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How to size a solar system for your home

13 Oct 2020
15 min
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Tatiana IvanovaAuthor

According to a Pew Research Center survey, most Americans favor expanding solar power. Nearly half of U.S. homeowners say they have given serious thought to adding solar panels at their home, but only 6% have a solar power system installed.

Why are they so few? Well, lots of them give up halfway, because they can't manage all the solar-related issues: How many panels do I need? What wattage to choose? Will it be enough? This article will answer all your questions and even more.

Understand the average solar power system size

Before we go to a real-life case with more precise calculations, let's have a look at what an average mid-size solar power system looks like. The US Energy Information Administration estimated that the average annual electricity consumption for a utility customer is 10,972 kWh, which means around 29.4 kWh per day. To cover 100% of these energy needs, an American homeowner will need 14 – 36 solar panels.

Note that the figures above are just a jumping-off point. They are here for reference purposes only, so that you can stay within the bounds of reason while sizing your own system. For example, if you have 5 or, on the contrary, 50 panels as a result of your calculations, you now understand that something must have gone wrong.

Find out the number of peak sun hours in your area

Peak sun hours are a really important thing to consider. Some people make a mistake thinking that peak sun hours (PSHs) are the hours of daylight. In fact, the PSH describes the intensity of sunlight in a specific area, defined as an hour of sunlight that reaches about 1,000 watts of power per square meter (around 10.5 feet).

It is logical that the number of peak sun hours you get per day depends on your location. The closer you are to the equator, the more PSHs you have. For the US, the average number is something between 4 or 5. You can use the PSH map above or simply enter your ZIP code into our calculator to find out how many sun peak hours you're going to benefit from in your area.

Calculate your home electricity usage

This step will shed light on the most popular question solar newcomers ask: 'What size solar panel system do I need?' Overall, there are three ways you can go to estimate your energy needs.

Option 1.
Calculate your daily load manually

To do that you need to decide what appliances (light, kettle, TV, etc.) you want to run and for how long. After that you should check the specification chart in your appliances for power rating (Watt), multiply each by the time (Hour) you typically use them per day and sum them up. Read How to design a solar system for home? | A1 Solar Store to learn how to figure out your daily energy consumption.

Option 2.
Find the arithmetic mean of your utility bills

If this option seems to work better for you, don't forget to take into account your solar power system type: grid-tied or off-grid. The first one is simpler to deal with, because there is no way you can miscalculate: you'll always be backed by the grid should anything happen. So, the arithmetic mean of any recent utility bills will be enough.

With an off-grid system, it is a little bit more complicated. Since you are fully independent and have no access to the grid, you need to be 120% sure your system is able to cover all your energy needs, and even more. This is especially important for regions with changeable or extreme weather. What's more, you might want to run a welding machine one day. Will your system still have enough capacity for that? These are good things to think about beforehand. That's why we recommend that you add 20% to the arithmetic mean of three-five highest utility bills for the past year. Better too much than too little.

You'll need to find your utility bills for at least the past three months. The best-case scenario is to take the electricity bills for the past year (12 months) and find their arithmetic mean. The thing is that our energy consumption varies depending on the season. For example, in winter your home electricity usage may be higher because of room heaters, christmas lights, etc.

Grid-tied system

Take your recent utility bills for any three-five months
Find the arithmetic mean of kWh/day

Off-grid system

Take three-five highest utility bills for the past year
Find the arithmetic mean of kWh/day
Add 20% to this number

Here is an example for you to show how it works:

Typically your bill will contain the information on the amount of energy (kWh) used during the billing period, as well as per day (kWh/day). The utility bill above shows that the daily consumption in September was 76 kWh. Let's assume that it is the average energy usage for this utility customer and take it as a basis for our further calculations. We are going to size a grid-tied system, just because it is a way more popular with homeowners than an off-grid one.

9 in 10

Americans choose grid-tied solar systems

First, divide your average daily consumption by the peak sun hours (5.7 PHS for Florida) to get the kW output:

Daily kWh ÷ PHS = kW output
76 ÷ 5.7 = 13.3 kW

The number you get here is a gross output of the solar system needed to cover 100% of energy needs. However, we can't leave it as it is. As a grid-tied system is connected to the utility grid, losses are inevitable. They are usually related to wiring, connection, and system availability losses and amount to about 15% (consult your utility provider to find out your potential system losses). It means that the net solar PV system output will be around 85%. To compensate for these losses, let's increase the system capacity by 15%.

kW output х 1.15 efficiency factor =
DC solar system size
13.3 kW х 1.15 = 15.3 kW

Option 3.
Use A1 Solar calculator

A1 Solar has created a calculator that can significantly economize your time. All you need to know is your ZIP code (to calculate the peak sun hours in your area) and your average monthly electricity consumption. This calculator won't only size your system in a second, but will also say how much you are going to save if you go solar.

As you can see, the calculator suggests a 10.24 kW system, which is about 30% less than what we've got before (15.3 kW). Why is it so? First, A1 Solar calculator uses only up-to-date variables. For example, PHS can vary throughout the year and differ from the average specified in the table or PHS map above. Secondly, the calculations above are for a medium size system, which covers up to 80% of electricity consumption. In fact, that will be more than enough. With such a system your house will be powered by solar energy during the daylight when the electricity rates are the highest. Electricity is often cheaper late at night or early in the morning, so that is when you can switch to the utility grid supply. That'll be less costly than to install a full-size system and then think what to do with the excess energy your panels have generated.

A1 Solar calculator is integrated with the NERL database, so we always know current electricity rates.

Calculate how many solar panels you need

Whichever option you have chosen to estimate your home electricity usage, the most important thing now is to accurately calculate the number of panels needed. A1 Solar calculator has already proposed 32 panels 325 W each for the 10.24 kW medium size solar PV system. The formula is very simple:

System capacity (W) ÷ solar panel output (W) = number of panels
10240 ÷ 325 = 32 panels

There is one variable in the formula above – solar panel output. It means that you can design a system consisting of 26 panels 390 W each, for example. There are two things which are going to influence your final decision on how many panels to install: the type of mounting and the space available.

West
California

554 kWh

2 kW

Northeast
Massachusetts

583 kWh

3 kW

Midwest
Nebraska

957 kWh

4.5 kW

South
Louisiana

1,187 kWh

5 kW

Choose between ground and roof mount

There is no solid answer as to what the best solar panel system for your home is: ground-mounted or roof-mounted. The majority of homeowners opt for roof mounts, because they are less expensive to install and take up less room on the property. On the other hand, ground mounts are a way easier to access for installation and repair. But there is one more thing to keep in mind: the alignment.

Every solar array works best when getting as much sunlight as possible. Since the United States is in the northern hemisphere, the sun will lean south as the Earth orbits. Thus, if your panels face true south, they will capture the most daylight and produce the best results. Needless to say, getting the perfect alignment can be quite tricky for a roof-mounted system. So, to cover the energy needs of one and the same house, the rooftop power station should be of a slightly bigger capacity than the one on the ground. Here are some strong points about each system for you to consider:

Ground Mount

Easy to access
Easy to clean
Easy to align properly
Stronger racking

Roof Mount

Relatively cheap
Installation labor cost is lower
Don't require any additional space
Easier to permit

Make sure you have enough space for the solar system

What we're going to discuss further is crucial for those who choose roof mounts. The thing is that even if you've calculated the amount of solar panels you need, it won't help if your roof can't accommodate them. Here is one more factor to consider in this respect – solar panel efficiency.

Solar panel efficiency shows solar panel's ability to convert sunlight into usable electricity. Given the same amount of sunlight shining for the same duration of time on two different solar panels, the more efficient panel will produce more electricity than the less efficient one.

Most solar panels are between 15% and 20% efficiency, but high-quality solar panels can exceed 22% efficiency. There are several factors that determine how efficient a solar panel is:

Material (monocrystalline or polycrystalline silicon, cadmium telluride, etc.)
Wiring and busing (busbars)
Reflection (glass layer on top of silicon solar cells).

These factors largely influence the price, so high efficiency solar panels tend to be 20-50% more expensive than their less efficient counterparts. On the other hand, the more efficient panels are, the fewer of them you'll need. That also means fewer wires and lower installation cost.

15 panels × 330W
17% efficiency
approx $2700 + auxiliary equipment

12 panels × 330W
20% efficiency
approx $3900 + auxiliary equipmen

Although the higher efficiency solar panel system is about 30% more expensive, they will allow you to economize in a long-term perspective. Firstly, they are high quality panels with low degradation rate. Secondly, they'll save you some valuable space on your roof. Finally, they can be all arranged on the south-facing side of the roof to harvest the most power.

To wrap it up, low and medium efficiency panels are popular with the majority of homeowners because of their relatively low price, while high efficiency panels are a good option for limited spaces.

Study the chart below to get an idea of how many square feet your rooftop would need for a 5 kW, 10 kW, and 15 kW system.

18.6%

is the average solar panel efficiency

5 kW

313

278

227

10 kW

625

556

455

15 kW

938

833

682

Once you know the kW size of your solar panel system, you can estimate the amount of space low, medium or high efficiency panels will take up on your roof. For example, the 10.24 kW system consisting of medium efficiency 32 panels 325W each will occupy about 550 square feet. It means that the homeowner can buy more lower efficiency panels and save money since they are cheaper than higher efficiency ones.

Reduce your solar system cost by 26 percent with Federal tax credit

The Federal Solar Tax Credit, also known as the Solar Investment Tax Credit (ITC), is an important solar incentive available in the United States, according to which the federal government rewards you with a tax credit for investing in solar energy. In fact, 26% of your total project costs (including equipment, permitting and installation fees) can be claimed as a credit on your federal tax return if you start the construction before December 31, 2020.

To qualify for 26% credit level, work must have started by December 31, 2020. All work commenced in 2021 qualifies for a 22% credit level. The permanent 10% credit level starts for projects that begin construction after December 31, 2021, or are put in service in 2024 or later. The good news is that you'll be still able to claim the ITC till 2022 (fairly speaking, the chances it'll be further extended are high). That bad news is that you can obviously deduct less with each passing year, so be quick to install your solar power system!

A tax credit is a dollar-for-dollar reduction of the income tax you owe. For example, if your system costs $10,000 and you have installed it before the end of 2020, you will owe $2,600 less in taxes the following year. Unfortunately, the Solar tax credit program can't issue you a refund check. It is only applied against your tax liability, or the money you owe the IRS at income tax time.

$1 credit

= $1 less you pay in taxes

Calculate your solar system

Our Grid-Tied Solar System Calculator will help you choose the right solar panels and accessories to cover your energy needs.

Calculate your PV system

Generally, it isn't a problem for a homeowner to claim these solar incentives. There are only two reasons why you may NOT be eligible for the Federal Solar Tax Credit:

You don't owe any taxes. Well, it's unlikely, because most of us do. Even if you don't have enough tax liability to claim the entire credit in one year, you can 'roll over' the remaining credits into the following years.
You don't own your system. That is why solar leasing isn't always a good idea. If another company leases you the system, they are the owners, so they are the ones who are able to claim the incentives.

If you've already bought your solar equipment and checked you're eligible for the ITC, it's the time for some paper work. In fact, claiming the ITC is easy. All you need to do is complete IRS Form 5695 and include the final result of that form on IRS Schedule 3/Form 1040.