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 available peak sun hours by location in a given area when determining the size and location of a solar installation.
To determine the optimal size and configuration of a solar energy system for a given customer, factors such as peak sun hours by location must be considered, as these directly impact the amount of solar energy that can be harvested and utilized.
Peak Sunlight Hours: An Easy Definition
A peak sun hour or peak sunlight 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 averages 1,000 watts of photovoltaic power per square meter. Typically, the sun's intensity 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 offer incentives for solar homeowners, ranging from equipment rebates to special rate plans for homeowners who produce clean energy. Installers should understand the 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. Electricity providers sometimes pay the full retail value for electricity that homeowners add to the grid.
In locations with fewer sunny days, generous incentives can still make solar panels worth it. Illinois, for example, has a low sun number. However, the state government’s aggressive push for renewable energy can make solar panels a solid proposition for ratepayers.
Regions With the Most Peak Sun Hours
The southwestern United States, including Arizona, California, Nevada, New Mexico, and parts of Texas, have some of the highest peak sunlight hours. These regions benefit from abundant sun intensity 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.
Regions With the Fewest Peak Sunlight 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 sunlight 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.
Peak Sunlight Hours by Location
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 sunlight 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, which is 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:
At What Times Does a Location Experience Peak Sun Hours?
A location generally receives the most solar energy during clear skies when the sun is high in the sky, usually around midday. This is when sun intensity by hour is at its peak, resulting in the most direct and intense sunlight, which maximizes solar irradiance and energy production.
Locations closer to the equator also experience more consistent and intense sunlight throughout the year than those farther away. However, factors such as cloud cover and atmospheric pollution can impact the amount of solar energy received at any given time.
Do Solar System Production Estimates Take Peak Sun Hours Into Account?
Solar installation companies should factor in peak sun hours when estimating solar system output. Many companies use solar software or resource data incorporating peak sun hours and geographic information to make accurate production estimates. For instance, the  PVWatts calculator utilizes average solar radiation data specific to the location to predict solar energy production.
Do Peak Sun Hours Vary by Season?
Peak sunlight hours can vary by season due to changes in the sun's angle and the length of daylight. For instance, summer months typically offer more peak sun hours as the sun is higher in the sky and days are longer, while winter months provide fewer peak sun hours due to lower sun angles and shorter days.
Other Factors That Affect Peak Sunlight Hours
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 in Relation to Peak Sunlight Hours?
Solar radiation refers to the energy emitted by the sun through electromagnetic waves, including ultraviolet (UV), visible light, and infrared (IR) radiation. It is the primary energy source for the Earth’s climate system, driving processes such as photosynthesis, weather patterns, and the water cycle.
Solar radiation is closely tied to peak sunlight hours, which refers to the number of hours during the day when solar irradiance, or the amount of solar radiation received per unit area, is at its highest. These peak sun hours typically occur when the sun is at its highest point in the sky and when there are minimal obstructions like clouds or atmospheric haze, ensuring maximum solar radiation intensity. Understanding peak sunlight hours is essential for optimizing solar energy systems to capture the most energy from the sun.
How Many Solar Hours Per Day Do Solar Panels Need?
Solar panels generate electricity whenever sunlight is available, but their solar output depends on the intensity of sunlight. "Full sun" or "peak sun" conditions occur when sunlight provides around 1,000 watts per square meter. Solar panels can still generate power during lower light conditions, but the output will be less.
To evaluate solar performance, peak sun hours and peak sunlight hours are used to measure the number of hours per day when sunlight intensity is equal to or greater than 1,000 watts per square meter. The number of peak sun hours varies depending on location, weather, and time of year.
On average, a solar panel system might produce significant power during 4 to 6 peak sun hours per day, although this can vary by region and site conditions. Even on cloudy days, solar panels can still generate electricity, but at a lower output than during peak sunlight hours.
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. However, these areas may need more solar panels to meet homeowners' power needs.
While fewer 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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