Year	Hours
1970-01-01	1970
1971-01-01	1969
1972-01-01	1918
1973-01-01	1904
1974-01-01	1873
1975-01-01	1847
1976-01-01	1874
1977-01-01	1837
1978-01-01	1806
1979-01-01	1804
1980-01-01	1795
1981-01-01	1776
1982-01-01	1703
1983-01-01	1685
1984-01-01	1679
1985-01-01	1645
1986-01-01	1640
1987-01-01	1653
1988-01-01	1663
1989-01-01	1648
1990-01-01	1644
1991-01-01	1635
1992-01-01	1637
1993-01-01	1623
1994-01-01	1614
1995-01-01	1590
1996-01-01	1592
1997-01-01	1583
1998-01-01	1570
1999-01-01	1560
2000-01-01	1523
2001-01-01	1514
2002-01-01	1476
2003-01-01	1473
2004-01-01	1501
2005-01-01	1495
2006-01-01	1473
2007-01-01	1485
2008-01-01	1492
2009-01-01	1472
2010-01-01	1478
2011-01-01	1476