You heard correctly: peak sunlight hours (or just peak sun hours for short) are in fact an important number, one that determines if solar panels will be worth it for you. Peak sun hours are an easy way to measure and compare the amount of sunlight between locations.
But what exactly does the term mean? What are peak sun hour numbers like in your state? And most importantly, what are the peak sun hours for the roof of your specific home? We’ll answer all these questions in this blog.
A peak sun hour is defined as one hour in which the intensity of solar irradiance (sunlight) reaches an average of 1,000 watts (W) of energy per square meter (roughly 10.5 feet).
Another way to put it: A peak sun hour is the equivalent of 1000 W/m² of sunlight for an hour.
A peak sun hour represents a lot of sunlight. Solar panels are only likely to receive around that much sunlight when facing directly towards the sun when the sun is at its strongest, at midday.
That amount of sunlight – 1000 W/m² over an hour – also happens to be the exact amount of sunlight used to test and rate solar panels in the lab. That means that over the course of a peak sun hour, a solar panel should be producing – before system losses due to temperature and other factors – at close to its specified output rating.
In other words, before system losses, during a peak sun hour, you can expect a 300-watt solar panel to produce roughly 300-watt–hours of electricity, and a 6-kilowatt system to produce roughly 6 kilowatt–hours of electricity.
Now that we know what a peak sun hour represents, let’s look at how it’s used to measure sunlight for a particular location.
The sun, as we know, doesn’t shine at peak sun hour-intensity all day long. Instead, the intensity of sunlight hitting the panels constantly fluctuates depending on the time of day, as well as the weather.
During the early mornings and late afternoon, sunlight is likely to be less than 500 W/m². Conversely, during ideal conditions - midday during a clear, sunny day - you might actually receive more than 1000 W/m², say 1,100 W/m².
No worries, we can simply express these amounts in terms of peak sun hours. For example:
It may sound complicated, but the concept is actually relatively simple to apply. For example, if a given location receives a total of 6,650 Wh/m² of solar radiation over the course of a day, then that location gets 6.65 peak sun hours.
As the image shows, adding up the total solar irradiance for a given location gives you the total peak sun hours value.
That is why the concept of 'peak sun hours' has been developed. It allows you to precisely measure the amount of irradiance (sunlight) that will hit solar panels installed in a given location. This, in turn, allows you to calculate the expected energy production for a given solar system size installed at that location.
In other words, peak sun hours tell you how much power a solar installation on your roof will generate. They also allow you to compare sunlight availability between locations.
In the United States, any place that gets 4 peak sun hours or more is considered a good location to produce useful amounts of solar energy.
However, even if you get fewer peak sun hours, solar might still be worth it for you. There are several other key factors that affect the viability of home solar, such as electricity rates and the availability of incentives on the federal level (like the 30% solar tax credit) and state level.
For instance, people living in locations with below-average peak sun hours but expensive utility rates and net metering stand to save a lot of money by going solar.
New York is an excellent example. The state only receives an average of 3.0-3.5 peak sun hours per day. However, the availability of terrific solar incentives means that even with less energy production, the average solar payback period for homeowners there is just 2.8-3.4 years - a level of return that’s hard to beat.
Summer months and locations farther south will generally see more peak sun hours than wintry times and areas farther north. That's because regions closer to the equator receive more direct sunlight.
On the other hand, in latitudes farther north, when the sun is closer to the horizon, the sunlight is filtered through more layers of the atmosphere. In those places, the sunlight isn’t as strong by the time it reaches your solar panels, which results in lower peak sun hours.
Let’s see how seasonal and latitudinal variations play out in two different cities - one southern one (Albuquerque, NM) and one northern one (Chicago, IL).
Located in the southwest, Albuquerque gets plenty of sunshine! There are only three months - November through January - where it receives less than four peak sun hours per day on average. Data source: PVEducation.org
Chicago, located at a more northerly latitude, receives less sunlight, especially during winter. It receives less than four peak sun hours per day for half the year. Data source: PVEducation.org
Of course, it’s not just about latitude; weather patterns play a key role in determining peak sun hours, as well. States in the southwest - where climate conditions create many cloud-free days - experience more peak sun hours than locations further south, such as Florida.
Peak sun hours also vary depending on where your solar panels are placed.
The peak sun hours' value for one part of your roof may be substantially different from the peak sun hours on another section of your roof. These factors can affect the number of peak sun hours your panels get.
