I'm getting a 12kW system installed at a cost of about $2.75/watt. I was very happy with that cost compared to other costs in the area.
The one huge thing with solar is you need to educate yourself. Solar installers possess the knowledge so they will suggest a system (or panels, or a specific invertor) that may or may not be what you want. Panel costs vary widely as does quality and efficiency at the end of the panels life (warranty) about 25 years. Panels do degrade in efficiency to the 80%s at the end of their life. Payback numbers I've seen are generally between 8-12 years depending on a variety of assumptions (these assumptions are almost always skewed in the installers behalf so again,) buyer beware.
On average, according to Solar Reviews - the average cost to install a system in New Hampshire is about $2.83/w.
https://www.solarreviews.com/solar-panel-cost
Get a couple opinions and quotes if you can.