To understand the issue, you need to understand the history of this credit.
For 2006 and 2007, there was a 10 percent credit for qualifying improvements, subject to a $500 lifetime limit ($200 for windows). During 2008, there was no credit. For 2009 and 2010, the credit rate was 30 percent of the cost of qualifying improvements, subject to a $1,500 limit during those two years combined.
From a monetary perspective, the credit that was extended to 2011 is the 2006-2007 credit, not the 2009-2010 credit. So the rate of the credit available for 2011 is 10 percent, not 30 percent. And the lifetime maximum is $500 – $200 for windows; $50 for each advanced main air circulating fan; $150 for each qualified natural gas, propane, or oil furnace or hot water boiler; and $300 for each item of energy-efficient building property.
You may have already exceeded your lifetime limit in prior years