The main market driver is incentives. In the 1970s and 1980s, the U.S. saw a significant national market for SWHs, partly in response to the energy crises and partly because of a 40% federal income tax credit coupled with individual state incentives. Sales peaked at about 180,000 SWH systems in 1984, just before the end of federal tax credits, representing almost 2% of the total number of WHs installed that year. In 2006, a federal investment tax credit was passed, and sales have recently increased again. The key barriers in the U.S. are the cost of an installed system, the relatively low price of natural gas, and lack of consumer awareness.