Up to 21,000 WA businesses face a surprise 10 per cent power price hike from next month, in part because a surge in solar panels has reduced electricity demand and forced Western Power to lift its tariffs to claw back a slump in revenue.

However, the hit could have been worse, with the State’s economic watchdog rejecting a rearguard bid by Western Power for a rise of almost 20 per cent – in order to avoid hitting businesses with “price shock”.