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Belgium · Elia

How well does Elia see the next quarter coming?

We compared five months of Elia imbalance forecasts with the prices that were actually settled. These are the most striking findings.

Kiani Barbary
Logo of Elia, the Belgian transmission system operator

Anyone running a battery on the imbalance market makes a decision every quarter of an hour: charge, discharge or stand still. To support that, Elia publishes an imbalance forecast every minute, for the current and for the next quarter. We looked into how useful those forecasts really are. We compared the forecasts from 28 April to 24 September 2026 with the settled prices, and analysed the prices from 1 October 2025 to 24 September 2026. Together that is about 14,250 quarters.

First, the rules of the game

Imbalance is settled per quarter of an hour. When the Belgian grid is short, with too little power, the MIP applies: the price of the most expensive reserve Elia had to activate. When the grid is long, with too much power, the much lower MDP applies. So a forecast is mainly valuable when it picks the right side: short or long.

The side is usually right

At the start of the quarter, the current-quarter forecast picks the right side in 82% of quarters. After 5 minutes that is 88%, after 10 minutes 93%. The next-quarter forecast is at 75% a quarter of an hour ahead and at 81% one minute before the start.

When the forecast is made Side right Avg. error False long False short
15 min before the quarter75.0%103 MW15.3%9.6%
8 min before the quarter78.5%91 MW13.9%7.6%
1 min before the quarter81.4%80 MW12.1%6.5%
At the start of the quarter81.6%79 MW11.8%6.6%
5 min into the quarter87.6%53 MW9.1%3.3%
10 min into the quarter92.5%27 MW6.1%1.4%

The first three rows are the next-quarter forecast, the last three the current-quarter forecast. False long: forecast long, but the quarter ended short. False short: the other way round.

How often the forecast side is right

% of quarters, by minute relative to the start of the quarter. Before the start this is the next-quarter forecast, after it the current-quarter forecast.

70% 80% 90% 100% start of quarter 75% 82% 88% 93% measured so far Elia −15 min−10 min−5 min0 min+5 min+10 min −15 min: Elia forecast 75.0%−14 min: Elia forecast 76.3%, measured so far 73.6%−13 min: Elia forecast 76.9%, measured so far 74.6%−12 min: Elia forecast 76.8%, measured so far 75.0%−11 min: Elia forecast 77.0%, measured so far 75.5%−10 min: Elia forecast 77.3%, measured so far 76.0%−9 min: Elia forecast 78.0%, measured so far 76.6%−8 min: Elia forecast 78.5%, measured so far 77.2%−7 min: Elia forecast 79.2%, measured so far 77.6%−6 min: Elia forecast 79.9%, measured so far 78.0%−5 min: Elia forecast 80.3%, measured so far 78.6%−4 min: Elia forecast 80.8%, measured so far 79.0%−3 min: Elia forecast 81.2%, measured so far 79.6%−2 min: Elia forecast 81.1%, measured so far 80.1%−1 min: Elia forecast 81.4%, measured so far 80.7%0 min: Elia forecast 81.6%+1 min: Elia forecast 83.2%, measured so far 81.6%+2 min: Elia forecast 84.8%, measured so far 83.0%+3 min: Elia forecast 85.5%, measured so far 84.3%+4 min: Elia forecast 86.8%, measured so far 85.9%+5 min: Elia forecast 87.6%, measured so far 87.4%+6 min: Elia forecast 89.0%, measured so far 88.6%+7 min: Elia forecast 89.8%, measured so far 89.5%+8 min: Elia forecast 90.7%, measured so far 90.5%+9 min: Elia forecast 91.8%, measured so far 91.3%+10 min: Elia forecast 92.5%, measured so far 92.0%+11 min: Elia forecast 93.2%, measured so far 92.7%+12 min: Elia forecast 94.2%, measured so far 93.7%+13 min: Elia forecast 94.8%, measured so far 94.7%+14 min: Elia forecast 95.2%, measured so far 95.2%
Elia forecastImbalance measured so far (no forecast)

One caveat: before the quarter starts, the forecast is only 1 to 3 percentage points better than simply looking at the imbalance measured so far. Only within the quarter does it pull clearly ahead.

You can't see the price itself

Elia forecasts the imbalance, not the price. Quarters forecast short settle at €160 to €168/MWh on average, quarters forecast long at €72 to €79/MWh. But exactly how high the price gets depends on the bids on the balancing market, and those are not in the forecast. Converting the forecast into a price leaves you €54/MWh off on average. That is no better than taking the usual price for that hour. Even knowing the imbalance exactly still leaves you €48/MWh off.

The costliest mistake always goes the same way

The forecasts err towards long more often than towards short: false long happens 1.6 to 4.4 times as often as false short. And that is exactly the mistake that costs money. Anyone counting on a cheap long quarter that turns out short pays about €137/MWh on average, while a real long quarter settles at €59 to €72/MWh.

For the price, both forecasts are structurally about 15 MW too long. The probabilities Elia publishes also underestimate short: when the current-quarter forecast gives a 35% chance of short, 51% of those quarters actually end short. The good news is that this bias is stable from month to month, so you can correct for it.

Small imbalances are noise

Most misses happen in quarters with a small imbalance. Below 50 MW, which is 29% of quarters, the side is right in only 59 to 68% of cases: barely better than guessing. Above 100 MW it is 86 to 100%. A silver lining: with a small imbalance, MIP and MDP are also closest together, so a miss costs less than its frequency suggests.

The sun sets the daily rhythm

Imbalance price by hour of day

Settled price per quarter, October 2025 to September 2026.

0 80 160 240 €/MWh €55 at 13:00 €123 at 19:00 00:0006:0012:0018:0023:00 00:00: median €102/MWh, middle 80% €32/MWh to €180/MWh, 3.5% negative01:00: median €100/MWh, middle 80% €33/MWh to €171/MWh, 3.2% negative02:00: median €100/MWh, middle 80% €24/MWh to €162/MWh, 5.2% negative03:00: median €98/MWh, middle 80% €16/MWh to €157/MWh, 6.5% negative04:00: median €97/MWh, middle 80% €17/MWh to €156/MWh, 6.3% negative05:00: median €100/MWh, middle 80% €26/MWh to €164/MWh, 4.7% negative06:00: median €107/MWh, middle 80% €38/MWh to €181/MWh, 2.8% negative07:00: median €108/MWh, middle 80% €37/MWh to €199/MWh, 3.8% negative08:00: median €100/MWh, middle 80% €22/MWh to €203/MWh, 5.4% negative09:00: median €94/MWh, middle 80% €3/MWh to €191/MWh, 8.6% negative10:00: median €79/MWh, middle 80% €-9/MWh to €179/MWh, 15.3% negative11:00: median €72/MWh, middle 80% €-10/MWh to €162/MWh, 19.0% negative12:00: median €63/MWh, middle 80% €-19/MWh to €143/MWh, 20.9% negative13:00: median €55/MWh, middle 80% €-22/MWh to €156/MWh, 25.3% negative14:00: median €67/MWh, middle 80% €-22/MWh to €146/MWh, 22.1% negative15:00: median €77/MWh, middle 80% €-16/MWh to €164/MWh, 17.9% negative16:00: median €91/MWh, middle 80% €-1/MWh to €171/MWh, 10.7% negative17:00: median €104/MWh, middle 80% €18/MWh to €191/MWh, 6.5% negative18:00: median €119/MWh, middle 80% €49/MWh to €214/MWh, 2.9% negative19:00: median €123/MWh, middle 80% €63/MWh to €230/MWh, 1.5% negative20:00: median €120/MWh, middle 80% €58/MWh to €222/MWh, 1.3% negative21:00: median €110/MWh, middle 80% €53/MWh to €210/MWh, 1.9% negative22:00: median €112/MWh, middle 80% €52/MWh to €208/MWh, 1.3% negative23:00: median €100/MWh, middle 80% €39/MWh to €183/MWh, 3.3% negative
Median priceMiddle 80% of quarters
  • Midday is long. From 08:00 to 17:00 there is too much power on the grid, peaking at 13:00 (+118 MW, with only 26% of quarters short). The median price then drops to €55/MWh and a quarter of those quarters settle below zero.
  • Evening and night are short. From 18:00 to 02:00, and again from 06:00 to 07:00. At 20:00, 72% of quarters are short. The evening peak, with a median of €123/MWh at 19:00, is the most expensive time of day.
  • Weekends are longer and cheaper. A median price of €85/MWh against €101/MWh on weekdays, and 13.6% negative quarters against 6.3%.
  • Prices rise towards autumn. The monthly median price doubled from €80/MWh in April to €160/MWh in September.

Median imbalance price by month

From April to September 2026 the median price doubled.

0 60 120 180 €/MWh Oct 25: median €78/MWh, 11.2% negative quarters Oct 25 Nov 25: median €84/MWh, 3.9% negative quarters Nov 25 Dec 25: median €79/MWh, 3.3% negative quarters Dec 25 Jan 26: median €100/MWh, 2.6% negative quarters Jan 26 Feb 26: median €87/MWh, 6.7% negative quarters Feb 26 Mar 26: median €100/MWh, 8.4% negative quarters Mar 26 Apr 26: median €80/MWh, 13.8% negative quarters€80 Apr 26 May 26: median €106/MWh, 10.3% negative quarters May 26 Jun 26: median €111/MWh, 9.4% negative quarters Jun 26 Jul 26: median €118/MWh, 11.9% negative quarters Jul 26 Aug 26: median €145/MWh, 9.3% negative quarters Aug 26 Sep 26: median €160/MWh, 9.7% negative quarters€160 Sep 26

Within a quarter the imbalance is restless: in 53% of quarters it crosses zero at least once. Between quarters it is surprisingly steady: 81% of quarters land on the same side as the quarter before. The final price is only settled late: halfway through, a third of quarters are still more than €10/MWh away from their final price.

Extremes are rare. 8.3% of quarters were negative and 1.9% were above €300/MWh. On Easter Monday, 6 April, five quarters settled at the price floor of −€15,000/MWh. The highest price was €3,115/MWh, on 15 August. The alpha surcharge Elia adds when the imbalance is large applied in 37% of quarters, with a median of €6.60/MWh.

Source: Voltmasters' own analysis of the public imbalance data from Elia Open Data (datasets ODS136, ODS147, ODS161, ODS162, ODS169 and ODS134). Forecasts from 28 April to 24 September 2026, prices from 1 October 2025 to 24 September 2026.

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