Bill & Debbie wrote:

I'm purchasing a replacement for my failed A/C system. The contractor has
guaranteed in writing that I will realize a 30% savings in my electric bill.
The amount being saved will be calculated at the end of the year by
comparing the energy used during the last year for the old system and the
energy used during the first year with the new system. If the 30% savings
is not realized, the installer will give me a check to pay for twice the
difference between what I paid for energy and would have paid if the 30%
savings had been realized. Also, they will make whatever changes are
necessary to ensure that the 30% savings is achieved at no cost to me.

I would appreciate any comments/advice/warnings about this guarantee.

Sounds fine, from your point of view.

Nick