How Much Can I Save With Solar Panels?
The easy answer is that you save the amount your system produces times the cost of purchasing that amount of electricity from your local utility.
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