i got the final of the 3 bids for an install of a new central air system. we don't have an existing system to replace so there isn't a previous configuration that can be considered.

with the 3 bids i got the following:

3 ton 11 SEER unit
4 ton 12 SEER unit
2 1/2 ton 14 SEER unit

what is the relationship between the ton and the SEER rating of a particular unit?

i'm guessing the ton is a result of the load calculation done. i'm surprised by such varied results. 2 1/2 - 4 ton seems like to be a big difference. is it?