GCPD Demand Forecast Accuracy: Error, Bias and Worst Miss, 21 August to 20 September 2026
The day-ahead demand forecast for GCPD was off by 3.6% on average (mean absolute percentage error) over 681 hours from 21 August to 20 September 2026, and 76% of hours landed within 5% of the reading. Its bias was -3.4%: it ran low on average, an under forecast. The largest miss was 61 MW (7.1%) too low at 31 Aug 2026, 17:00 PDT (UTC-07:00). Dates and times are local (America/Los_Angeles).
Key Facts
- The day-ahead demand forecast for GCPD was off by 3.6% on average (mean absolute percentage error) over 681 hours from 21 August to 20 September 2026, and 76% of hours landed within 5% of the reading. Its bias was -3.4%: it ran low on average, an under forecast. The largest miss was 61 MW (7.1%) too low at 31 Aug 2026, 17:00 PDT (UTC-07:00).
- Across 53 US grids ranked by average error over their own latest 30 day windows, GCPD came 25 (1 is the smallest error).
- On average the forecast differed from reported demand by 27 MW, against average demand of 738 MW.
- The forecast was above reported demand in 7% of compared hours.
Forecast Error at a Glance
681 hours from 21 August to 20 September 2026Average error3.6%
Bias-3.4%
Hours within 5%76%
Worst miss61 MW
Average error is the mean absolute percentage gap between the forecast and reported demand. Bias is the signed mean (-26 MW per hour on average): the forecast ran low on average, an under forecast. The worst miss was 61 MW (7.1%) too low at 31 Aug 2026, 17:00 PDT (UTC-07:00): forecast 800 MW, reported 861 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 | 674 MW | 644 MW |
| 13 Sept, 20:00 | 672 MW | 648 MW |
| 13 Sept, 21:00 | 661 MW | 644 MW |
| 13 Sept, 22:00 | 650 MW | 632 MW |
| 13 Sept, 23:00 | 646 MW | 623 MW |
| 14 Sept, 00:00 | 638 MW | 620 MW |
| 14 Sept, 01:00 | 636 MW | 613 MW |
| 14 Sept, 02:00 | 640 MW | 617 MW |
| 14 Sept, 03:00 | 643 MW | 623 MW |
| 14 Sept, 04:00 | 644 MW | 628 MW |
| 14 Sept, 05:00 | 666 MW | 642 MW |
| 14 Sept, 06:00 | 679 MW | 660 MW |
| 14 Sept, 07:00 | 693 MW | 672 MW |
| 14 Sept, 08:00 | 695 MW | 684 MW |
| 14 Sept, 09:00 | 701 MW | 691 MW |
| 14 Sept, 10:00 | 700 MW | 693 MW |
| 14 Sept, 11:00 | 702 MW | 693 MW |
| 14 Sept, 12:00 | 699 MW | 695 MW |
| 14 Sept, 13:00 | 705 MW | 697 MW |
| 14 Sept, 14:00 | 705 MW | 701 MW |
| 14 Sept, 15:00 | 703 MW | 706 MW |
| 14 Sept, 16:00 | 706 MW | 706 MW |
| 14 Sept, 17:00 | 700 MW | 704 MW |
| 14 Sept, 18:00 | 698 MW | 699 MW |
| 14 Sept, 19:00 | 694 MW | 696 MW |
| 14 Sept, 20:00 | 692 MW | 692 MW |
| 14 Sept, 21:00 | 683 MW | 679 MW |
| 14 Sept, 22:00 | 661 MW | 662 MW |
| 14 Sept, 23:00 | 648 MW | 646 MW |
| 15 Sept, 00:00 | 644 MW | 630 MW |
| 15 Sept, 01:00 | 636 MW | 624 MW |
| 15 Sept, 02:00 | 635 MW | 621 MW |
| 15 Sept, 03:00 | 635 MW | 626 MW |
| 15 Sept, 04:00 | 640 MW | 628 MW |
| 15 Sept, 05:00 | 661 MW | 645 MW |
| 15 Sept, 06:00 | 679 MW | 657 MW |
| 15 Sept, 07:00 | 686 MW | 671 MW |
| 15 Sept, 08:00 | 686 MW | 677 MW |
| 15 Sept, 09:00 | 689 MW | 684 MW |
| 15 Sept, 10:00 | 692 MW | 680 MW |
| 15 Sept, 11:00 | 694 MW | 678 MW |
| 15 Sept, 12:00 | 695 MW | 677 MW |
| 15 Sept, 13:00 | 695 MW | 681 MW |
| 15 Sept, 14:00 | 690 MW | 679 MW |
| 15 Sept, 15:00 | 690 MW | 680 MW |
| 15 Sept, 16:00 | 697 MW | 679 MW |
| 15 Sept, 17:00 | 696 MW | 680 MW |
| 15 Sept, 18:00 | 696 MW | 686 MW |
| 15 Sept, 19:00 | 691 MW | 687 MW |
| 15 Sept, 20:00 | 694 MW | 684 MW |
| 15 Sept, 21:00 | 683 MW | 671 MW |
| 15 Sept, 22:00 | 670 MW | 653 MW |
| 15 Sept, 23:00 | 660 MW | 640 MW |
| 16 Sept, 00:00 | 653 MW | 627 MW |
| 16 Sept, 01:00 | 646 MW | 626 MW |
| 16 Sept, 02:00 | 642 MW | 620 MW |
| 16 Sept, 03:00 | 642 MW | 619 MW |
| 16 Sept, 04:00 | 649 MW | 627 MW |
| 16 Sept, 05:00 | 666 MW | 645 MW |
| 16 Sept, 06:00 | 683 MW | 660 MW |
| 16 Sept, 07:00 | 689 MW | 671 MW |
| 16 Sept, 08:00 | 693 MW | 677 MW |
| 16 Sept, 09:00 | 696 MW | 682 MW |
| 16 Sept, 10:00 | 703 MW | 685 MW |
| 16 Sept, 11:00 | 704 MW | 694 MW |
| 16 Sept, 12:00 | 710 MW | 700 MW |
| 16 Sept, 13:00 | 720 MW | 707 MW |
| 16 Sept, 14:00 | 721 MW | 706 MW |
| 16 Sept, 15:00 | 726 MW | 707 MW |
| 16 Sept, 16:00 | 728 MW | 712 MW |
| 16 Sept, 17:00 | 734 MW | 710 MW |
| 16 Sept, 18:00 | 727 MW | 704 MW |
| 16 Sept, 19:00 | 718 MW | 689 MW |
| 16 Sept, 20:00 | 716 MW | 689 MW |
| 16 Sept, 21:00 | 699 MW | 677 MW |
| 16 Sept, 22:00 | 674 MW | 664 MW |
| 16 Sept, 23:00 | 661 MW | 652 MW |
| 17 Sept, 00:00 | 656 MW | 636 MW |
| 17 Sept, 01:00 | 649 MW | 629 MW |
| 17 Sept, 02:00 | 648 MW | 620 MW |
| 17 Sept, 03:00 | 645 MW | 619 MW |
| 17 Sept, 04:00 | 646 MW | 628 MW |
| 17 Sept, 05:00 | 664 MW | 644 MW |
| 17 Sept, 06:00 | 680 MW | 655 MW |
| 17 Sept, 07:00 | 692 MW | 662 MW |
| 17 Sept, 08:00 | 699 MW | 670 MW |
| 17 Sept, 09:00 | 705 MW | 678 MW |
| 17 Sept, 10:00 | 713 MW | 682 MW |
| 17 Sept, 11:00 | 714 MW | 689 MW |
| 17 Sept, 12:00 | 724 MW | 696 MW |
| 17 Sept, 13:00 | 732 MW | 706 MW |
| 17 Sept, 14:00 | 742 MW | 710 MW |
| 17 Sept, 15:00 | 742 MW | 717 MW |
| 17 Sept, 16:00 | 749 MW | 723 MW |
| 17 Sept, 17:00 | 741 MW | 723 MW |
| 17 Sept, 18:00 | 738 MW | 711 MW |
| 17 Sept, 19:00 | 728 MW | 697 MW |
| 17 Sept, 20:00 | 722 MW | 687 MW |
| 17 Sept, 21:00 | 704 MW | 673 MW |
| 17 Sept, 22:00 | 684 MW | 654 MW |
| 17 Sept, 23:00 | 669 MW | 646 MW |
| 18 Sept, 00:00 | 658 MW | 638 MW |
| 18 Sept, 01:00 | 648 MW | 627 MW |
| 18 Sept, 02:00 | 648 MW | 624 MW |
| 18 Sept, 03:00 | 647 MW | 622 MW |
| 18 Sept, 04:00 | 653 MW | 627 MW |
| 18 Sept, 05:00 | 666 MW | 644 MW |
| 18 Sept, 06:00 | 680 MW | 654 MW |
| 18 Sept, 07:00 | 695 MW | 664 MW |
| 18 Sept, 08:00 | 706 MW | 671 MW |
| 18 Sept, 09:00 | 715 MW | 673 MW |
| 18 Sept, 10:00 | 718 MW | 676 MW |
| 18 Sept, 11:00 | 725 MW | 682 MW |
| 18 Sept, 12:00 | 730 MW | 686 MW |
| 18 Sept, 13:00 | 735 MW | 694 MW |
| 18 Sept, 14:00 | 744 MW | 696 MW |
| 18 Sept, 15:00 | 753 MW | 697 MW |
| 18 Sept, 16:00 | 758 MW | 698 MW |
| 18 Sept, 17:00 | 756 MW | 695 MW |
| 18 Sept, 18:00 | 742 MW | 687 MW |
| 18 Sept, 19:00 | 731 MW | 675 MW |
| 18 Sept, 20:00 | 722 MW | 667 MW |
| 18 Sept, 21:00 | 708 MW | 650 MW |
| 18 Sept, 22:00 | 695 MW | 638 MW |
| 18 Sept, 23:00 | 682 MW | 629 MW |
| 19 Sept, 00:00 | 670 MW | 632 MW |
| 19 Sept, 01:00 | 665 MW | 621 MW |
| 19 Sept, 02:00 | 663 MW | 621 MW |
| 19 Sept, 03:00 | 658 MW | 619 MW |
| 19 Sept, 04:00 | 657 MW | 620 MW |
| 19 Sept, 05:00 | 660 MW | 632 MW |
| 19 Sept, 06:00 | 661 MW | 634 MW |
| 19 Sept, 07:00 | 669 MW | 640 MW |
| 19 Sept, 08:00 | 683 MW | 647 MW |
| 19 Sept, 09:00 | 695 MW | 654 MW |
| 19 Sept, 10:00 | 700 MW | 656 MW |
| 19 Sept, 11:00 | 704 MW | 661 MW |
| 19 Sept, 12:00 | 712 MW | 669 MW |
| 19 Sept, 13:00 | 724 MW | 676 MW |
| 19 Sept, 14:00 | 733 MW | 678 MW |
| 19 Sept, 15:00 | 740 MW | 682 MW |
| 19 Sept, 16:00 | 743 MW | 692 MW |
| 19 Sept, 17:00 | 740 MW | 687 MW |
| 19 Sept, 18:00 | 727 MW | 672 MW |
| 19 Sept, 19:00 | 714 MW | 662 MW |
| 19 Sept, 20:00 | 702 MW | 650 MW |
| 19 Sept, 21:00 | 691 MW | 635 MW |
| 19 Sept, 22:00 | 669 MW | 625 MW |
| 19 Sept, 23:00 | 646 MW | 621 MW |
| 20 Sept, 00:00 | 634 MW | 634 MW |
| 20 Sept, 01:00 | 627 MW | 629 MW |
| 20 Sept, 02:00 | 626 MW | 622 MW |
| 20 Sept, 03:00 | 624 MW | 617 MW |
| 20 Sept, 04:00 | 625 MW | 616 MW |
| 20 Sept, 05:00 | 629 MW | 622 MW |
| 20 Sept, 06:00 | 629 MW | 622 MW |
| 20 Sept, 07:00 | 636 MW | 627 MW |
| 20 Sept, 08:00 | 651 MW | 639 MW |
| 20 Sept, 09:00 | 665 MW | 653 MW |
| 20 Sept, 10:00 | 675 MW | 658 MW |
| 20 Sept, 11:00 | 682 MW | 665 MW |
| 20 Sept, 12:00 | 696 MW | 673 MW |
| 20 Sept, 13:00 | 710 MW | 685 MW |
| 20 Sept, 14:00 | 722 MW | 698 MW |
| 20 Sept, 15:00 | 732 MW | 706 MW |
| 20 Sept, 16:00 | 737 MW | 709 MW |
| 20 Sept, 17:00 | 734 MW | 708 MW |
| 20 Sept, 18:00 | 722 MW | 694 MW |
Assumptions
- Only hours with both a forecast and a positive demand reading count (681 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.