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Choosing the Right-Sized Solar-Energy System for Your Home

Solar Panel Size

Solar energy is growing in popularity, especially as solar equipment prices fall and the cost of electricity continues to rise in most parts of the country. In a previous article, I discussed how to determine if switching to solar energy makes financial sense for you (while solar energy will save many homeowners money in the long run, it is not necessarily a good economic decision for everyone just yet).

Let’s assume that you have decided to go solar. The next question you may be asking is “how do I choose the right size solar photovoltaic (PV) system for my home?”

In addition to determining how much solar energy you can produce where you live, there are a number of other factors to consider, such as the amount of electricity you use, the size, orientation and design of your roof, and the state/municipal/utility policies that affect the cost of going solar.

Ultimately, a qualified solar installer will need to come to your home and go over the particularities of your situation with you, but it’s always a good idea to have a better understanding of the situation before this stage, so that you can follow the discussion and ask the right questions.

Electricity Use

One of the first things to consider is how much electricity you typically use, and how it varies with the seasons. For example, do you have an air conditioner that runs at full blast in the summer, or do you heat your home with electric baseboard heaters during the cold winter months?

The most accurate way to calculate how much electricity you use is to dig up your utility bills for the past 12 months (or longer if the past year was unseasonably warm or cold). If you don’t have your bills on hand, you can use our Power Consumption Calculator to estimate your electricity usage.

Once you determine your average annual electricity use, you can use our Solar Power Calculator to determine which size system, where you live, will best match that your needs. A general rule of thumb would be to try and optimize the system for your actual electricity use, and not to over-size or under-size it by too much. Obviously, every situation is different, so as you learn more you can come back to the calculator and fine-tune the parameters so that you get the most bang for your buck.

Here is an example for a south-facing roof in Jackson, New Jersey (see image below). Let’s assume that your average annual electricity use is 8,500 kWh, which is slightly above the state average of 8,036 kWh/year. Using the Solar Power Calculator, you can adjust the Annual Electricity Consumption slider so that it reflects your electricity use and then adjust the Number of Solar Panels slider until the solar electricity production matches (and the blue wedge in the orange circle is minimized).

It’s also important to adjust the utility rate if it is higher or lower than the state average ($0.16/kWh in the case of New Jersey), in order to more accurately estimate your savings. In this case, you would be looking at installing a system that is around 6.5 kW. But not so fast, you need to consider your roof now...

Solar Power Calculator

Your Roof

It’s all well and good to estimate what size system will match your electricity use, but what if the portion of your roof that is south-facing is very small, or is disrupted by skylights, or is partially shaded in the afternoon? Only a solar professional can actually determine what size system will fit on your roof and work optimally to meet your electricity needs.

When a solar installer visits your home, they do what is called a home site audit or assessment. A comprehensive assessment includes taking detailed measurements and making a thorough examination of your roof, looking at the condition, age and structure of your roof, as well as the placement of vents, attic fans, chimneys or other obstructions that will impact the system design and solar electricity production.

The assessment will also include shade measurements and an examination of your electrical panel. These steps are necessary for the installer to determine what type of system is possible and to propose the optimal design for your situation. It’s recommended that, whenever possible, you speak with multiple installers and get multiple quotes.

State/Municipal/Utility Policies

As I mentioned earlier, you generally want to choose your system size to match your electricity needs. However, there can be policies in place at the state, municipal or utility level that might impact your decision.

For example, a smaller system might be all you want (and be less expensive!) if your utility rate schedule is a tiered one and your main goal is to keep out of the most expensive tier. A small solar PV system that covers some of your electricity use can keep you below that critical threshold where the retail rate jumps to a more expensive rate. Or, if you are on a time-of-use schedule, you might be looking for a system that can offset your use during the peak electricity period when rates are highest.

Another reason to consider under-sizing your system would be if you live in a state where net-metering policies are such that excess electricity is only credited at the wholesale rate, rather than at the retail rate (which is what you as a consumer pay when you need extra electricity), such as is the case with the recent policy change in Nevada. Unless you have a battery system to store excess electricity, in a place like Nevada, you would not want to overbuild your PV system.

At the other end of the spectrum, an over-sized system might be beneficial if you want to ensure that you produce enough solar electricity to meet even your highest consumption days and are fortunate enough to live in a place where you are well compensated for your solar electricity production.

A number of states offer performance payments, in the form of Solar Renewable Energy Certificates (SRECs). In the District of Columbia, for example, homeowners earn SRECs for every MWh of electricity generated that are generously credited at the moment, although the rate of compensation is decreasing. Other places, such as Washington State, have other programs to compensate homeowners for their electricity production, and which increase if the solar equipment is made in the state, for example.


While installing a solar PV system is an environmental decision for some, for most it is a financial decision. Determining whether solar energy will save you money in the long run and finding the system size that will help you make the most of your investment are important considerations.

It pays to take the time to understand your situation, to speak with multiple installers, and research your options. Sunmetrix Discover can help you, by letting you adjust your system size, your utility rate, your electricity use, and more.

Simone Garneau is the co-founder of Sunmetrix, an online consumer education website for residential solar energy. The goal of Sunmetrix is to help homeowners go solar and save money with our Solar Cashback Program. In addition to the 200+ articles about solar energy, Sunmetrix offers homeowners three main resources: a Consumer Report for solar energy, Discover to preview solar energy for your home, and GO, the only solar energy test drive experience. Read all of Simone's MOTHER EARTH NEWS posts here.

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4/16/2016 1:08:09 PM

There's no question about whether a solar panel system will save you money over the long term. It's not an "if" it's a "how much". How much you save largely depends on which solar installer you use. It definitely pays to shop around. Nick Tedesco