Peak sun hours measure sunlight intensity, which is key for solar power. See if your home gets enough light to make solar panels worth it.
Despite what your TV listing says, it isn't always sunny in Philadelphia -- or anywhere else. That means your solar panels won't always be soaking up the sun's rays.
Solar panels are rapidly growing in popularity as people try to offset rising energy costs and take advantage of new tax breaks. But whether going solar makes financial sense depends on more than tax incentives. How much sunlight your solar panels receive plays a huge role in how much electricity the panels can generate.
That's why the optimal sunlight your location receives daily is an important metric to understand before going solar. Peak sunlight hours (commonly called peak sun hours) is a standardized measure used to compare sunlight intensity across different locations.
"With solar energy, it's crucial to know what peak sun hours are and what they're not," said Rohit Kalyanpur, CEO of Optivolt, a Silicon Valley-based solar technology company. "Peak sun hours are not the hours between sunrise and sunset. Instead, it refers to measuring the intensity of sunlight at a given location."
Understanding this concept will help you make a good decision when deciding if a solar energy system will meet your needs. In addition, you'll be able to confidently assess any solar system's efficiency, cost savings, and environmental impact.
A handy definition of a peak sun hour is a one-hour period during which sunlight (solar irradiance) generates 1,000 watts (equivalent to 1 kilowatt) of energy per square meter of surface area. For example, if your home receives 500 W/m² of solar irradiance between 8 a.m. and 10 a.m., those two hours combined would be considered one peak sun hour.
If that location receives 5,000 watt-hours per square meter (Wh/m²) over the course of a 12-hour day, those 5,000 Wh/m² equals five peak sun hours. It's important to remember that just because there are 10 hours of sunlight in a day does not mean that there are 10 peak sun hours. The sun's intensity changes throughout the day for various reasons, and these changes affect the performance of solar panels.
We can simplify the definition of a peak sun hour to a precise equation:
Peak Sun Hours (PSH) = Solar Irradiance (kW/m²) × Duration (h)
In the case of a "peak sun hour," the solar irradiance is defined explicitly as 1 kW/m², and the duration is defined as one hour. Therefore, the mathematical expression for a peak sun hour becomes:
This means that during a peak sun hour, an area of one square meter receives 1,000 watt-hours (or 1 kilowatt-hour) of solar energy.
Now that you've learned all about peak sun hours, how would you calculate the peak sun hours in your neighborhood to determine if investing in a solar system is worthwhile?
There are several online tools available that can help you calculate the peak sun hours at your home, including the National Renewable Energy Laboratory's PVWatts Calculator and the European Commission's Photovoltaic Geographical Information System (PVGIS).
Another tool is the World Bank Group's Global Solar Atlas portal, which uses a different terminology -- Global Horizontal Irradiation (GHI) -- to describe peak sun hours. GHI measures the total amount of solar radiation absorbed on a horizontal surface over a specific time. The unit of measuring GHI is in kilowatt-hours per square meter (kWh/m²).
"If your home receives an average of four peak sun hours per day, then you're a good candidate for a solar system," Kalyanpur said.
Those online tools can help you tell if a solar system is a good investment, but there are other factors to consider before making your decision, including:
Peak sun hours vary widely from state to state. For example, Nevada receives 4.6 to 5.8 peak sun hours, making it one of the best locations in the US for owning a solar system. Conversely, Washington receives 2.8 to 4.4 peak sun hours, making it one of the worst locations in the US for owning a solar system. However, even though Washington gets less sunlight, owning a solar system could still be beneficial.
Other factors to consider include incentives, tax credits and rebates available to residents who invest in solar systems.
Here's a table for the average daily peak sun hours by state, according to the Global Solar Atlas portal:
|State||Peak sun hours||State||Peak sun hours|
|Alabama||4.4 - 5||Montana||3.4 - 4.4|
|Alaska||2 - 3||Nebraska||4.1 - 4.7|
|Arizona||5 - 6||Nevada||4.6 - 5.8|
|Arkansas||4.4 - 4.7||New Hampshire||3.4 - 3.9|
|California||4.1 - 6||New Jersey||3.9 - 4.3|
|Colorado||4.2 - 5.4||New Mexico||5 - 6|
|Connecticut||3.9 - 4.1||New York||3.5 - 4.1|
|Delaware||4.1 - 4.3||North Carolina||4.2 - 4.7|
|Florida||4.8 - 5.4||North Dakota||3.7 - 4.1|
|Georgia||4.4 - 4.9||Ohio||3.7 - 4.1|
|Hawaii||3.5 - 6.6||Oklahoma||4.5 - 5.3|
|Idaho||3.4 - 4.8||Oregon||3.3 - 4.8|
|Illinois||3.9 - 4.4||Pennsylvania||3.6 - 4.1|
|Indiana||3.9 - 4.3||Rhode Island||3.9 - 4.1|
|Iowa||3.9 - 4.3||South Carolina||4.5 - 4.9|
|Kansas||4.3 - 5.1||South Dakota||3.9 - 4.5|
|Kentucky||4 - 4.4||Tennessee||4.1 - 4.5|
|Louisiana||4.6 - 5.1||Texas||4.6 - 5.9|
|Maine||3.4 - 3.9||Utah||4.4 - 5.5|
|Maryland||3.8 - 4.4||Vermont||3.4 - 3.8|
|Massachusetts||3.7 - 4.1||Virginia||4 - 4.5|
|Michigan||3.5 - 3.9||Washington||2.8 - 4.4|
|Minnesota||3.5 - 4||West Virginia||3.8 - 4.2|
|Mississippi||4.5 - 4.9||Wisconsin||3.6 - 4|
|Missouri||4.2 - 4.5||Wyoming||3.9 - 4.9|