How do US-tax credits work?

[again]

Hello everyone,

I'm from Germany and I have to calculate an US-investment for a German Corporation which wants to establish an US corporation in the fields of photovoltaic.

I read a lot about US tax credits and other incentives. What I don't understand so far is, do I have to deduct the amount of tax credit from the taxable income (and then multiply the result with the resp. tax rate) or do I have to caculate the regular tax burden on the regular taxable income first and then reduce the tax burden itself by the amount of tax credit?

(I hope you do understand what I want to ask you with my poor little English...!)

Thank you for your help!

Best regards from Germany!
again