A tax deduction reduces the amount of income subject to tax. For example, if your taxable income is $25,000 and you have a $500 tax deduction, your taxable income would be reduced to $24,500.

A tax credit reduces the amount of tax you have to pay. For example, if you owe $800 in taxes at the end of the year, and you get a $300 tax credit, then you will only owe $500. If you are getting a refund at the end of the year, you can still most likely collect the entire tax credit, as long as the total amount of income taxes that you have paid throughout the year (check your W2 for this amount) is more than the total amount of tax credits that you are claiming. You generally can't get more money back from the government than you paid in income taxes with the exception being "refundable" tax credits, such as the Earned Income Tax Credit.

If you don't pay any taxes, then you can't get the tax credit for energy efficiency.