Peak sun hours are an important factor for homeowners who want to install a solar energy system. The efficiency of solar panels is directly influenced by the amount of solar irradiance they receive. Therefore, solar installers need to consider the peak sun hours available in a given area when determining the size and location of a solar installation.
Let’s take a look at what you need to know about peak sun hours so you can explain the concept and its importance to prospective clients.
Peak Sunlight Hours: An Easy Definition
A peak sun hour isn’t a measure of time or how many hours of sunlight a location receives. It’s a measurement of the amount of sunlight that falls on a given area – a concept also known as solar irradiance or solar insolation.
A peak sun hour is when the sun’s intensity is an average of 1,000 watts of photovoltaic power per square meter. Typically, the intensity of sunlight is greatest in the middle of the day. That’s also when solar panels receive the most direct sunlight.
Illinois, for example, averages 3 – 4 peak sun hours per day. During those hours, solar panels will receive close to 1,000 watts of solar energy per square meter. Texas averages 4.5 – 6 peak sun hours per day, so a solar array in Austin could produce more energy than the same-sized system in Chicago.
Why Peak Sun Hours Are Important for Solar Energy
Knowing the peak sun hours of your service area can help you determine whether solar panels will offer a good return on investment for a client. That’s just one piece of the puzzle, though. A few other factors you’ll want to discuss with clients include:
Government incentives such as the federal solar tax credit: Incentives can reduce the cost of a solar panel system, potentially shortening the payback period. We suggest installers advise clients to consult a tax professional to see if they qualify for certain incentives.
Rebates from a utility company: Utility companies may have incentives for solar homeowners, and these incentives can range from rebates on equipment to special rate plans for homeowners who produce clean energy. Installers should have a good understanding of offers available from utility companies in their area.
Utility company net metering policies: Some utility companies have policies that compensate customers for the energy they generate. In some cases, electricity providers may pay the full retail value for electricity that homeowners add to the grid.
In locations that don’t have as many sunny days as Arizona, generous incentives can make solar panels still worth it. Illinois, for example, has a low sun number. But the state government’s aggressive push for renewable energy can make solar panels a solid proposition for ratepayers.
What Regions of the U.S. Have the Most Peak Sun Hours?
The southwestern United States, including Arizona, California, Nevada, New Mexico, and parts of Texas, experiences some of the highest peak sun hours. These regions benefit from abundant sunlight and favorable weather conditions, making them ideal for solar energy production. The combination of high solar insolation and clear skies contributes to the optimal performance of solar installations in these areas.
What Regions of the U.S. Have the Fewest Peak Sun Hours?
Regions with the fewest peak sun hours in the United States are typically found in the Pacific Northwest and parts of the Midwest and Northeast. States like Washington, Oregon, Alaska, Maine, New Hampshire, and parts of the Great Lakes region may experience fewer hours of direct sunlight due to higher latitudes, more frequent cloud cover, and varying weather patterns. Therefore, solar panels in these states will not receive as much sunlight as systems in sunnier climates.
How to Calculate Peak Sun Hours
Peak sun hours are not simply the number of daylight hours. They are the equivalent number of hours when the sun is shining at its maximum intensity. The formula for calculating peak sun hours is:
Peak Sun Hours = Total Daily Solar Insolation/Solar Noon Duration
Find the average daily solar insolation (sunlight intensity) for your location. This information is often available from solar resource maps or databases.
Solar noon is the time when the sun is at its highest point in the sky.
What Are The Peak Sun Hours In Different States?
Some clients may ask you how many peak sun hours they get. Peak sun hours vary widely by location due to latitude and climate.
STATE | AVERAGE PEAK SUN HOURS |
Arizona | 7-8 |
California | 5-7.5 |
Colorado | 5-6.5 |
Florida | 3.5-4.5 |
Georgia | 4-4.5 |
Illinois | 3-4 |
Maryland | 3-4 |
Massachusetts | 2.5-3.5 |
Michigan | 2.5-3.5 |
Missouri | 4-4.5 |
Nevada | 6-7.5 |
New Jersey | 3.5-4 |
North Carolina | 4-4.5 |
Ohio | 2.5-3.5 |
Pennsylvania | 2.5-3.5 |
Rhode Island | 3-4 |
South Carolina | 4-4.5 |
Texas | 4.5-6 |
Utah | 6-7 |
Virginia | 3.5-4 |
A few factors, including latitude and weather conditions, can impact how many peak sun-hours an area receives. If you look at a map of peak sun hours in the U.S., you’ll notice that the areas with the highest peak sun hours are in the Southwest. The desert areas have clearer skies than areas like Florida, that are further south, but they often have cloudier conditions.
Peak sun hours will also vary by season. In the summer months, when the sun stays up longer, there will be more peak sun hours than the shorter periods of daylight in the winter months. For example, Austin, Texas, ranges from an average of 2.73 peak sun hours in December to 6.77 peak sun hours in July.
Here’s a sample of peak sun hours at locations throughout the country from solar company Palmetto:
When Does a Location Receive the Most Solar Energy?
A location typically receives the most solar energy during periods of clear skies when the sun is high in the sky, which typically occurs around midday. This is when the sun's rays are most direct, leading to maximum solar irradiance and energy production. Additionally, locations closer to the equator tend to receive more consistent and intense sunlight throughout the year compared to those farther away. However, weather conditions, such as cloud cover and atmospheric pollution, can also affect the amount of solar energy received at any given time.
Do Solar System Production Estimates Take Peak Sun Hours Into Account?
Yes, solar installation companies should use peak sun hour data when estimating solar system output. Many solar companies use solar software or solar resource data when estimating solar production and output, both of which factor in peak sun hours and the geographic information for a potential solar system. The PVWatts calculator uses the average solar radiation data specific to the location to estimate solar energy production.
Other Factors That Affect Average Daily Peak Sun Time
A solar panel system should operate at its peak output rating in lab conditions. In real-world conditions, the system will have some output loss caused by high temperatures, among other factors. Other factors that impact how efficient and productive a solar system can be include:
Roof orientation: South-facing roofs typically have the most optimum angle for sunlight.
Shading: Properties with large trees or buildings obstructing the roof can block some sunlight from reaching a photovoltaic system.
Time of day: When the sun is low in the sky, sunlight has to filter through more of the atmosphere. This reduces the intensity of solar radiation.
What is Solar Radiation?
Solar radiation refers to the energy emitted by the sun in the form of electromagnetic waves. It encompasses a broad spectrum of wavelengths, including ultraviolet (UV), visible light, and infrared (IR) radiation. Solar radiation is the primary source of energy for the Earth's climate system and drives processes such as photosynthesis, weather patterns, and the water cycle.
Solar radiation is directly related to peak sun hours, as peak sun hours refer to the number of hours in a day when solar irradiance (the amount of solar radiation received per unit area) is at its highest level. During peak sun hours, the intensity of solar radiation is maximized, typically occurring when the sun is at its highest position in the sky and when there are minimal obstructions such as clouds or atmospheric haze.
How Many Hours of Sunlight Do Solar Panels Need?
Solar panels can generate electricity whenever there is sunlight, but the solar output depends on the intensity of sunlight. "Full sun" or "peak sun" conditions are considered to be around 1,000 watts of sunlight per square meter. However, solar panels can still generate power under lower light conditions.
In terms of hours, solar panels are often rated based on "peak sun hours," which represent the number of hours during a day when sunlight intensity is equivalent to or greater than 1,000 watts per square meter. The actual number of peak sun hours can vary based on geographic location, weather patterns, and other factors.
As a rough estimate, a typical solar panel system might generate significant power during 4 to 6 peak sun hours per day on average. This can vary depending on the region and specific solar site conditions. Keep in mind that solar panels can still produce some electricity during cloudy or overcast days, but the output will be lower compared to bright, sunny days.
Peak Sun Hours Help Determine The Number Of Solar Panels
Solar power systems are viable nearly anywhere in the United States, even in areas with fewer peak sun hours. These areas may need more solar panels to meet a homeowner’s power needs, though.
While a lower number of peak sun hours may increase the payback period of a PV array, homeowners will still reduce their carbon footprint, lower their electric bill, and raise the value of their homes.
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