What intraday liquidity is worth to a renewable portfolio

Intraday markets get sold to renewable owners on the strength of their spreads. The pitch cites the volatility between the day-ahead price and the last intraday trades, points at the hours where that spread was enormous, and implies that a portfolio with the right desk could have been on the right side of it.

Validation work that compares an intraday trading signal against a portfolio’s actual production, run in shadow mode over an extended period, tells a quieter story. Most of the value came from one activity: adjusting the day-ahead position when an updated forecast showed that the nomination had been too aggressive, or too cautious, before imbalance settlement priced the error. Speculative spread trading contributed little and cost a great deal of variance. Intraday earned its keep as an error-correction tool for forecast risk, and very little else.

Why the value is smaller than the spread suggests

Three constraints sit between a headline spread and a portfolio’s bank account.

The first is volume. A portfolio can only trade the position it actually has left to trade. If the updated forecast says production will fall 15 MWh short of nomination in a given hour, the sensible intraday trade is roughly 15 MWh. Being right about price direction with 15 MWh is a very different thing from the spread multiplied by the whole portfolio’s output, which is what a spread statistic implicitly assumes.

The second is timing. The forecast uncertainty that justifies a trade resolves gradually as delivery approaches. Trade early and you are acting on an update that may reverse. Wait until the update is reliable and the remaining time to gate closure is short, the book is thinner and the price has usually already moved towards where the balancing price will settle. By the time you know enough to trade confidently, some of the value has gone.

The third is liquidity itself. Intraday depth varies by hour, by day and by bidding zone. In the hours where a portfolio most needs to correct a large error, typically during a fast-moving weather event, everyone else with the same exposure is on the same side of the book. The spread that looks attractive in hindsight was partly the price of being able to trade at all.

The comparison most owners get wrong

The most common way to size intraday’s value is to take the day-ahead price against the balancing price for each hour, multiply by the portfolio’s forecast error, and call the total “what intraday could have saved.” This overstates the answer, sometimes substantially.

Imbalance settlement is more than a penalty. When a portfolio is short in an hour where the balancing price is below day-ahead, buying the shortfall back through settlement is cheap; occasionally it is a gain. When it is long in an hour where the balancing price is above day-ahead, the surplus settles well. Part of the gap between day-ahead and realized settlement would have been captured by the balancing mechanism with no trade placed. The incremental value of intraday is only the part where trading produced a better price than settlement would have, net of what it cost to trade.

Working with large renewable portfolios has shown that once the comparison is made against balancing settlement, the incremental value of intraday is real, positive and considerably more modest than the first calculation suggested. It is a worthwhile line in realized value per MWh, and rarely the largest one.

A decision framework

The intraday-versus-imbalance question in any given hour comes down to four things, assessed in order.

How large and how confident is the forecast update? A small revision inside normal forecast noise is not worth trading; the cost of being wrong about the revision exceeds the cost of settling it. A large revision backed by fresher weather input is a different matter.

Where does the expected balancing price sit relative to the tradeable intraday price? If the error will settle at a price close to, or better than, what the book offers, there is nothing to gain. The trade only makes sense when expected settlement is meaningfully worse than the intraday price, after fees.

Is there enough depth on the right side of the book to move the position without moving the price against yourself? For a small portfolio, usually yes. For a large one, the volume needing correction may be a large fraction of what the book will absorb in that hour.

What does the trade do to the portfolio position, as opposed to the asset position? Two assets in the same portfolio may have offsetting forecast revisions, and trading each in isolation pays fees twice to correct an error that would have netted to nothing. Intraday decisions belong at the portfolio’s net position.

Where the framework stops working

It breaks in thin markets. In zones or hours where intraday liquidity is poor, the book cannot be relied on to exist when the portfolio needs it, and a strategy that assumes it does will produce good backtests and bad months. There, imbalance settlement is the portfolio’s real route to market for its forecast error, and the effort is better spent on nominating well in the first place.

It also breaks when it is asked to do more than correct errors. The moment intraday activity is expected to generate returns beyond fixing the portfolio’s own position, it has become proprietary trading with a renewable asset as collateral. That may be a reasonable business, with its own capital, governance and staffing, and it should be judged on its own P&L.

Practical implications

Measure intraday’s contribution against balancing settlement. Measuring it against day-ahead flatters it.

Size expectations by the volume the portfolio has to correct and the liquidity available in the hours it needs to correct it, and ignore market-wide spread statistics.

Trade the portfolio position, so that errors which net across assets are never paid for twice.

Related reading

• How the Polish Balancing Market Changes Realized Value per MWh — the settlement mechanism intraday trading is correcting against
• Shadow-Mode Optimization — how to test an intraday strategy before committing real trading risk to it

If your intraday result were restated as the difference between the price you traded at and the price the same volume would have settled at, would it still look like a profit centre?