As with everything in life, the lowest price is not necessarily the best one. Rather it might be a sign of an inexperienced company or that a company will not be in business when you need them.
Design is very important in a renewable energy system. A bit too much shade or a bit too little wind is a killer.
A system which is shoddily or incorrectly installed is never a bargain. Quality and experience should rank high on your list when a contractor is chosen.
Ask how long they have been in business.
Ask how many systems they have installed.
You are relying on your contractor to be familiar with Code requirements in your area, although most are based on the National Electrical Cod (NEC).Your contractor will also get permits where needed and these permits should be included in the bid.
The warranty offered by the company is another very important factor when evaluating bids. You also needa company that will be in business to support that warranty.