By Stephen Boscarino
There is not one answer to this question. It is mathematical and the numbers involved for each house are unique.
The pertinent numbers are, of course, the amount of electricity you are consuming over the course of a year and the cost of that energy and the amount of energy the solar can produce. You will pay the utility for any additional energy that needs to be supplied after your solar system becomes your primary source of electrical power.
The amount of electricity a Photo Voltaic (PV) system can produce is impacted by a number of factors including the square footage of available roof, the direction and pitch of the roof and, importantly, shading conditions. The more favorable these characteristics are, the greater the production of electrical power will be.
Keep all of the above stated factors constant, another important factor is the efficiency of the panels being used in the project. Panels are rated in watts and the number of watts that a panel can generate is an indication of its efficiency. The higher the efficiency the panels have, the higher the amount of power they will be generate. The more power created by solar, the less has to be bought off the utility.
An important thing to consider is whether the consumption of the house likely to go up or down in the future or will it stay the same?
For example, a young married couple with a baby on the way and plans for two more children and an electric car in their future can safely assume that the first part of the equation, the total amount of energy needed to run the house will go up. The mid 50 year-old couple whose 3 children will be leaving the house over the next couple of years, on the other hand, can certainly expect their consumption go down. Over time, the specifics of your situation will be a determining factor in how much you will pay the utility each year.
To stay with this thought, besides the fluctuation in the amount of energy a house will require another important factor is the amount of power the panels will continue to make over time. All panels will make less power as they age compared to when they were brand new. Not that you should expect to have to buy new panels any time soon. Solar lasts a long time. The standard warranty period for the energy production of solar is 25 years. The typical warranty will state that you will receive no less than 90% of the starting production in power per year over the first 10 years. After that, the warranty drops in a plateau fashion to 80%, so that your production could be substantially lower in the last 15 years covered by the warranty. Not every warranty works this way but most do.
For a higher level of predictability in the amount of money that will be owed to the utility each year, look for a warranty that does not have the sudden decline at any point and pay attention to where the production could potentially be over the course of the warranty period before your protection kicks in. For people with a longer term perspective on things it definitely makes sense to seek out the manufacturers with slower, steadier permitted declines in production. The fancy term used in the industry for this is “Rate of Degradation.” The slower the rate of degradation, the less you will pay to the utility over time. The strongest warranties in solar promise that the panels they cover will be producing close to 90% after 25 years.
An ideal solar electric system is one that covers 100% of your electric usage; maximizing roof space and panel production to achieve that goal. In cases such as these, the remaining PSEG bill should average about $11.00 a month.
So it’s all just arithmetic but there are a number of variables involved and those variable are unique to each house. Be certain to take all these things into consideration or make sure you are working with an installer who can guide you through these factors when you are thinking about your solar project.