You need to call on some pros. There are many issues that need to be addressed, before an answer can be given.
Major issues include:
-Who designed the system?
-Who selected the equipment?
-Was the equipment used correctly, installed correctly?
-Were proper methods used to test the switch-over?
-How long has the equipment been in use?
-Was any other word performed on the system?
-What did the customer ask for? Did he get it?
-What promises were made?
-Did the customer accept the equipment, and the testing?
-Was there anything that would have prevented the power failure? Was they 'thing' part of the job?
If you have "errors and omissions" insurance, then you need to talk to your insurance carrier.
Otherwise, you need to get an attorney involved. You need his professional opinion as to what your area recognises as to express and implied warranties, damage limitations, etc.
There are literally centuries of experience that help us determine what is reasonable, and what is not. The "Uniform Commercial Code" offers some guidance- but you have to understand the context. Likewise, standard insurance policies are based upon this sort of litigation.
I would not place a lot of faith in their 'after the fact' second guessing. Were they competent enough to do this, they would also be competent enough to specify things correctly- and therefore be responsible for giving you a bad spec.
