The ruling underscores the tricky balance US regulators face as utilities wade into the business of retailing solar energy, with many in the PV industry claiming that utilties possess a unique ability to stifle the competition.

This spring Xcel, which is among the largest US power utilities, proposed its “Solar*Connect” programme in Colorado, one of eight states it serves.

Xcel’s plan was to buy the output from a 50MW utility-scale PV plant to be sited somewhere in Colorado, and then sell the power for a premium to its retail customers who wanted to use solar electricity in their homes.