There are 4 main criteria you should look at which I explain below:
Obviously this is very important, but it may not be the most significant factor in your purchase. Remember, this is a system that you will depend on every day for up to 20 years, so it is critical that you have a system that you are happy with, has the features you need, and is suited to your individual circumstances.
What performance warranties does the installer provide? Your installer should provide warranty of the panels, controller (if a split system) and boost system for at least 5 years.
Also ask what guarantees do they provide on the rest of the system and their workmanship (over and above the manufacturers warranties)?
Ask them how you will know if your system is performing properly.
Ask them what maintenence is needed and what it will cost.
A well fitted solar hot water panel can be a beautiful thing. What kind of mounting system does your contractor use and how do they look.
Ask for some referrals and go look at previous installations. Inspect the workmanship, and overall simplicity of the installation. Do they use stainless steel nuts and bolts to prevent galvanic corrosion with any aluminium framing? If there is an aluminium frame on a tin roof, have they used an insulating spearator to prevent glavanic corrosion?
(Note: Galvanic Corrosion is when dissimilar metals react to each other and corrode.)
Having a good relationship with your contractor matters a lot for such an important project. A good relationship will ensure any problems are fixed up quickly and in a mutually beneficial way. Choose a contractor that you are compatible with. For example: If you like to ask loads of detailed questions, go with someone who enjoys explaining things in terms you can understand!
Here’s a list of questions you may like to ask (source: Home Power Magazine Issue 127):