I have more than average experience and training in this area. I have a degree as thermal technician and worked in the solar industry as a designer/installer.
It saddens me to see the industry both unjustifiably maligned and on the other hand over promising financial rewards.
To cut through the haze, first consider the location available for panels. If your roof is facing due south and you have a steep 12/12 pitch and no trees or buildings in the way, then awesome, skip to next step. If not, then your "window" needs to be assessed and quantified. Most installers are honest enough to not recommend a system if less than 80% of the potential can be harvested. The additional panels necessary rarely justify the results. Many tax credits also have a minimum window definition. You have this calculated by a professional (electronic devices purpose made for this).
OK, lets say you have a more typical pitch and roof faces 20 degrees away from due south, you now know that the panels will produce about 80% of their peak to begin with.
Next, simply take the size of the proposed grid tied system (system is sized by KW peak output of sum of panels). Let's say you have room and budget for a 6KW system (this is average rooftop size).
So take 6(KW) times .80 (your window) times that by the average annual peak hours equivalent of sun per day for your area (available online in my area it is 4hrs) and then times that by .7 if you live in snow country, .8 if not (average losses from snow/dirt/degradation combined). Take that and multiply the number of days in 20 years (7300) that result is the estimated KWH of production reasonably expected in 20 years. Times that by what your USAGE rate is per/kwh. (average in U.S. is .15/kwh. Assume increases in that rate over those 20 years (inflation) so add about 10%. So in my example use a rate of $0.18/kwh to get a reasonably good idea of what to expect in savings.
6 x .8 x 4hrs =19.2 kwh/day. Times that by incidental losses (I get fair amount of snow) 19.2KWH x .7=13.44 KWH/day. Times that by the average lifespan of the panels of 20 years (7300 days). 13.44 x7300=98,112 KWH/20 years. Times that by your inflation adjusted usage rate. 98,112 x .18= $17,660. So in 20 years you could reasonably expect around $17,660 production of electricity in my area.
Unfortunately, that is not the end of it. Typically, peak output is during midday, but peak usage may or not coincide with your production. This then requires we know what the net metering rates are in your specific area. In my area each power company sets its own policy. When you are producing more than your demand (e.g. no one is home, ac is off) the power goes to the grid, and this is recorded on a bidirectional meter. One company pays more for power than they change, another pays less. Many power companies actually want to have residential production as it lowers their need to meet peak demand, usually during the day for industry. Other companies want to discourage residential solar/wind and so keep usage rates low while raising service charges. Unless you want to spend buckets of money with an off-grid system (add $20-30,000 battery storage), you will be still receiving a bill for service charges, even if you meet all your electrical needs with your system. That is why we only used the usage rate for a meaningful calculation of the savings. Conveniently my power company pays about the same as it charges, consult your provider.
Then there is the matter of the inverter. Panels are likely to last 20 years. On average inverters last about 10 years, and on a 6KW system they will cost about $2500-3000 installed. Warrantees are usually expired by then so you will be paying for it. So subtract that from the $17,660. (17,660-3000=$14,660) The average (and they vary a lot by region) cost per KW rooftop is about $3. Your cost to install a 6KW rooftop system is therefore about $18,000. Ground mount system adds about 20% ($21,600).
Soooooo....In short, an average homeowner living in snow country can expect a solar system to produce enough to almost pay for the system over 20 years. This is where tax credits and incentives come into play. Whatever the incentives are, they are pretty much your profit/savings. In the US the Federal tax credit is now 30% which is the highest it's ever been. There may be some state and other incentives as well but they rarely add up to more than a thousand. So in our example, the incentives might be around $6000 (18,000x.3=5,400 + local/state=6,000). Leaving you with a $12,000 cost to install the 6KW rooftop system for a net advantage of about $2600.
The reasons for promoting solar is that, all said, the environmental impacts are less than the average environmental impacts of large scale production of electricity. If your power provider is getting their power from coal especially. If your grid power was produced with solar/wind and nuclear, there is basically no advantage environmentally to residential power and the financial benefits are realistically not that much.
