Commenting on the move by ScottishPower, consumer watchdog Which? said: "With household budgets under pressure, it seems incredible that energy suppliers can effectively use their customers' money as interest-free loans."
The group added that the other big six energy firms should follow suit, and anyone unhappy with the service they are being offered by their current supplier should simply switch tariffs.
ScottishPower said 3.2 million of its customers could be affected by the move, which will see the energy giant pay back £1 for every £33 a customer has in credit over £100.
The calculation will be made every 12 months and the maximum balance a customer can claim interest on will be £500.
ScottishPower has also introduced a pay-in-advance option which promises to save customers five per cent on their standard tariff.
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