The late Mike Parker had been putting together a list of all the "fallen flag" utilities that he had come across in his research on Mountain States Power and Pacific Power & Light Co. So, with his work in mind (now lost), here's my "fallen flag" list:

MDU Resources Group, Inc. family:
Montana-Dakota Utilities Co. (Minneapolis / Bismarck)
Dakota Public Service Co. (Bismarck)
North Dakota Light & Power Co. (Bismarck)
Hughes Electric Co. (Bismarck, ND area) - also owned Glendive & Fargo franchises
Hughes & Deiters Co. (Dickinson, ND area)
Hazen Power Co. (Hazen, ND)
Hebron Electric Light & Power Co. (Hebron, ND)
Northern Electric Co. (Dickinson, ND)
Carson Electric Co. (Carson, ND)
Center Light Co. (Center, ND)
Chrysler Light & Power Co. (Belfield, ND)
Glen Ullin Electric Co. (Glen Ullin, ND)
Hannes Light & Power Co. (Glen Ullin, ND)
Mandan Electric Co. (Mandan, ND)
Equity Elevator & Trading Co. (Werner, ND)
Northern Power & Light Co.
Ashley Electric Co. (Ashley, ND)
Dakota Utilities Co. (Eureka, SD) - also LaMoure, ND - see OTP
Ellendale Electric Co. (Ellendale, ND)
Forbes Electric Co. (Forbes, ND)
Gettysburg Light & Power Co. (Gettysburg, SD)
Krein Electric Co., Inc. (Wishek, ND area)
Berreth Brothers Co. (Zeeland, ND)
Lehr Light & Power Co. (Lehr, ND)
Kulm Milling Co. (Kulm, ND)
Mobridge Electric Power & Heating Co. (Mobridge, SD)
Napoleon Light & Power Co. (Napoleon, ND)
Steele Light & Power Co. (Steele, ND)
Emmons County Light & Power Co. (Linton, ND)
Linton Light & Power Co. (Linton, ND)
Montana-Dakota Power Co. (Minneapolis)
Eastern Montana Light & Power Co.
Glendive Light, Heat, & Power Co. (Glendive, MT)
Hughes Electric Co. (Glendive, MT)
Hanks Hardware Co. (Hanks, ND)
Kenmare Light & Power Service (Kenmare, ND area)
Bowbells Light & Power Co. (Bowbells, ND)
Minnesota Northern Power Co. (Minneapolis)
Minnesota Electric Light & Power Co. (see OTP)
Northern Utilities Corporation (Jamestown, ND) - see OTP
Bowman Electric Co. (Bowman, ND area)
Reeder Electric Co. (Reeder, ND)
Hettinger Electric Light & Power Co. (Hettinger, ND)
Plentywood Electric (Plentywood, MT)
Public Utility Co. of Stanley (Stanley, ND)
Ray Electric Co. (Ray, ND)
Terry Light & Power Co. (Terry, MT)
Tolley Electric Co. (Tolley, ND)
United Power Co. (Crosby, ND area)
Crosby Power & Light Co. (Crosby, ND)
Crosby Auto Co. (Crosby, ND)
Mountain States Power Co. (eastern MT division)
Ismay Light & Telephone Co. (Ismay, MT)
Midwest Public Service Co. (Baker, MT)
Baker Light & Power Co. (Baker, MT)

NorthWestern Energy family:
Northwestern Public Service Co.
Aberdeen Light & Power Co. (Aberdeen, SD)
Andover Electric Light Co. (Andover, SD)
Consumers Electric Co.
Consumers Utility Co.
South Dakota Light & Power Co.
Eastern Dakota Electric Co.
Faulkton Electric Co. (Faulkton, SD)
Iroquois Light & Power Co. (Iroquois, SD)
Mettler Brothers Light Co. (Menno, SD)
Mitchell Power Co. (Mitchell, SD)
Spink County Power Co.
Ashton Power Co. (Ashton, SD)
Springfield Power & Light Co. (Springfield, SD)
Union Light & Power Co.
Union Power & Light Co.
Western Power & Gas Co. (SD) - also NSP
Central Gas & Electric Co. (SD)
Central West Public Service Co. (NE / SD / ND / IA)
People’s Gas & Light Co. (Chamberlain, SD)

Otter Tail Corporation family:
Otter Tail Power Co.
Bottineau Electric Light Plant (Bottineau, ND)
Benson Hardware Co. (Pillsbury, ND)
City Electric Co. (Milnor, ND)
Dakota Utilities Co. (LaMoure, ND) - also see MDU
Fordville Garage & Electric Light Co. (Fordville, ND)
General Service Co. (Sanish, ND)
Gwinner Electric Co. (Gwinner, ND)
Lidgerwood Cooperative Milling Co. (Lidgerwood, ND)
Mantador Grain Co. (Mantador, ND)
Midwest Power Co.
General Utilities Corporation
Motor Service Co. (Litchville, ND)
Northern Light Electric Co. (Wahpeton, ND)
Phelps Electric Co. (Hankinson, ND)
Rutland Garage & Repair Co.
Sheyenne Valley Light & Power Co. (Valley City, ND)
Tri-State Power Co.
Wind Electric Corporation (Wyndmere, ND)
Western Electric Co. (Jamestown, ND)
Jamestown Electric Light & Power Co. (Jamestown, ND)
Roberts County Power Co.
Commonwealth Service Co.
New Effington Electric Co. (New Effington, SD)
Central Light & Power Co. of ND
Bacon & Burr Co. (Granville, ND)
Bowdon Light Plant (Bowdon, ND)
Central Power Co.
Fessenden Light & Power Co. (Fessenden, ND)
Harvey Electric & Light Power Co. (Harvey, ND)
Harvey Milling Co. (Harvey, ND)
Union Public Service Co.
Dakota Light & Power Co. (Flandreau, SD area)
Flandreau Light & Power Co. (Flandreau, SD)
Central Light & Power Co. (SD)
Minnesota Utilities Co. - NOT Minnesota Power’s predecessor
Federal Public Service Co.
Roberts County Power Co. (sold 1940)
Western Power & Light Co.
Wheaton Electric Light Co.
Central Electric & Telephone Co.
Central West Public Service Co.
Erickson Power Co.
Northern Utilities Corporation
Northern Light & Telephone Co.
Public Service Corporation
Drake-Jones Co.
Interstate Power Co. (Bemidji / Crookson divisions only)
International Public Service Corporation
Pembina Light & Power Co. (Pembina, ND)
Minnesota Electric Distributing Co.
Minnesota Electric Light & Power Co. - owned by MN Northern Pwr Co. (see MDU)
Warfield Electric Co. (Bemidji, MN)

Xcel Energy family:
Northern States Power Co. (ND / SD / MN)
Minot Light & Telephone (Minot, ND)
Red River Power Co. (Grand Forks, ND)
Buxton Electric Light & Power Co. (Buxton, ND)
Union Light Heat & Power Co. (Fargo, ND)
Hughes Electric Co. (Fargo, ND)
Western Power & Gas Co. (SD)
Central Gas & Electric Co. (SD)
Central West Public Service Co. (NE / SD / ND / IA)
St. Cloud Public Service Co. (St. Cloud, MN)
Minneapolis General Electric Co. (Minneapolis)
Consumers Power Co. (1910s-1916)
Washington County Light & Power Co. (1909-1910)
Northern States Power Co. - Wisconsin
Mississippi Valley Power Co. (Winona, MN area)
Public Service of Colorado (CO)
Southwestern Public Service Co. (TX)