Generating your own electricity really isn't that much far below grid prices - you'd typically be looking at a central estimate of 15 years to get a return on investment for a large Combined Cooling and Power cogeneration setup.

But, crucially, there's a huge level of uncertainty. You're making bets on the relative cost of nat gas and grid power, both of which have historically shown extreme volatility over that sort of timescale.

The level of risk means that very few proposed schemes go ahead in full unless there's some other factor involved (lack of sufficient grid connectivity, availability of subsidies).