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The correct credit is $600 if you spent $2,000.  It is not a dollar-for-dollar credit.  The credit is 30% of your cost, to a maximum of $2,000.  So, in order to be eligible for the maximum of $2,000, you'd need to have spent $6,667 or more.

See this article for a summary of the amounts available: Energy Efficient Home Improvement Credit (EEHIC)