If you are leasing a system: you save the difference between what you pay the leasing company and what you would pay the local utility company.

If you are purchasing a system: this answer is much more complex. To come up with an estimated dollar figure you must know the amount of energy your system will produce. You must factor in items such as number of sunny days, length of sun exposure on your panels, etc. to come up with that figure. You must also consider the current and future value of electricity from your local utility company, the cost of the system, the life expectancy of the system, along with your current and future electricity usage. If it sounds complicated, that’s because it is.

If you are connected to the grid, chances are you will have a dual metering system. One of the meters will measure the amount of electricity you pull from the grid (you’ll pay for this at the retail rate from your local utility company); the other meter will measure the amount of electricity you provide back to the grid (you’ll be paid for this at the normal wholesale rate from your utility company) – yes you are going to pay more to get the electricity than you are going to be able to sell it for. The contract for this with your local utility company will need to be considered when calculating savings.

When vetting possible companies to purchase your system from, ask them to estimate your cost savings. They have experience with systems that will be similar in size, quality, and production capacity as the one you are considering and they have experience in the locale.

Sources:

http://www.seattle.gov/light/solarenergy/solarfaq.asp#save

http://energy.gov/energysaver/articles/planning-home-solar-electric-system

http://energy.gov/energysaver/articles/grid-connected-renewable-energy-systems

http://www.nrel.gov/docs/fy12osti/54738.pdf

Typical solar panels weigh between 25-40 pounds each. They are generally installed (on pitched roofs) by attaching them to the rafters. Small holes are drilled and after the installation is complete, installers generally add sealants to fill any gaps between the fitting and the hole that was drilled. Most installers will also add flashing to further protect against the elements. If your roof is flat, most installers will secure the panels to ballasts, such as concrete blocks, so there are no holes drilled in the roof.

Since solar panels have long life spans, if your roof is in need of repair, it will be less costly to repair the roof prior to installation.

Your roof makes a difference in the installation of panels. A composite shingle roof is the easiest type of roof for installers to work on. Wood shake roofs and tile roofs can be difficult due to the splitting and breaking of tiles since these types of roofs are not made to be walked on. Slate roofs are generally not compatible with solar panel installation.

Sources:

http://www.energy.gov/eere/femp/downloads/solar-ready-buildings-planning-quide

http://www.solarenergyenvironments.com/faq.php#link24

• The year the solar panels were produced
• The quality of the panel (higher quality panels tend to last longer)
• Time panel gets direct sun each day

The EU Energy Institute estimates that 90% of panels purchased in the last ten years will last 30 years. Panels manufactured in 2013 or later should last 40 years. They also noted that about 90% of all panels will outlast the estimated lifespan – albeit with a drop in performance.

It should be noted that while the lifespan of the solar panels are getting longer, inverters still have a life span of approximately 15 years.
Sources:
http://www.ul.com/global/documents/newscience/journal/sustainableenergy/New_Science_SE_Journal_Issue_3.pdf
http://www.solar-rating.org/facts/system_ratings.html
http://www.ee.institute.org