http://www.nationalgrid.com/uk/electricity/data/realtime/demand/demand8.htm
http://en.wikipedia.org/wiki/Energy_in_the_United_Kingdom
In the whole of UK, there is an average of 35800MW, yet on the online game, there is far more demand then that, despite having a smaller area.
I would suggest 1/6 to 1/8 of current values.
possibly even 1/10 for larger maps with more cities.