By Sam Wasson
Updated Dec 2, 2022
Solar energy is becoming more popular each year, but for the uninitiated, there can be a steep learning curve in understanding it. For example, the simple question of “how many solar panels do I need to power my house?” has a frustratingly complicated answer. Between calculating production ratios and finding the annual electricity your home uses, it’s easy to get overwhelmed by all the math. In this article, we aim to help by going over everything you need to know when calculating the number of solar panels for your home.
First, we want to establish that everything in this guide will be for estimating the number of solar panels needed to power your home. You should schedule an appointment with a solar company to speak with a professional installer. These industry pros can determine how much power your home needs from your monthly electric bill and recommend the right panels.
To estimate the number of solar panels needed to power your home, you need to use the following equation: Number of Panels = System Size / Production Ratio / Panel Wattage. For this equation to be useful, you must first understand what these variables mean and know how to find them.
System size represents the number of solar panels needed to fulfill your home’s energy needs over one year. The size of your solar panel system directly reflects the power your household consumes in one year, measured in kilowatt-hours.
Kilowatt-hours are units of measurement that represent how much power a system uses. Specifically, it measures how long it takes an appliance or system to consume 1,000 kilowatts. For example, a 200-watt electric blanket would take five hours to consume a kilowatt-hour of electricity, and a 2,000-watt air conditioner would only take half an hour to consume a kilowatt-hour.
So, to know the appropriate size of your solar panel system, you’ll need to know how many kilowatt-hours of electricity your home consumes. This number will represent the total electricity consumed by all household appliances and systems, from each light bulb to your air conditioning system and beyond. In 2020, the average American home consumed 10,715 kilowatt-hours of power, with an average of 893 kilowatt-hours per month.
Thankfully, finding the number of kilowatt-hours your household consumes is pretty easy. It’s typically listed in your electric bill, presented by a number denoted with kWh or kW h. Occasionally, some utility bills do not list your yearly energy usage in kilowatt-hours. When this happens, you can get a rough estimate by adding up the electricity usage of each appliance and system with an online calculator. You can also purchase an at-home energy monitoring system that will give you a monthly energy consumption total.
We will use 2020’s average household energy consumption of 10,715 for a home’s power requirements and needed system size.
Solar panel wattage, also called power rating, is how efficiently your solar panels absorb sunlight under ideal conditions, with an output measured in watts. Basically, it’s a numerical value representing how good your solar panels are at absorbing sunlight and producing power. Commercially available solar panels have a wattage ranging between 170 to 400 watts, with the national average wattage ranging from 250 to 400 watts. For our purposes, we will use solar panels rated at 325 watts.
A solar system’s production ratio represents the ratio of the estimated energy output of a system over time (measured in kilowatt-hours) to the total system size (measured in watts). In layman’s terms, this represents a system’s estimated actual power output accounting for variables like nighttime, overcast days, and winter months. Summer locations with longer days and shorter winter months will have a better production ratio, producing more power. Regions like the Northeastern portion of the U.S., such as New York, have more overcast days and long winters and will have a lower ratio representing less overall power produced from a system.
To determine your production ratio, you must divide the system’s size (capacity in kilowatts) by its total output over time (output in kilowatt-hours). For example, a 10-kilowatt system that outputs 16 kilowatt-hours of power over a year has a production ratio of 1.6. This ratio is on the higher side, representing a solar system located somewhere like Hawaii or Arizona. However, a 15-kilowatt system that outputs only 14 kilowatt-hours of power over a year would only have a production level of 1.07, representing a more overcast area like Maine. The average production ratio of American solar panel systems ranges from 1.3 to 1.7, and we will use an average of 1.5 for our purposes.
Now that we’ve explained the elements of the equation, let’s put it into practice. Assuming we have a typical house with a system size of 10,715 kilowatt-hours, located in a location granting us a production ratio of 1.5, with panels rated at 325 watts, we get the following equation:
Number of solar panels = 10,715 / 1.5 / 325, giving us an estimated 22 (rounded up) solar panels. Coincidentally, the number of solar panels needed to power most homes in America ranges from 20 to 24, depending on region and, therefore, production ratio.
When estimating the number of solar panels needed to power your home, keep in mind the following key factors:
The amount of sunlight your home is exposed to is the largest additional factor to consider when calculating the size of your solar panel system. The more sunlight you have, the fewer solar panels you’ll need. If you live in geographic locations with fewer sunny days, you’ll need more solar panels or higher efficiency panels to compensate.
Solar panel wattage, or how efficient your solar panels are, will determine how many you need to power your home. For example, if we take the equation we used to calculate your total panels needed for a home with a capacity of 10,715 kilowatt-hours and instead use a 175-watt panel, you’ll need 40 panels to reach the same amount of electricity for one year.
There are several kinds of solar panels, each with specific benefits, costs, and downsides. When signing up with a solar panel company, you can usually choose from one of these three common types of solar panels:
These are the most expensive, efficient, and difficult-to-produce solar panels. They’re created from a single sheet of crystalline silicon, have a dark black appearance, are more efficient than other panels, and have a higher wattage.
These solar panels are created from shards of silicone crystal that have been melted and fused together. These panels have a lower wattage and are less efficient than monocrystalline but are much less expensive. While you can save money with these panels, you may need more of them to fulfill your home’s electrical capacity.
There are several thin-film panels to choose from, each made from different materials. Most solar companies offer one or more of the following: cadmium telluride, copper indium gallium selenide, or amorphous silicon. The exact cost of thin-film panels will depend on their material, but on average, they cost less than monocrystalline due to their reduced installation labor cost. These panels have a lower wattage and efficiency than standard panels but are a viable option due to their lightweight and malleable nature. Thin-film panels are usually the best option for homes with odd-shaped roofs or older homes whose roofs cannot bear the weight of traditional panels.
The larger your roof and the better its total weight capacity, the more panels you’ll be able to install. Smaller roofs or older homes with poorly reinforced roof supports will not be able to handle the additional weight of the solar panel system.
Figuring out how to calculate the total number of solar panels needed to power a home is like going back to school, as you need to learn a little vocabulary and do some basic math. However, with the information presented in this article, you can easily estimate the number of panels needed for your solar energy system. We still recommend talking with a professional and getting an official quote before making any major decisions regarding your installation.
Yes, while difficult, time-consuming, and expensive, it’s possible to use residential solar panels to go completely off-grid.
While solar panel technology has come a long way in the last decade, it’s still an expensive technology. As of writing this article, the average cost of solar panels in the U.S. is around $16,000. This steep upfront cost can be a barrier to entry for many homeowners, and thankfully there are plenty of federal and state incentive programs to help offset the cost.
Most photovoltaic solar panels last about 25 years.
While solar panels are less efficient during the winter, they will still gather sunlight and produce electricity. Solar panels are, on average, 40% to 60% less effective during winter than in summer. This lack of efficiency is not due to cold weather or snow but shorter days resulting in fewer sun hours and shorter peak sunlight hours.
Yes, the larger the square feet of a home, the more electricity it will need. Larger homes have higher electricity needs, mostly from their heating and cooling systems. Thankfully, larger homes also have more roof space, allowing for larger solar arrays.
If the energy production of your solar panels exceeds your home’s electricity consumption, one of two things will happen to the excess power. First, it can be sold back to a local utility company (a process called net metering) and sent into the electric grid. When resold to a utility company, you get solar credits. These credits can then reduce the cost of future electricity bills if your solar system does not produce enough energy to cover your home’s needs. Second, you can store it in a solar battery for later use.
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