| Year | Usage in Gallons |
| 2018 | 558,287,000 |
| 2017 | 503,499,000 |
| 2016 | 531,958,000 |
| 2015 | 522,732,000 |
| 2014 | 525,787,000 |
| 2013 | 498,228,000 |
| 2012 | 598,960,000 |
| 2011 | 537,792,000 |
| 2010 | 562,681,000 |
| 2009 | 527,006,000 |
| 2008 | 616,854,000 |
| 2007 | 608,719,000 |
| 2006 | 665,231,000 |
| 2005 | 642,236,000 |
| 2004 | 595,130,000 |
| 2003 | 634,192,000 |
| 2002 | 766,610,000 |
| 2001 | 747,911,000 |
| 2000 | 796,885,000 |
| 1999 | 729,237,000 |
| 1998 | 752,701,000 |
| 1997 | 714,063,000 |
| 1996 | 746,889,000 |
| 1995 | 656,145,000 |
| 1994 | 740,668,000 |
| 1993 | 674,975,000 |
| 1992 | 652,712,000 |
| 1991 | 614,633,000 |
| Month | Average |
| January | 34,000,000 |
| February | 34,000,000 |
| March | 31,000,000 |
| April | 35,000,000 |
| May | 38,000,000 |
| June | 58,000,000 |
| July | 67,000,000 |
| August | 74,000,000 |
| September | 68,000,000 |
| October | 49,000,000 |
| November | 37,000,000 |
| December | 34,000,000 |