Calculate the Number of Solar Panels You Need

If you’re considering solar power, one of the first questions on your mind is likely to be: “How many solar panels do I need?” In this guide, we’ll walk you through how to calculate the number of solar panels needed for your energy goals and other factors to take into consideration. 

Key takeaways: 

• The average U.S. household needs between 15 to 21 solar panels to cover its daily energy usage. 
• To calculate the number of solar panels you need, you’ll have to know your average electricity usage, how much sunlight your location receives, and the wattage of the solar panels you’re considering. 
• Other factors to consider are solar panel efficiency, the size and angle of your roof, and your budget. 

How many solar panels do you need?

Calculating how many solar panels you need involves four steps: 

• Step 1: Calculate your home’s average energy requirements

• Step 2: Find out how much sunlight your location receives

• Step 3: Calculate the capacity your solar panel system needs

• Step 4: Bring all that information together to find out how many solar panels you’ll need in total. 

If this all sounds like too much math, the easiest way to find out how many solar panels you need is to contact a professional solar installer for a free solar evaluation and quote. If you’re ready to roll up your sleeves and do the calculations yourself, let’s look at each step in detail below. 

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Step 1: Find out how much energy you currently use

The first step when calculating how many solar panels you need is to understand how much electricity your household currently uses (and might need in the future). The easiest way to find this out is to look at past electricity bills to know how much power you’ve used in the last year. 

Find your yearly average

If possible, we recommend looking at your electricity usage over an entire year to get a more accurate estimate. The amount of power you use is likely to vary from month to month and your most recent bill might not necessarily reflect average usage (think of using air conditioners in summer or heating in winter). 

Once you’ve calculated how much electricity you’ve used over the last 12 months, you can then divide it by 365 to find out your average daily usage. If you have a smart meter, this process should be easier as your account should already state daily usage. 

Power consumption is measured in kilowatt-hours (kWh). According to the U.S. Energy Information Administration, the average U.S. household consumes about 10,500 kWh of electricity per year, which equals about 875 kWh per month or 29 kWh per day. That said, every household is different. 

Your usage may vary depending on:

• The number of people you live with
• The area you live in
• Whether you use gas for cooking or hot water.  

Consider when you use electricity

While calculating how much energy you use, it’s also worth considering when you use electricity the most. If your home is mostly empty during the day, it’s likely that your main power consumption happens later in the day and evening. If so, you might want to consider pre-cooling your home in the summer, installing panels facing west to produce energy later in the day, and consider investing in battery storage. 

Take future needs into account

It’s also a good idea to consider your future power needs. Your solar system is hopefully going to be with you for years to come, so it can be helpful to anticipate how much electricity you might need in the future. 

Some things to consider with future needs include: 

• Getting an electric vehicle (EV)
• Starting a family
• Switching from gas hot water and cooking to electric
• Adding a swimming pool
• Working from home more often. 

Of course, none of your future needs have to be set in stone, but it’s helpful to consider how much power you might need in the future so you can accommodate that from the beginning. If you can’t afford to invest in a solar system that can handle your potential future, that’s okay - you can always upgrade or expand your residential solar panels later on.

Give yourself a buffer

Because solar panels generate power from unobstructed sunlight exposure, they won’t be operating at maximum efficiency 24 hours a day. Weather conditions and other factors can temporarily reduce your solar panel’s output. For that reason, it’s a good idea to add a buffer of about 25% to your calculated daily usage to make sure you’re able to generate all the clean energy you need. 

Add it all up

Once you’ve calculated your daily average electricity usage from your utility bills, factored in your future power needs, and added a 25% cushion, you’ll have the number of kWh you need your solar panels to generate each day (or year) to cover your electricity needs. 

Let’s go over that again: 

1. Calculate your daily energy usage from utility bills
2. Factor in any future power needs
3. Add a 25% cushion.

Step 2: Find out how much sunlight your area gets

The next step is to think about how much sunlight you get in your particular location. The amount of sunshine that hits your solar panels will determine how much electricity you can expect to produce and how many solar panels you’ll need. 

Homes in sunnier locations, such as Phoenix, are likely to have more peak sunlight hours than homes in cloudier areas, like Pittsburgh. That doesn’t mean you can’t generate solar power if you live in Pittsburgh, it just means you might need more solar panels. 

The amount of sunlight a location gets is measured in ‘peak sun hours’. Sunny states, like Arizona, can get up to 222 peak sun hours a month while cloudier states, like Ohio, are estimated to get around 120 peak sun hours per month. 

The below map from the National Renewable Energy Laboratory (NREL) shows the annual average daily total solar resource for each state in kWh per day. 

Step 3: Calculate the capacity of your solar panel system

The next step is to calculate how much capacity you need your solar panel system to hold. To do this, you’ll take your average daily usage you calculated in Step 1 and compare it to the number of daily peak sunlight hours for your location using the map provided in Step 2. This will give you the amount of power you need your solar system to generate each day to meet your needs. 

Here’s a step-by-step process using an average U.S. household using 10,500 kWh per year.

1. Find your daily usage: Divide your annual usage by 365 to get your daily kWh needs. For the average U.S. household, this would be 29 kWh per day. 
2. Find your daily peak sunlight hours: Find your state on the map to know your daily total solar resource per day. Let’s say you live in Texas and get 5 kWh per day. 
3. Divide your usage by daily solar resource: Divide your daily energy usage by the daily peak sun hours. In this case, it would be 29 divided by 5 which totals 5.8. 
4. Convert to wattage: Multiply your result by 1,000 to convert from kWh to wattage, which is the rating used for solar panels. In this case, we’d get 5,800 watts. 

The final number you get in watts (in this case 5,800) is the size you need your solar panel system to be in order to generate enough power. 

Step 4: Calculate how many solar panels you need

The final step is to divide the solar system capacity you need with a solar panel’s power output to calculate how many panels you need altogether. Generally, the higher a solar panel’s output, the fewer solar panels you’ll need all up.  

The average residential solar panel has an output of 250 to 400 watts. If you’re looking at a particular solar panel, you can find its wattage on the datasheet where it can be labeled rated power, maximum power, Pmax, or nominal power. 

To calculate the amount of solar panels you need, you’ll be dividing your target solar system capacity with the solar panel’s output. Let’s continue with our example using the average U.S. household. 

1. Your solar energy system needs to generate 5,800 watts per day.
2. You’re looking at a solar panel wattage of 300. 
3. Divide 5,800 by 300 to get the number of panels you need. In this case, it’s around 19. 

With this example, you’d need 19 solar panels with a 300 watts power rating.

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Other factors to consider

The above steps can be used as a guide to help you calculate how many solar panels you need. In reality, there are a number of other factors to consider, including solar panel efficiency, the size of your roof, and your budget. 

Solar panel efficiency

Solar panels vary in quality and efficiency. Not all panels will work the same and it's difficult to give an exact approximation of how many solar panels you’ll need for your home without getting an expert opinion. Generally, the more efficient and high quality the solar panels are, the more wattage they’ll produce and the less you’ll need.  

The size & direction of your roof

Depending on the size and shape of your roof, you may be able to fit more or less solar panels. If you have a large roof, you may be able to sacrifice efficiency and buy larger panels at a lower cost to achieve your target output. If you have a smaller roof area, or it’s partially shaded, then it may be better to invest in a fewer number of more efficient solar panels. 

The direction your roof faces is another consideration and can impact the amount of energy your solar panels generate. South-facing roofs in the northern hemisphere are optimally positioned to get maximum sun exposure, which means you’ll need fewer panels. The angle of your roof can also play a role in how much solar energy your panels harvest. 

This is why consulting with a solar expert is so useful - they can take all these considerations into account to give you an accurate estimate of how many solar panels you need and how much power you can expect to generate. 

Your budget & financial incentives

Budget is a big consideration when it comes to making the switch to solar. You may not always have the amount of cash on hand to cover the number of solar panels you need to invest in. That said, don’t forget to factor in solar financial incentives that can make purchasing solar panels a lot more affordable. 

In the United States, the Federal Solar Investment Tax Credit allows you to take a tax credit off your personal income tax return of 30% of the solar system cost. Depending on your state, you may be eligible for more tax credits or rebates on top of that amount. In Arizona, for example, no sales tax is charged on solar panels and you can receive a state income tax credit of up to $1,000 for a solar purchase. 

Beware of wholesale panels

Some homeowners are tempted to purchase wholesale solar panels as they seem more cost-effective and may allow you to invest in more panels. However, we advise against wholesale panels as they are fraught with risks: 

• Uncertain of quality: You might end up with subpar or damaged panels. They can also lack warranty.

• Difficulty with installation: Many reputable installers refuse to install panels that they didn’t supply, which can complicate the process.

• Pay more later on: Investing in low-quality solar panels might mean you end up paying more later on to have them repaired. 

• Interconnection: It might be significantly more difficult to get approved for interconnection with the grid in your are; especially if not installed by a licensed electrician

It’s always best to source solar panels through certified solar companies that can ensure quality and reliability into the future. 

Summary: How many solar panels do I need?

The above steps should help you understand roughly how many solar panels you’ll need to generate enough power for your average usage. The next step is to consult with a professional solar installer who’ll evaluate your home and provide you with a customized solar solution. 

At Sun Valley Solar, all our systems are fully custom-made to meet your goals. We start every installation with an assessment of your specific energy needs, including analyzing your home, location, and daily energy usage. We'll then design a personal energy system around your needs and provide you with an estimated return on investment (ROI). 

Schedule a no-obligation consultation with Sun Valley Solar by filling in this quote form or giving our friendly team a call on 480-576-5693. As Arizona's most-trusted solar company, we've helped bring solar power to more than 10,000 homeowners and countless businesses. With our help, you’ll enjoy a solar system that's not just fitted to your roof but tailored to your life.