Power companies are warning the road to fully renewable energy will be a bumpy one.  

The big four power companies will co-invest in an energy stockpile which will include 600-thousand tonnes of coal.  

Mercury Chief Executive Stew Hamilton says power companies have a commitment to renewables.  

He told Mike Hosking Mercury has invested a billion dollars into geothermal and wind power, but there needs to be a backup.  

Hamilton says when the wind isn't blowing and the sun isn't shining, there needs to be a more solid source of energy. 

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