IID Demand Forecast Accuracy: Error, Bias and Worst Miss, 21 August to 20 September 2026
The day-ahead demand forecast for IID was off by 5.1% on average (mean absolute percentage error) over 718 hours from 21 August to 20 September 2026, and 70% of hours landed within 5% of the reading. Its bias was +1.5%: it ran high on average, an over forecast. The largest miss was 394 MW (116.2%) too high at 20 Sept 2026, 17:00 PDT (UTC-07:00). Dates and times are local (America/Los_Angeles).
Key Facts
- The day-ahead demand forecast for IID was off by 5.1% on average (mean absolute percentage error) over 718 hours from 21 August to 20 September 2026, and 70% of hours landed within 5% of the reading. Its bias was +1.5%: it ran high on average, an over forecast. The largest miss was 394 MW (116.2%) too high at 20 Sept 2026, 17:00 PDT (UTC-07:00).
- Across 53 US grids ranked by average error over their own latest 30 day windows, IID came 33 (1 is the smallest error).
- On average the forecast differed from reported demand by 28 MW, against average demand of 638 MW.
- The forecast was above reported demand in 49% of compared hours.
Forecast Error at a Glance
718 hours from 21 August to 20 September 2026Average error5.1%
Bias+1.5%
Hours within 5%70%
Worst miss394 MW
Average error is the mean absolute percentage gap between the forecast and reported demand. Bias is the signed mean (+4 MW per hour on average): the forecast ran high on average, an over forecast. The worst miss was 394 MW (116.2%) too high at 20 Sept 2026, 17:00 PDT (UTC-07:00): forecast 733 MW, reported 339 MW.
Source: U.S. Energy Information Administration (EIA). (US Government work, public domain) Data as of 2026-09-20. Accuracy is computed by World Power Monitor from the operator's hourly forecast and reported demand.
Forecast and Reported Demand, Last 7 Days
Local time, to 20 Sept 2026, 18:00 PDT (UTC-07:00)Data to 2026-09-20
| Local hour | Reported demand | Day-ahead forecast |
|---|---|---|
| 13 Sept, 19:00 | 772 MW | 761 MW |
| 13 Sept, 20:00 | 715 MW | 707 MW |
| 13 Sept, 21:00 | 656 MW | 657 MW |
| 13 Sept, 22:00 | 588 MW | 596 MW |
| 13 Sept, 23:00 | 526 MW | 549 MW |
| 14 Sept, 00:00 | 481 MW | 497 MW |
| 14 Sept, 01:00 | 452 MW | 466 MW |
| 14 Sept, 02:00 | 424 MW | 438 MW |
| 14 Sept, 03:00 | 411 MW | 418 MW |
| 14 Sept, 04:00 | 408 MW | 418 MW |
| 14 Sept, 05:00 | 414 MW | 428 MW |
| 14 Sept, 06:00 | 436 MW | 442 MW |
| 14 Sept, 07:00 | 449 MW | 465 MW |
| 14 Sept, 08:00 | 483 MW | 510 MW |
| 14 Sept, 09:00 | 544 MW | 554 MW |
| 14 Sept, 10:00 | 588 MW | 604 MW |
| 14 Sept, 11:00 | 638 MW | 653 MW |
| 14 Sept, 12:00 | 712 MW | 716 MW |
| 14 Sept, 13:00 | 767 MW | 778 MW |
| 14 Sept, 14:00 | 824 MW | 814 MW |
| 14 Sept, 15:00 | 852 MW | 859 MW |
| 14 Sept, 16:00 | 880 MW | 865 MW |
| 14 Sept, 17:00 | 877 MW | 855 MW |
| 14 Sept, 18:00 | 821 MW | 816 MW |
| 14 Sept, 19:00 | 767 MW | 762 MW |
| 14 Sept, 20:00 | 710 MW | 704 MW |
| 14 Sept, 21:00 | 654 MW | 648 MW |
| 14 Sept, 22:00 | 594 MW | 581 MW |
| 14 Sept, 23:00 | n/a | 525 MW |
| 15 Sept, 00:00 | 502 MW | 484 MW |
| 15 Sept, 01:00 | 462 MW | 448 MW |
| 15 Sept, 02:00 | 435 MW | 427 MW |
| 15 Sept, 03:00 | 418 MW | 410 MW |
| 15 Sept, 04:00 | 422 MW | 404 MW |
| 15 Sept, 05:00 | 436 MW | 411 MW |
| 15 Sept, 06:00 | 457 MW | 424 MW |
| 15 Sept, 07:00 | 471 MW | 447 MW |
| 15 Sept, 08:00 | 515 MW | 483 MW |
| 15 Sept, 09:00 | 566 MW | 519 MW |
| 15 Sept, 10:00 | 615 MW | 562 MW |
| 15 Sept, 11:00 | 657 MW | 608 MW |
| 15 Sept, 12:00 | 712 MW | 663 MW |
| 15 Sept, 13:00 | 766 MW | 720 MW |
| 15 Sept, 14:00 | 788 MW | 758 MW |
| 15 Sept, 15:00 | 833 MW | 797 MW |
| 15 Sept, 16:00 | 849 MW | 810 MW |
| 15 Sept, 17:00 | 845 MW | 806 MW |
| 15 Sept, 18:00 | 780 MW | 769 MW |
| 15 Sept, 19:00 | 734 MW | 717 MW |
| 15 Sept, 20:00 | 673 MW | 667 MW |
| 15 Sept, 21:00 | 619 MW | 619 MW |
| 15 Sept, 22:00 | 565 MW | 553 MW |
| 15 Sept, 23:00 | 515 MW | 501 MW |
| 16 Sept, 00:00 | 471 MW | 459 MW |
| 16 Sept, 01:00 | 441 MW | 426 MW |
| 16 Sept, 02:00 | 413 MW | 403 MW |
| 16 Sept, 03:00 | 402 MW | 390 MW |
| 16 Sept, 04:00 | 399 MW | 384 MW |
| 16 Sept, 05:00 | 411 MW | 393 MW |
| 16 Sept, 06:00 | 434 MW | 411 MW |
| 16 Sept, 07:00 | 449 MW | 427 MW |
| 16 Sept, 08:00 | 477 MW | 461 MW |
| 16 Sept, 09:00 | 518 MW | 496 MW |
| 16 Sept, 10:00 | 556 MW | 534 MW |
| 16 Sept, 11:00 | 624 MW | 579 MW |
| 16 Sept, 12:00 | 673 MW | 628 MW |
| 16 Sept, 13:00 | 721 MW | 676 MW |
| 16 Sept, 14:00 | 769 MW | 721 MW |
| 16 Sept, 15:00 | 798 MW | 754 MW |
| 16 Sept, 16:00 | 816 MW | 766 MW |
| 16 Sept, 17:00 | 795 MW | 756 MW |
| 16 Sept, 18:00 | 755 MW | 723 MW |
| 16 Sept, 19:00 | 697 MW | 675 MW |
| 16 Sept, 20:00 | 641 MW | 627 MW |
| 16 Sept, 21:00 | 589 MW | 576 MW |
| 16 Sept, 22:00 | 536 MW | 516 MW |
| 16 Sept, 23:00 | 477 MW | 466 MW |
| 17 Sept, 00:00 | 443 MW | 444 MW |
| 17 Sept, 01:00 | 418 MW | 414 MW |
| 17 Sept, 02:00 | 401 MW | 393 MW |
| 17 Sept, 03:00 | 381 MW | 380 MW |
| 17 Sept, 04:00 | 390 MW | 376 MW |
| 17 Sept, 05:00 | 403 MW | 387 MW |
| 17 Sept, 06:00 | 415 MW | 405 MW |
| 17 Sept, 07:00 | 425 MW | 418 MW |
| 17 Sept, 08:00 | 454 MW | 452 MW |
| 17 Sept, 09:00 | 479 MW | 484 MW |
| 17 Sept, 10:00 | 526 MW | 518 MW |
| 17 Sept, 11:00 | 565 MW | 558 MW |
| 17 Sept, 12:00 | 644 MW | 607 MW |
| 17 Sept, 13:00 | 700 MW | 654 MW |
| 17 Sept, 14:00 | 736 MW | 695 MW |
| 17 Sept, 15:00 | 780 MW | 732 MW |
| 17 Sept, 16:00 | 782 MW | 743 MW |
| 17 Sept, 17:00 | 773 MW | 747 MW |
| 17 Sept, 18:00 | 730 MW | 715 MW |
| 17 Sept, 19:00 | 695 MW | 673 MW |
| 17 Sept, 20:00 | 638 MW | 627 MW |
| 17 Sept, 21:00 | 582 MW | 580 MW |
| 17 Sept, 22:00 | 526 MW | 524 MW |
| 17 Sept, 23:00 | 469 MW | 474 MW |
| 18 Sept, 00:00 | 436 MW | 433 MW |
| 18 Sept, 01:00 | 403 MW | 404 MW |
| 18 Sept, 02:00 | 375 MW | 382 MW |
| 18 Sept, 03:00 | 365 MW | 368 MW |
| 18 Sept, 04:00 | 359 MW | 365 MW |
| 18 Sept, 05:00 | 371 MW | 377 MW |
| 18 Sept, 06:00 | 391 MW | 394 MW |
| 18 Sept, 07:00 | 400 MW | 409 MW |
| 18 Sept, 08:00 | 426 MW | 444 MW |
| 18 Sept, 09:00 | 447 MW | 480 MW |
| 18 Sept, 10:00 | 490 MW | 521 MW |
| 18 Sept, 11:00 | 538 MW | 569 MW |
| 18 Sept, 12:00 | 581 MW | 622 MW |
| 18 Sept, 13:00 | 641 MW | 670 MW |
| 18 Sept, 14:00 | 695 MW | 718 MW |
| 18 Sept, 15:00 | 736 MW | 754 MW |
| 18 Sept, 16:00 | 757 MW | 764 MW |
| 18 Sept, 17:00 | 739 MW | 758 MW |
| 18 Sept, 18:00 | 698 MW | 720 MW |
| 18 Sept, 19:00 | 643 MW | 668 MW |
| 18 Sept, 20:00 | 589 MW | 615 MW |
| 18 Sept, 21:00 | 546 MW | 565 MW |
| 18 Sept, 22:00 | 495 MW | 512 MW |
| 18 Sept, 23:00 | 443 MW | 464 MW |
| 19 Sept, 00:00 | 409 MW | 432 MW |
| 19 Sept, 01:00 | 376 MW | 399 MW |
| 19 Sept, 02:00 | 354 MW | 375 MW |
| 19 Sept, 03:00 | 331 MW | 360 MW |
| 19 Sept, 04:00 | 336 MW | 350 MW |
| 19 Sept, 05:00 | 327 MW | 352 MW |
| 19 Sept, 06:00 | 337 MW | 352 MW |
| 19 Sept, 07:00 | 340 MW | 363 MW |
| 19 Sept, 08:00 | 370 MW | 413 MW |
| 19 Sept, 09:00 | 404 MW | 453 MW |
| 19 Sept, 10:00 | 443 MW | 508 MW |
| 19 Sept, 11:00 | 500 MW | 560 MW |
| 19 Sept, 12:00 | 565 MW | 611 MW |
| 19 Sept, 13:00 | 635 MW | 661 MW |
| 19 Sept, 14:00 | 675 MW | 702 MW |
| 19 Sept, 15:00 | 722 MW | 743 MW |
| 19 Sept, 16:00 | 738 MW | 756 MW |
| 19 Sept, 17:00 | 732 MW | 754 MW |
| 19 Sept, 18:00 | 671 MW | 714 MW |
| 19 Sept, 19:00 | 609 MW | 663 MW |
| 19 Sept, 20:00 | 553 MW | 613 MW |
| 19 Sept, 21:00 | 517 MW | 563 MW |
| 19 Sept, 22:00 | 472 MW | 510 MW |
| 19 Sept, 23:00 | 428 MW | 466 MW |
| 20 Sept, 00:00 | 339 MW | 431 MW |
| 20 Sept, 01:00 | 339 MW | 399 MW |
| 20 Sept, 02:00 | 339 MW | 376 MW |
| 20 Sept, 03:00 | 339 MW | 359 MW |
| 20 Sept, 04:00 | 339 MW | 352 MW |
| 20 Sept, 05:00 | 339 MW | 348 MW |
| 20 Sept, 06:00 | 339 MW | 345 MW |
| 20 Sept, 07:00 | 339 MW | 356 MW |
| 20 Sept, 08:00 | 339 MW | 400 MW |
| 20 Sept, 09:00 | 339 MW | 442 MW |
| 20 Sept, 10:00 | 339 MW | 490 MW |
| 20 Sept, 11:00 | 339 MW | 541 MW |
| 20 Sept, 12:00 | 339 MW | 593 MW |
| 20 Sept, 13:00 | 339 MW | 642 MW |
| 20 Sept, 14:00 | 339 MW | 682 MW |
| 20 Sept, 15:00 | 339 MW | 720 MW |
| 20 Sept, 16:00 | 339 MW | 732 MW |
| 20 Sept, 17:00 | 339 MW | 733 MW |
| 20 Sept, 18:00 | 339 MW | 700 MW |
Assumptions
- Only hours with both a forecast and a positive demand reading count (718 of 720 hours in the window). Missing and zero values, and readings under 30% or over 3 times the grid's own median for the window, are treated as reporting faults and left out.
- Error is (forecast minus reported demand) divided by reported demand; average error uses absolute values so high and low misses do not cancel. Bias keeps the sign and does cancel.
- Reported demand is preliminary and may be revised. A 30 day window is a short sample and one heat wave or holiday can move these figures.
- US figures are for the EIA balancing authority or region named on this page. US and European grids are ranked separately.