* Market responds to low renewables forecast
* Spot contracts still hold onto some gains
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UK prompt prices fell Monday on easing supply pressures as more thermal plant capacity came online, while lingering margin concerns curbed losses, sources said.
Baseload power for Tuesday delivery was last heard trading at GBP42/MWh before the 11 am London time market close, more than GBP23/MWh lower than Friday's close for Monday.
No peakload (block 3+4+5) contracts were seen trading Monday and was valued GBP36.40/MWh lower than Friday's close at GBP48.60/MWh using block 3+4 power price of GBP42.50/MWh and block 5 power price of GBP60.75/MWh.
Also reflecting the supply squeeze in the UK electricity market, Sunday's spot auction on N2EX and EPEX Spot exchanges saw the block 5 power price, which is typically the peak demand period, settle just short of GBP100/MWh.
However, a trader said Monday the block 5 price in the "intraday market is is now below GBP90/MWh. Coal is coming in and I think we have almost 1 GW of coal extra in this morning".
A higher UK clean dark spread -- the theoretical margin for a 35%-efficient coal-fired power plant including emissions -- meant there was more incentive for UK coal-fired power generators to produce electricity.
At midday Monday, coal output rose to nearly 2 GW, up from around 600 MW at the same time Friday, National Grid data showed. Since last Monday, coal output during the morning peakload hours has stayed well below 1 GW, grid data showed.
N2EX and EPEX Spot's auction saw the block 5 clearing at GBP56.98/MWh, lower than the settlement value of GBP99.70/MWh on Sunday, data showed.
"Wind seems to be a strong driver, almost zero forecast for tonight, down from 4 GW in the morning," a trader said.
By Tuesday, peak wind power generation was forecast to jump to 5 GW but solar power output to decline by 1 GW to around 3.5 GW.
Although both the day-ahead contracts fell sharply on Monday, they remained above than the average price seen June 1-23, which stood at GBP39.25/MWh for the base and GBP43.35/MWh for the peak, according to S&P Global Platts pricing data showed.
Further on the supply side, gas-fired power production stood at 17.7 GW, wind output was at negligible levels of less than 200 MW, and nuclear power generation was a touch lower at 7.7 GW, National Grid data showed.