How Many Solar Panels Do I Need?

Installing a solar energy system to power your house is a long-term investment in both your financial and environmental future. If you want to do it right the first time, you’ll need to crunch some numbers and install enough solar panels to maximize your return.

Assuming you want to install enough solar panels to power your entire home, you first need to understand how much energy your home consumes, the number of peak sun hours in your area, and the power wattage of the panels you choose. From there, you can estimate how many panels you’ll need to power your home.

Factors That Determine How Much Solar Energy You Need

We don’t often think about how much energy we’re using when it’s seemingly free-flowing on demand. Installing your own micro-grid of solar power requires some reflection on your personal energy consumption, coupled with the potential for energy production based on the amount of sunlight in your area.

Before we can calculate the number of panels you need, we need to understand the factors that influence it and some key terms.

Energy Consumption

In 2020, the average American home used 893 kilowatt-hours (kWh) per month, or 30 kWh per day, according to the U.S. Energy Information Administration (EIA). That statistic, however, may be meaningless to you without understanding what a kWh is. To better understand that statistic, it might help to know the definition of a kWh and how it’s different from a kilowatt (kW).

A kilowatt (kW)measures how much power an appliance consumes. A light bulb, for example, may require 100 watts (.1 kW), whereas a clothes dryer could require 2,000 watt (2 kW). A kilowatt-hour (kWh) measures the amount of time (in hours) it takes for an appliance to consume a kW of energy.

So, that 100 W light bulb would require 10 hours to consume one kWh whereas the 2,000 W clothes dryer would only take 30 minutes. To give you perspective, that clothes dryer would have to run for over 18 days straight to consume the national average of 893 kWh consumed per month.

We’ll get into calculations later on in this article once you have all the context.

Amount Of Sunlight Available

You’ll want to know how many kWh your house uses per day because that’s how you’ll measure the potential to replenish that energy by the number of peak sunlight hours your home receives. If you live in a cloudy environment, like Portland, Oregon, you’re going to have fewer peak sunlight hours than a desert city like Palm Springs, California. All that means is that you’ll need more solar panels to capture the same amount of energy.

Solar Panel Choice

The number of solar panels you’ll need will hinge on the quality of those panels, which can be measured in both power rating and efficiency. Most residential solar panels have a power rating range between 250 and 400 watts, though the solar panel size doesn’t vary. For our ongoing example, we’re going to use the average 320 watt panel. As far as quality goes, beware of solar panels with grid lines, which can lower the panels’ overall efficiency and can lead to peeling.

It’s important to note that the average solar panel dimensions are 5.4 feet by 3.25 feet. If the average American home requires 19 to 27 solar panels to fully power their electricity needs, you’d need close to 475 square feet of roof space at the high end of the spectrum.

Also, solar panels weigh an average of 40 pounds each, so it’s important to consider the weight-bearing limits of your roof when selecting a higher- or lower-efficiency panel. Homes with smaller roofs and high-consumption appliances like an electric car or hot tub will likely need to invest in high-efficiency panels to get the most energy possible from the space available.

How To Calculate Your Solar Panel System Needs

As you begin to seriously consider installing a solar power system for your home, you’re likely to consult a professional installer who can accurately estimate your needs. The following exercise is useful if you want to better understand their process or install solar panels yourself.

We’re going to do an example calculation for the average American home, according to EIA data.

Step 1: Determine Your Daily Wattage Requirements

The easiest way to get a snapshot of your energy usage is to take a look at your past utility bills and get an average number of kWh used per day. Because energy consumption varies by season, make sure you’re basing your estimates off seasons when the A/C or heat is consuming the most energy.

In 2020, the average American home consumed about 10,715 kWh per year. For simplicity, let’s round that up to 30 kWh per day (10,715 kWh / 365 days @ 30 kWh/day).

Step 2: Find Your Daily Peak Sunlight Hours

The easiest thing to do now is Google the average daily peak sunlight hours for your area.

The number of panels you’ll need are influenced by the number of peak sunlight hours your area gets per day. Generally, you can bet that Northeastern homes will require more solar panels than Southwestern homes to bring in the same solar power. Areas like Southern California will get around 5 to 6 peak sunlight hours a day, on average, while the New York City metropolitan area may average about 4 hoursa day.

The Global Solar Atlas offers an interactive map with solar energy output data. Most residential solar systems use photovoltaic (PV) solar cells, so look for information on the PV output of kWh per day.

Step 3: Do Some Math

For the sake of our example, let’s say you live in an area of Arizona with 6 peak sun hours per day and plan to install the average 320 W solar panel. When you multiply the number of hours by the wattage of the panel, you’ll find out how many watt-hours (Wh) that panel will produce.

Panel wattage x number of peak hours = watt-hours produced per day

320 watts x 6 peak sun hours = 1,920 Wh per day

We know we want to produce 30 kWh per day, so let’s get that 1,920 Wh figure into the same measurement (kWh).

1,920 Wh / 1,000 = 1.92kWh per day

You can find out how many solar panels you need by dividing your goal kWh production by the predicted kWh production of a single panel. So, if one panel produces 1.92 kWh per day, and you need an average of 30 kWh per day to meet your energy needs, you will need 15.6 panels.

Daily goal kWh / kWh produced by a single panel = Number of solar panels required

30 kWh / 1.92 kWh = 15.6 panels

By this measurement, our sample Arizona home would need 16 solar panels to cover the electrical bill of the average American home. If that same house was placed in the Northeast with a hypothetical 4 hours of sun hours per day, it would take 23 solar panels to produce the same amount of electricity.

The Bottom Line: Solar Panels Are More Than A Numbers Game

Your personal energy use and local sun are the biggest factors in determining how many solar panels you need. With a record of your utility bills and an interactive solar energy output map, you can get a rough calculation.

It’s a good idea to know what size solar energy system you’re expecting to invest in. That said, professional consultants will factor in many more technical variables that affect solar panel efficiency, like the angle of your roof toward the sun.

Check your work or skip it altogether and get an estimate from a Rocket Solar℠ advisor.