Friday as it braces for an extreme heatwave.

The problem is likely to get worse in the medium term with the closure of Victoria's Hazelwood brown coal power station.

That state is expected to join South Australia next summer in reaching regular electricity reserve shortfalls.

What's the solution?

South Australia needs more sources of power that can be dispatched on demand.

Local authorities are already investigating the possibility of an additional interconnector to NSW, Victoria or Queensland.

If they decide to proceed, the solution will take years and cost power users billions of dollars.

Other potential options include storage of renewable energy — through batteries, pumped hydro or other technology.

The SA Government is also seeking to stimulate a new market entrant, in all likelihood a gas generator, by offering a long-term contract to supply 75 per cent of its own power needs.