WALC Demand Forecast Accuracy: Error, Bias and Worst Miss, 21 August to 20 September 2026
The day-ahead demand forecast for WALC was off by 14.0% on average (mean absolute percentage error) over 688 hours from 21 August to 20 September 2026, and 22% of hours landed within 5% of the reading. Its bias was -0.7%: it ran low on average, an under forecast. The largest miss was 627 MW (77.6%) too high at 31 Aug 2026, 19:00 MST (UTC-07:00). Dates and times are local (America/Phoenix).
Key Facts
- The day-ahead demand forecast for WALC was off by 14.0% on average (mean absolute percentage error) over 688 hours from 21 August to 20 September 2026, and 22% of hours landed within 5% of the reading. Its bias was -0.7%: it ran low on average, an under forecast. The largest miss was 627 MW (77.6%) too high at 31 Aug 2026, 19:00 MST (UTC-07:00).
- Across 53 US grids ranked by average error over their own latest 30 day windows, WALC came 48 (1 is the smallest error).
- On average the forecast differed from reported demand by 142 MW, against average demand of 1,022 MW.
- The forecast was above reported demand in 44% of compared hours.
Forecast Error at a Glance
688 hours from 21 August to 20 September 2026Average error14.0%
Bias-0.7%
Hours within 5%22%
Worst miss627 MW
Average error is the mean absolute percentage gap between the forecast and reported demand. Bias is the signed mean (-21 MW per hour on average): the forecast ran low on average, an under forecast. The worst miss was 627 MW (77.6%) too high at 31 Aug 2026, 19:00 MST (UTC-07:00): forecast 1,435 MW, reported 808 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 MST (UTC-07:00)Data to 2026-09-20
| Local hour | Reported demand | Day-ahead forecast |
|---|---|---|
| 13 Sept, 19:00 | 1,010 MW | 1,129 MW |
| 13 Sept, 20:00 | 842 MW | 1,011 MW |
| 13 Sept, 21:00 | 790 MW | 911 MW |
| 13 Sept, 22:00 | 948 MW | 837 MW |
| 13 Sept, 23:00 | 806 MW | 812 MW |
| 14 Sept, 00:00 | 914 MW | 716 MW |
| 14 Sept, 01:00 | 862 MW | 703 MW |
| 14 Sept, 02:00 | 884 MW | 664 MW |
| 14 Sept, 03:00 | 854 MW | 697 MW |
| 14 Sept, 04:00 | 842 MW | 687 MW |
| 14 Sept, 05:00 | 792 MW | 648 MW |
| 14 Sept, 06:00 | 782 MW | 777 MW |
| 14 Sept, 07:00 | 859 MW | 790 MW |
| 14 Sept, 08:00 | 946 MW | 813 MW |
| 14 Sept, 09:00 | 999 MW | 870 MW |
| 14 Sept, 10:00 | 1,055 MW | 896 MW |
| 14 Sept, 11:00 | 1,100 MW | 973 MW |
| 14 Sept, 12:00 | 1,175 MW | 970 MW |
| 14 Sept, 13:00 | 1,232 MW | 978 MW |
| 14 Sept, 14:00 | 1,376 MW | 1,044 MW |
| 14 Sept, 15:00 | 1,413 MW | 1,063 MW |
| 14 Sept, 16:00 | 1,033 MW | 939 MW |
| 14 Sept, 17:00 | 1,183 MW | 933 MW |
| 14 Sept, 18:00 | 1,267 MW | 1,192 MW |
| 14 Sept, 19:00 | 1,021 MW | 1,225 MW |
| 14 Sept, 20:00 | 1,031 MW | 1,093 MW |
| 14 Sept, 21:00 | 1,172 MW | 947 MW |
| 14 Sept, 22:00 | 1,267 MW | 935 MW |
| 14 Sept, 23:00 | 1,178 MW | 935 MW |
| 15 Sept, 00:00 | 979 MW | 842 MW |
| 15 Sept, 01:00 | 1,007 MW | 841 MW |
| 15 Sept, 02:00 | 1,009 MW | 874 MW |
| 15 Sept, 03:00 | 979 MW | 882 MW |
| 15 Sept, 04:00 | 968 MW | 890 MW |
| 15 Sept, 05:00 | 952 MW | 863 MW |
| 15 Sept, 06:00 | 855 MW | 859 MW |
| 15 Sept, 07:00 | 1,057 MW | 988 MW |
| 15 Sept, 08:00 | 1,252 MW | 979 MW |
| 15 Sept, 09:00 | 1,157 MW | 1,004 MW |
| 15 Sept, 10:00 | 1,121 MW | 1,072 MW |
| 15 Sept, 11:00 | 1,201 MW | 1,099 MW |
| 15 Sept, 12:00 | 1,105 MW | 1,105 MW |
| 15 Sept, 13:00 | 1,117 MW | 1,174 MW |
| 15 Sept, 14:00 | 1,161 MW | 1,193 MW |
| 15 Sept, 15:00 | 1,176 MW | 1,102 MW |
| 15 Sept, 16:00 | 930 MW | 1,101 MW |
| 15 Sept, 17:00 | 1,115 MW | 1,267 MW |
| 15 Sept, 18:00 | 1,369 MW | 1,289 MW |
| 15 Sept, 19:00 | 1,254 MW | 1,194 MW |
| 15 Sept, 20:00 | 1,113 MW | 1,132 MW |
| 15 Sept, 21:00 | 1,324 MW | 998 MW |
| 15 Sept, 22:00 | 1,197 MW | 978 MW |
| 15 Sept, 23:00 | 1,138 MW | 985 MW |
| 16 Sept, 00:00 | 956 MW | 902 MW |
| 16 Sept, 01:00 | 929 MW | 863 MW |
| 16 Sept, 02:00 | 967 MW | 863 MW |
| 16 Sept, 03:00 | 970 MW | 907 MW |
| 16 Sept, 04:00 | 990 MW | 933 MW |
| 16 Sept, 05:00 | 986 MW | 919 MW |
| 16 Sept, 06:00 | 1,107 MW | 862 MW |
| 16 Sept, 07:00 | 1,044 MW | 762 MW |
| 16 Sept, 08:00 | 1,072 MW | 908 MW |
| 16 Sept, 09:00 | 1,199 MW | 1,005 MW |
| 16 Sept, 10:00 | 1,263 MW | 1,058 MW |
| 16 Sept, 11:00 | 1,160 MW | 1,079 MW |
| 16 Sept, 12:00 | 1,143 MW | 1,137 MW |
| 16 Sept, 13:00 | 1,175 MW | 1,158 MW |
| 16 Sept, 14:00 | 1,271 MW | 1,192 MW |
| 16 Sept, 15:00 | 1,336 MW | 1,267 MW |
| 16 Sept, 16:00 | 1,207 MW | 1,225 MW |
| 16 Sept, 17:00 | 1,244 MW | 1,378 MW |
| 16 Sept, 18:00 | 1,426 MW | 1,366 MW |
| 16 Sept, 19:00 | 1,371 MW | 1,324 MW |
| 16 Sept, 20:00 | 1,280 MW | 1,161 MW |
| 16 Sept, 21:00 | 1,353 MW | 1,151 MW |
| 16 Sept, 22:00 | 1,135 MW | 1,348 MW |
| 16 Sept, 23:00 | 976 MW | 1,185 MW |
| 17 Sept, 00:00 | 928 MW | 1,032 MW |
| 17 Sept, 01:00 | 963 MW | 921 MW |
| 17 Sept, 02:00 | 987 MW | 969 MW |
| 17 Sept, 03:00 | 994 MW | 897 MW |
| 17 Sept, 04:00 | 971 MW | 893 MW |
| 17 Sept, 05:00 | 884 MW | 851 MW |
| 17 Sept, 06:00 | 920 MW | 780 MW |
| 17 Sept, 07:00 | 1,063 MW | 906 MW |
| 17 Sept, 08:00 | 849 MW | 968 MW |
| 17 Sept, 09:00 | 945 MW | 919 MW |
| 17 Sept, 10:00 | 905 MW | 967 MW |
| 17 Sept, 11:00 | 996 MW | 1,046 MW |
| 17 Sept, 12:00 | 869 MW | 1,083 MW |
| 17 Sept, 13:00 | 969 MW | 1,140 MW |
| 17 Sept, 14:00 | 1,002 MW | 1,170 MW |
| 17 Sept, 15:00 | 1,188 MW | 1,248 MW |
| 17 Sept, 16:00 | 1,138 MW | 1,076 MW |
| 17 Sept, 17:00 | 1,364 MW | 1,517 MW |
| 17 Sept, 18:00 | 1,574 MW | 1,505 MW |
| 17 Sept, 19:00 | 1,620 MW | 1,413 MW |
| 17 Sept, 20:00 | 1,535 MW | 1,359 MW |
| 17 Sept, 21:00 | 1,326 MW | 1,343 MW |
| 17 Sept, 22:00 | 1,139 MW | 1,169 MW |
| 17 Sept, 23:00 | 1,011 MW | 1,163 MW |
| 18 Sept, 00:00 | 964 MW | 1,069 MW |
| 18 Sept, 01:00 | 931 MW | 999 MW |
| 18 Sept, 02:00 | 953 MW | 962 MW |
| 18 Sept, 03:00 | 973 MW | 961 MW |
| 18 Sept, 04:00 | 992 MW | 987 MW |
| 18 Sept, 05:00 | 953 MW | 920 MW |
| 18 Sept, 06:00 | 800 MW | 916 MW |
| 18 Sept, 07:00 | 782 MW | 1,036 MW |
| 18 Sept, 08:00 | 967 MW | 1,158 MW |
| 18 Sept, 09:00 | 1,083 MW | 1,233 MW |
| 18 Sept, 10:00 | 1,218 MW | 1,184 MW |
| 18 Sept, 11:00 | 1,293 MW | 1,182 MW |
| 18 Sept, 12:00 | 1,373 MW | 1,300 MW |
| 18 Sept, 13:00 | 1,416 MW | 1,301 MW |
| 18 Sept, 14:00 | 1,402 MW | 1,253 MW |
| 18 Sept, 15:00 | 1,375 MW | 1,344 MW |
| 18 Sept, 16:00 | 1,123 MW | 1,242 MW |
| 18 Sept, 17:00 | 1,230 MW | 1,556 MW |
| 18 Sept, 18:00 | 1,399 MW | 1,521 MW |
| 18 Sept, 19:00 | 1,506 MW | 1,412 MW |
| 18 Sept, 20:00 | 1,282 MW | 1,137 MW |
| 18 Sept, 21:00 | 1,235 MW | 1,163 MW |
| 18 Sept, 22:00 | 1,226 MW | 1,225 MW |
| 18 Sept, 23:00 | 1,130 MW | 1,326 MW |
| 19 Sept, 00:00 | 918 MW | 1,081 MW |
| 19 Sept, 01:00 | 866 MW | 1,085 MW |
| 19 Sept, 02:00 | 922 MW | 1,031 MW |
| 19 Sept, 03:00 | 922 MW | 1,035 MW |
| 19 Sept, 04:00 | 937 MW | 1,059 MW |
| 19 Sept, 05:00 | 912 MW | 1,049 MW |
| 19 Sept, 06:00 | 764 MW | 948 MW |
| 19 Sept, 07:00 | 757 MW | 1,010 MW |
| 19 Sept, 08:00 | 927 MW | 1,132 MW |
| 19 Sept, 09:00 | 985 MW | 1,208 MW |
| 19 Sept, 10:00 | 1,105 MW | 1,237 MW |
| 19 Sept, 11:00 | 1,234 MW | 1,225 MW |
| 19 Sept, 12:00 | 1,319 MW | 1,364 MW |
| 19 Sept, 13:00 | 1,361 MW | 1,443 MW |
| 19 Sept, 14:00 | 1,424 MW | 1,453 MW |
| 19 Sept, 15:00 | 1,406 MW | 1,286 MW |
| 19 Sept, 16:00 | 1,240 MW | 985 MW |
| 19 Sept, 17:00 | 1,390 MW | 995 MW |
| 19 Sept, 18:00 | 1,407 MW | 1,349 MW |
| 19 Sept, 19:00 | 1,310 MW | 1,227 MW |
| 19 Sept, 20:00 | 1,159 MW | 970 MW |
| 19 Sept, 21:00 | 1,176 MW | 946 MW |
| 19 Sept, 22:00 | 1,061 MW | 883 MW |
| 19 Sept, 23:00 | 968 MW | 900 MW |
| 20 Sept, 00:00 | 873 MW | 1,000 MW |
| 20 Sept, 01:00 | 823 MW | 929 MW |
| 20 Sept, 02:00 | 839 MW | 902 MW |
| 20 Sept, 03:00 | 849 MW | 893 MW |
| 20 Sept, 04:00 | 848 MW | 883 MW |
| 20 Sept, 05:00 | 844 MW | 897 MW |
| 20 Sept, 06:00 | 779 MW | 868 MW |
| 20 Sept, 07:00 | 729 MW | 882 MW |
| 20 Sept, 08:00 | 876 MW | 993 MW |
| 20 Sept, 09:00 | 923 MW | 1,038 MW |
| 20 Sept, 10:00 | 1,043 MW | 1,122 MW |
| 20 Sept, 11:00 | 1,160 MW | 1,189 MW |
| 20 Sept, 12:00 | 1,235 MW | 1,166 MW |
| 20 Sept, 13:00 | 1,291 MW | 1,166 MW |
| 20 Sept, 14:00 | 1,311 MW | 1,291 MW |
| 20 Sept, 15:00 | 1,297 MW | 1,380 MW |
| 20 Sept, 16:00 | 1,073 MW | 1,058 MW |
| 20 Sept, 17:00 | 1,333 MW | 1,197 MW |
| 20 Sept, 18:00 | 1,295 MW | 1,034 MW |
Assumptions
- Only hours with both a forecast and a positive demand reading count (688 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.