It's not unusual in Ireland to use CHP (combined heat and power) units in reasonably large commercial premises some of which switch over when peak usage rates etc kick in to cut costs.
Ireland's generation capacity is also being streched to its limits and the ESB seems to be actively encouraging people to get off the grid during peak times where possible.
In large commercial applications CHP can be extremely efficient from an over-all energy consumption point-of-view and when implemented correctly can cut-down on the quantities of CO2 produced as they don't waste as much energy as conventional large scale powerplants and distribution systems.
On a small scale for a single house it wouldn't really work.
One or two large industrial operations actually generate more power than they need and sell the excess back to the grid.
In Cork for example the sewage treatment plant and the landfill (dump) produce their own power from methane produced as a byproduct of their processes and sell the rest back into the grid.
I've seen shopping centres (Malls) that use CHP too though as it's cheaper than buying electricity (at least at peak times) and buying natural gas to heat seperately.
