Simply, would suggest a criteria matrix of what’s include and not included from each proposal. You may wish to compare energy estimates with actuals. Energy is asymptotic over time. The modules are typically clean when first installed, some may choose to use a water hose ~once or ~twice a year at specific times. Estimates on Mean Time Between Failure (MTBF) of various components may be discuss including any service level agreements, shading, … and degradation projections.

Suggest purchasing a system you are able to afford. With the use of micro-inverters, smaller systems may be implemented. For string inverters, there are best practice with regards to string sizing based on the lowest ambient temperature and NEC 690.7 to maximize the photovoltaic source circuit. With derate factors, depending on location, one module may provide as much energy as one month of usage. Options for PV may be the roof, ground, … Some ground-mounted structures may be beautiful alternative, if the roof would not be a viable option.

For some locations there are both requirements and issues:

http://www.dsireusa.org/solar
http://www.newenergychoices.org/uploads/FreeingTheGrid2009.pdf

If California, a contractor requires “four (4) full years of journeyman-level or higher experience in the classification”

For example, install and use PV systems ~5 years, with only one situation. A tree fell on and switched off the visible “readily accessible” AC disconnect means Switched back on.

Sunlight, a “good and perfect gift is from above”

You are indeed correct there are programs where you can sell excess electricity back to the utility. That’s called “net-metering”. CL&P and UI are both required to buy back excess energy you produce with solar. This law was in effect in 1998 and is still in force today.