Yep, it's a fairly complex (as in, I don't understand it fully, although I've read a little) interaction of subsidies, varying prices, and the same of standby c

Yep, it's a fairly complex (as in, I don't understand it fully, although I've read a little) interaction of subsidies, varying prices, and the same of standby capacity (i.e. not actual capacity, but the ability to ramp up to generate x GW in y seconds should it be needed, to cover spikes - generally by gas power stations, which are able to vary output power far more quickly than most types, and by pumped storage, which can release up to a certain amount of energy per day for very short, large spikes, for example one one caused by people putting the kettle on when popular TV shows finish). Most of the trading is automated and humans aren't involved, as prices are being constantly renegotiated as demand changes, and supply may also be throttled down or up for fuel-consuming non-nuclear sources (nuclear runs as close to max output as they can due to running costs being consistent through output levels, and fuel costs being negligible in any case) to maximise profit - generally by increasing output as prices rise, but for wind and other renewables, sometimes output is artificially limited as a way of manipulating subsidies, which can actually be more profitable for the operators if less power is sold under the right grid conditions. All the data is open, which lets some interesting sites appear, such as http://www.gridwatch.templar.co.uk/index.php http://www.gridwatch.templar.co.uk/index.php . Mousing over the source numbers shows a brief overview, and the site also includes international purchase/sale data.