Western Australia will spend $126 million preparing the state's electricity grid for a massive surge in green energy demand.

The state government funding, to be included in Thursday's 2023/2024 budget, will fund network planning in WA's southwest.

It comes as the McGowan government released an assessment of the potential change in electricity demand over the next 20 years.

Energy Minister Bill Johnston said expanding the electricity grid was the most cost-efficient way to support decarbonisation as the state moved towards net zero emissions by 2050.

He said it would be extended to wind and solar projects and be able to transmit the green energy they generated.

"This modelling builds on the 2020 whole of system plan, recognising the urgency of action on climate change and the accelerated decarbonisation plans for the state's major industries," he said.

"Transmission infrastructure is a critical enabler of our energy transition, being essential for connecting users with renewable energy resources."

Initial modelling anticipates that by 2042 the state's southwest could require up to 51.1 gigawatts of new generation and storage capacity, almost 10 times the amount of generation currently on the region's grid.

The funding will allow WA's electrical network operator, Western Power, to commence planning works in the northern, central, southern and eastern corridors of the southwest interconnected system.

It will also support planning and purchase of long-lead items for initial upgrades, which is expected to see a boom in new energy projects.

Opposition spokeswoman for energy Steve Thomas said the McGowan government had failed to prepare or adequately fund the state's energy transition plan.

He said the assessment identified that 4000km of new high-capacity transmission lines could be needed over the next 20 years and this was likely to cost billions of dollars.

"That is currently unfunded, as is much of the rest of the transition process," Dr Thomas said.

"How does the government expect to pay for this, what costs will be passed on to energy users in WA, and will that reflect an increase in energy bills?"