How much profit is too much? Perhaps it depends on the industry and perhaps it depends on your political views. However, when you see figures such as Tesco making £2,188,000,000 (just over £2 Billion), remember that it depends on how this relates to their total sales - in Tesco's case it is just £3.3% or £3.30 per £100 of stuff that they sell.
Below are some examples of Operating Profit Margins. SSE, nPower and Centrica (British Gas) are all power companies - you will recognise the rest of them.