Offer On Table

The Offer On Table is the primary screen that should be used to review how high offer prices (and so potentially system prices) could be if National Grid needs to bring a plant on to fill a gap or to build strategic margin.

For this chart the ability to filter is particularly useful and so can be used to both view the plants available to provide capacity above that provided by already online plants (this uplift form online plants can be viewed in the Availability > System Margin Summary chart) within the next 30 minutes.

This chart has been filtered to only show plants with an NDZ (Notice to Deviate from Zero) less than 30 minutes and sorted to find the cheapest plants if the gap proves to be very small by sorting by the Minimum Run Cost.

This Minimum Run Cost is the cost of a run to the Stable Export Limit (SEL – minimum level of generation able to be maintained for a prolonged period) and then to zero, with the return to zero coming as soon as is physically possible.

This shows the options to National Grid to flexibly manage any brief shortfalls, but on tighter days the focus for National Grid is more about boosting the levels of flexible capacity during an evening peak.

This is achieved by holding back plants with short response times, as well as starting up larger and slower plants to run at their Stable Export Limit. This is achieved by backing down existing plants in the market.

The effect this has is that where the market had one 500MW unit running at 495MW with 5MW headroom, it could instead have two running at 250MW with a combined 500MW of headroom, whilst only seeing a 5MW increase in net generation; with one being bid down by 245MW and another offered up by 250MW.

The list of plants available to perform this service can be seen via the following setup:

This has been achieved by filtering for an NDZ (Notice to Deviate from Zero) greater than 45 minutes and a MNZT (Minimum Non-Zero Time) also greater than 45 minutes.

This shows that National Grid could currently bring on nine plants for up to £60/MWh, suggesting limited high-priced potential beyond the typical pumped storage range of around £110/MWh.

With such a long list, generators are effectively being encouraged by market fundamentals to submit low prices, where the list shortens this effect does not apply and so on the tight days, prices on this list can rise into the hundreds or thousands of pounds per MWh.

On this chart it is possible to examine the information at the accepted/declared bids/offers at a selected group of units. By default the first 20 units on the table will be selected.

Then information about these units can be seen for those selected by using 'Pricing Unit Details' > 'See Declared Offers...'. The selected units can be changed by clicking on the cell containing the unit name without click on the link on the name itself.

Included Data

The data on this chart is as follows:

• Av. Price (SEL) – The average price of getting a unit from zero up to the SEL (Stable Export Limit) – the minimum amount of generation possible from the unit.
• Av. Price (MEL) – The average price of getting a unit from zero up to the MEL (Maximum Export Limit) – the maximum amount of generation possible from the unit.
• Min Run Cost – Considers the cost, factoring for dynamic data such as ramp rates and minimum non-zero times to work out the lowest overall cost to the system of bringing the unit online following its shortest run profile.
• MRC/MEL – Divides the minimum run cost by MEL to produce a metric that compares the cost of bringing the unit online at its minimum output against its maximum output. This is used since the unit once online can then provide a volume of power between SEL and MEL that the system can use to balance supply and demand.
• Latest Call (MEL) - The last point in time the unit can be called at in order to reach MEL by the Required Time. This can exclude units unable to respond in a given time frame.
• Earliest Offline - The earliest a unit can come offline having been called at the latest call time. Filtering this value can exclude units that will have to generate for too much time.
• SEL - Stable Export Limit in MW.
• MEL - Maximum Export Limit in MW.
• BOA - Bid/Offer Volume in MW.
• MNZT - Minimum Non-Zero Time in Minutes.
• NDZ – Notice to Deviate from Zero in Minutes.
• To SEL – Time (not factoring for NDZ) to get from zero to the SEL.
• To MEL – Time (not factoring for NDZ) to get from zero to the MEL.
• MZT – Minimum Zero Time in Minutes.

Note that moving left and right when target time has been chosen by period will move from one settlement period to the next. This will take the average values across the settlement period. Within the settlement period levels of generation will change minute-to-minute and so LIVE and Custom Time allow for the selection of either the current position or the position at a particular time.

Colours Guide

The following details the colours used in this chart:

Data Service

The Offer On Table data can be extracted from the API. The parameters used as follows: