Ofgem is probing two companies to find out if they overcharged the grid to lower their production levels.
The energy regulator said it was looking into whether EP SHB, which owns the South Humber Bank gas power plant, and SSE Generation broke the terms of their licences in a way that could have ultimately pushed up household energy bills.
A key part of ensuring that the lights stay on across Britain is to balance the grid.
This means it is important that power plants do not generate more electricity than households and businesses use at any given point, and vice versa.
To do this the grid can ask electricity generators to stop producing, or reduce the amount of electricity they are putting into the grid.
In return the grid will often pay these producers to turn off or reduce their production.