rebate calculator

I am trying to put together a worksheet to figure out some local solar rebates. I am a solar installer in CO. Anyway I have the following "tiers" under the new rebate structure:
-Small customer owned system 3KW and under: $1/watt and $.0945 per kWh produced over 9 years
-Small customer owned 3.01KW-6KW: $.75/watt and $.0945 per kWh produced
-Small systems 6.01-10KW: $4500 rebate and $.0945 per kWh produced

This I have all figured out on my spreadsheet, and it all works to show payback period, money saved, tax write off, etc.

The problem I have is I would like to use the same worksheet to do the other rebate tiers. The other two categories have a total of 5 tiers (different rebate structure). I was thinking of having a cell that I type in which category it is, and then having the cell that has my equations in it figure out which equation to use....but have no idea how to do that. ANY HELP WOULD BE APPRECIATED!!! I attached a locked worksheet to view.

Thanks,
Jeff