How Many Solar Panels Do I Need? A Guide to Solar Power Systems

Thanks to tremendous technical developments over the years, solar panels are by far one of the cheapest, safest, and most reliable ways of producing home electricity.

If you consider getting solar panels to partially or fully cover your electricity needs, the first question that will come to mind is, “How many solar panels do I need?”

In this article, I will guide you in developing your perfect solar project. By the end of it, you will be able to:

• Accurately size your system.
• Estimate your financial gain.
• Find the perfect spot to install them.

7 Things to Consider Before Installing Solar Panels

In 2020, solar energy became the cheapest source of electricity in the world. You can now reduce your environmental impact while saving money by installing a solar system on your rooftop.

However, bear in mind that you are dealing with nature here. Therefore, your solar power production will vary depending on many factors, including time of day or season, weather condition, exposure to peak sun hours, and more.

If you’re renting someone else’s property, you will likely need permission before installing solar panels. SpareRoom can help you find properties to rent with solar panels all over the US, from LA to Austin. Try it now.

For your solar system to be profitable, it has to be optimized. To get a clearer view, I have listed below the 7 most important factors that will influence the size of your solar energy system.

1. What Is Your Solar Energy Production Potential?

Solar intensity greatly varies depending on your position on the planet. For example, with the same number of solar modules, you will produce two times more electricity in California than in Alaska. Therefore, your investment will be doubled in Alaska for the same amount of electricity output.

The main factor of influence is the peak sunlight hour that your location receives yearly. Global Solar Atlas is a comprehensive solar database that will give you an accurate estimation of your solar energy production potential.

When you enter your address into the system, you will get the specific photovoltaic (PV) power output in kWh/kWp per year.

For example, if you get a value of 1000 kWh/kWp per year, it means a solar energy system of 1kW has the potential to produce 1000 kWh in a year.

2. How Much Energy Do You Need?

To accurately determine your number of solar panels, you have to know your daily energy needs.

Your utility bill measures your electricity consumption in kWh on a monthly basis. Then, divide it by 30 to obtain your daily energy needs.

You now have all the information to get a first estimate of your solar energy system size.

Let’s have a look at the example below:

In the USA, an average house uses 30 kWh per day. With this information, go back to the Global Solar Atlas and get your solar energy production potential (e.g., 4.9 kWh/kWp per day in Los Angeles).

Divide your average daily energy needs by your solar production potential. You will get the total power output of your future solar system to cover your consumption.

Here we find: 30/4.9= 6.12 kWp.

You will need a 6.12kWp solar energy system to cover 100% of your electricity needs. Given an average solar panel power of 400W, your system will have 16 solar panels.

3. What Will Be the Power Output of Your Solar Modules?

As mentioned before, solar PV technology is extremely versatile, and you will find solar modules with power outputs from 100W to 500W.

Keeping our previous example of an average house in Los Angeles, you will need 16 solar panels of 400W to cover your needs. Five years ago, the standard was the 275W solar panel, and for the same installation, you would have needed 23 panels! 

Solar energy systems with a large number of solar panels will occupy more space, are more complex, are more expensive, and are less efficient than systems with a reduced number.

Therefore, we recommend going for solar modules with the highest output, like the new market standard – 400W solar panels.

4. Where Will You Install Your Solar Energy System?

Solar panels love direct sunlight. The technology is developed to work best under full sun during peak sunlight hours.

Peak sunlight hours only occur a few hours during a day – around noon. Consequently, you have to be sure that your solar panels are properly installed to get the most of the peak sunlight hours.

Two tips for the best place to install your solar panels:

• Make sure that there is no local shading.
• Make sure that your panels are facing south.

The direction that your solar panel array faces is called the azimuth angle. A 180° angle means that your solar modules are facing south, whereas a 0° angle means that your panels are facing North.

As an example, for the same location, a system facing south (180° azimuth angle) will produce two times more electricity than a system facing north (0° azimuth angle).

In the northern hemisphere, increasing the azimuth angle will give you more production in the afternoon, and decreasing the angle will give you more production in the morning.

Most of the domestic installations are on rooftops, because they usually have large, unused spaces where panels can be set without being an eyesore. In addition, they are mostly free of local shading.

5. How Will You Install Your Solar Panels?

Don’t lay your solar panels flat or vertical because it will greatly affect their production.

Like most domestic solar energy systems, you might have to install your solar panels on the roof.

The pitch of your roof is of the same importance as the orientation. Depending on your locations’ latitude, solar panels need to be tilted to a certain angle to maximize full sun exposure and electricity production over the year.

Find your ideal tilting angle with the Global Solar Atlas (by entering your address).

A typical tilting angle in the Northern Hemisphere is 20°. This means your solar module array has a 20° angle horizontally.

If you are going for a rooftop installation, you might need to add an extra structure on your roof to reach the optimal tilting angle.

Note that a lower tilt angle will get you more production during the summer and a higher tilt angle provides more production during the winter.

6. What Type of Solar Panels Will You Choose?

There are two types of solar panels available: monocrystalline and polycrystalline.

With recent technical developments, both technologies offer similar efficiencies at room temperature. However, one main difference is the thermal behavior of the solar modules.

All solar modules lose efficiency when their temperature rises, typically from -0.3%/°C to -0.5%/°C, and panels can get as hot as 80°C.

The monocrystalline technology shows better performance at higher temperature compared to polycrystalline (up to 20% more energy produced).

Go for monocrystalline solar modules as they will maximize your system production.

7. What Will You Do With Your Excess Solar Energy Production?

Your solar energy production is seasonal. Likewise, if your system is sized to cover 100% of your needs, you will certainly have an excess of production. This happens when you receive the highest number of peak sun hours during the summer months.

Don’t worry, this energy won’t be wasted, and you will even be paid for your electricity injected in the network thanks to net metering.

Net metering allows you to sell your excess of solar energy to the local electricity grid. Usually, you will receive a credit on your future utility bill. Have a look at this article to see if your state allows net-metering.

Polycrystalline vs Monocrystalline – What Type of Solar Panel Is Best to Power an Average House?

The heart of your solar energy system is the solar panel. You might be hesitating between the two types commercially available: polycrystalline and monocrystalline.

They both have silicon as their active material. However, the monocrystalline technology is more recent and advanced. It offers several benefits compared to polycrystalline:

• Better thermal coefficient, more electricity production
• Better overall efficiency, smaller size
• Smooth and uniform appearance

In the end, I would recommend monocrystalline solar cells as they would take less space (for the same power) and offer better efficiency during sun peak hours.

In terms of size, you will find solar panels ranging from 100W to 530W. You might want to consider smaller modules if the geometry of your roof doesn’t allow large module installation. Bear in mind that the more modules you install, the more complex and less efficient your system is.

In the chart below, I have summarized the most common solar panel sizes and power output.

Solar panel power	Solar panel size	Solar panel power/square foot
100W	42 x 19.5in (5.54sq feet)	18W
200W	25.9*63.8in (10.93sq feet)	18.3W
300W	64.96*38.9in (17.13sq feet)	17.5W
400W	79*39in (21.39sq feet)	18.7W
530W	89*44in (27.19sq feet)	19.5W

How to Calculate the Number of Solar Panels You Need

You can easily estimate your number of solar panels by using a simple solar panel calculation formula combining three variables:

• Yearly energy needs (kWh)

Find this information on your utility bill.

• Solar energy production potential (kWh/Wp)

Enter your location in the Global Solar Atlas to get this information.

• Solar panel max. power (Wp)

Solar modules are ranked in terms of Watt peak (Wp). This is the maximum power they can produce in ideal situations. You will find modules ranging from 100 Wp to more than 500 Wp.

The formula to calculate your number of solar modules is as follows:

The only fixed number is the production potential.

You can adjust your energy needs (e.g., if you don’t want to cover 100% of your electricity bill) and the peak power of the solar panels you will buy.

Note that this formula gives you a good estimate, but as mentioned before, other factors will influence your production – don’t neglect them!

Solar Panel Calculators and Their Importance

Solar calculators help you accurately determine the number of panels you will need for your solar energy system.

The sun is a natural source of energy and is therefore intermittent. A passing cloud, a rainy day, local shading by trees, and surface dusts are some of the factors that can influence the power output of your panel.

Solar calculators use weather and solar insulation databases to predict with a 95% accuracy the solar production potential in any part of the Earth.

They will help you size your solar energy system according to multiple parameters. The simplest calculators will only give you an estimate of your system size, and the most advanced provides a complete technical and economic picture of your solar system.

4 Best Solar Calculators To Size Your Solar Energy System

I have selected the best solar calculators to accompany your project: from the roughest estimate for a quick look to the most accurate calculator that takes into account your roof’s geometry and your system’s loss, combined with a huge weather pattern and satellite measurement database.

1. Go Green Solar – Get a First Estimate

This is a fast and straightforward calculator.

What do you need?

Just enter your address (zip code) and your monthly energy consumption (in kWh).

What will you get?

The calculator will provide a rough estimate of the number of solar modules needed to cover 100% of your daily needs.

2. Sunwatts – Get a More Advanced Evaluation

This calculator gives you a more accurate estimate of the number of modules needed based on the solar potential production map.

What do you need?

Enter the number of sun-hour per day and your yearly energy consumption (in kWh). Adjust the percentage of the electricity bill you would like to offset with your system.

What will you get?

It will provide your solar array size estimate.

3. Google Project Sunroof – Get a Personalized, Advanced Analysis of Your Solar System

An innovative approach to calculate your solar power production potential and your number of solar modules, the Google Project Sunroof is a technology developed by Google based on Google Earth imagery. It analyzes the shape of your roof, its orientation, and nearby shading objects and combines all the information with a weather database.

What do you need?

This calculator is very easy to use. You just need to enter your address and your average monthly electricity bill.

What will you get?

You’ll get a personalized estimation of your recommended solar installation size, installation upfront costs, state and federal incentives, and potential cost savings over 20 years. They even included an estimate of your positive environmental impact in terms of CO₂ savings.

4. NREL PVWatts Calculator – Get a Professional Evaluation of Your Solar Project

This is the most advanced solar calculator. It is based on a weather and insulation database combined with mathematical equations, taking into account all potential losses of the system. It is well-suited for preliminary studies.

NREL, or the National Renewable Energy Laboratory, is the world’s most renowned institute for solar energy. They offer a professional solar calculator that is accessible to anyone.

What do you need?

Enter your address, and adjust the parameters, such as solar panel orientation, loss coefficient, module type, and many more.

What will you get?

You’ll get the most accurate estimate of your solar energy production per month and money saved thanks to the system.

How Many Solar Modules Do You Need to Power a House?

To determine the number of solar modules needed to power an average house, you will need two information:

• The daily energy consumption
• The daily solar production potential

In 2020, according to the EIA (US Energy Information Administration), an average house in the USA uses 30 kWh of energy per day. This figure is influenced by the number and total power of appliances in use. For a personalized answer, look at your utility bill to get your exact daily energy consumption.

According to the Global Solar Atlas, the daily average PV production potential in the USA is 4.5 kWh/kWp.

Let’s do the math – Average consumption/Average potential production = 30/4.5 = 6.7.

Therefore, a solar panel array of 6.7 kW is needed to cover 100% of the daily needs of an average house. If you choose the new standard 400W panel, it means that you will need 17 solar modules (of 400W) to cover 100% of the electricity needs of your house.

Of course, all houses are different, and the energy needs greatly vary depending on the size of your house and the number of electric appliances as mentioned before.

Check the chart below for the number of solar panels needed to cover 100% of your electricity bills depending on the size of your house:

House size	Number of 400W solar panels needed	Solar module total surface	Total cost of solar modules ($200 per module)
1500 sq. ft.	11	235 sq. ft.	$2,200
2300 sq. ft. (average family house in the USA)	17	364 sq. ft.	$3,400
2500 sq. ft.	19	406 sq. ft.	$3,800

Bear in mind that those figures are average numbers in the USA and greatly influenced by many factors such as the number of peak solar hours received at your location and solar panel orientation.

Do you want a more accurate estimate? Have a look at our solar calculator selection.

How Many Solar Modules Do You Need to Live Off The Grid?

When living off the grid, you will be disconnected from the electricity supplier. Therefore, you have to be sure that your solar system is perfectly sized to cover your needs at any time of the year.

Off-grid systems have oversized solar panel arrays compared to grid-connected systems.

Whereas a grid connected house will only need 17 solar panels to cover its needs, an off-grid house will require 25 solar panels.

To choose your number of solar modules for off-grid living, you will need to know:

• Your average monthly energy consumption (on your electricity utility bill)
• Your monthly solar energy production potential (with NREL PVWatts calculator)

During winter months, you will produce less than during summer. Thus, you will need to oversize your system to cover low-production months.

For example, in California, you will produce 60% more solar energy during summer!

Adjust the solar array size until your lowest production month (usually December) matches with your average monthly consumption.

Let’s see how many solar panels an average house in the USA would need to live off grid in California:

Average electricity use per month	912kWh
Highest production month for a 10kW system	1693kWh (July)
Lowest production month for a 10kW system	986kWh (December)

This shows a 10kW system is enough to cover your needs at all times (even during the lowest production month).

For this system, you will need 25 solar panels of 400W.

The main drawback of living off grid is that you will be overproducing electricity most of the year.

Tips and Future Trends

Over the last five years, thanks to tremendous price decrease, solar energy became the new king of electricity. This is now a great time to install a solar energy system in your house. Aside from reducing your electric bill, you can also reduce your environmental impact.

Let me summarize the most important tips to optimize your solar production:

• Find the most sunny spot with no shade (roof or garden).
• Know your energy needs.
• Estimate your solar energy production with a solar calculator.
• Consider selling your excess of solar energy with net metering.

In the coming years, thanks to recent technical developments and increased capacity production of solar panels, the cost of solar systems will be even further reduced, and solar energy will be fully democratized.

Domestic solar energy producers will organize themselves in micro energy cooperatives, sharing their energy on a local scale and selling the excess to the utility network. This will lead to a more reliable and sustainable energy grid across the country.