Why can't you find a written warranty or guarantee on their
site?  Home Depot does not actually warranty or guarantee
any of the work done by one of their outside contractors.  
Their guarantee is that their sub-contractor will warranty the
work for 1 year.  How do you know that the company or
person that Home Depot hires to do your electrical work will
even be in business in 1 year?  If there is an issue with
whatever was installed you will get no support or help from
Home Depot and you are left to fight with the sub-contractor
on your own.