EDMONTON -- Green energy groups have joined an environmental think tank in calling on the Alberta government to legislate firm targets for renewable power.

The Pembina Institute and a number of green energy firms say in a letter to Premier Rachel Notley and others in her government that the province has shown strong leadership in its Climate Leadership Plan.

But the letter says there are questions about how the targets will be enforced and what considerations would lead to a deviation from the targets.

The government is moving to phase out coal-fired power by 2030 and simultaneously triple renewable energy's share of the power supply to 30 per cent.

The groups that signed the letter are asking the Alberta government to turn that target into law.

The letter says more companies will enter the market if there's certainty, meaning bids for green energy projects will be lower.

Alberta Environment Minister Shannon Phillips has said an auction-style call for renewable generation proposals will be announced in November.

She has also said support could come in the form of government purchases of renewable energy credits from the projects on long-term contracts, or through "contract-for-differences" agreements that would top-up developer revenue when prices fall.