| AEP Ohio | 2009 |
| Alliant Energy | 2004 |
| Ameren Illinois | 2010 |
| Cedar Falls Utilities | 2011 |
| CenterPoint Energy | 2004 |
| City Utilities of Springfield, MO | 2015 |
| City Water, Light & Power | 2007 |
| Columbia Water and Light | 2008 |
| Commonwealth Edison | 2000 |
| Consumers Energy | 2010 |
| Dayton Power & Light | 2011 |
| DTE Energy | 2009 |
| Duke Energy | 2006 |
| East Kentucky Power Cooperative | 2012 |
| Evergy (Formerly KCP&L + Westar) | 2016 |
| Indianapolis Power & Light Company | 2003 |
| Louisville Gas & Electric Company and Kentucky Utilities (LGE KU) | 2012 |
| MidAmerican Energy Company | 2006 |
| Minnesota Energy Resources Corporation | 2007 |
| Minnesota Power | 2014 |
| Nicor Gas | 2012 |
| NIPSCO | 2014 |
| North Shore Gas | 2010 |
| Ohio's Electric Cooperatives | 2014 |
| Peoples Gas | 2010 |
| SEMCO Energy Gas Company | 2014 |
| Southern Minnesota Municipal Power Agency | 2004 |
| Spire | 2018 |
| Upper Peninsula Power Company | 2019 |
| Vectren Indiana South | 2016 |
| Wabash Valley Power Association | 2004 |
| WPPI Energy | 2018 |
| Xcel Energy | 2000 |
