What are peak sun hours?

Peak sun hours explained

Peak sun hours, sometimes also called “daily peak sun hours,” “peak irradiance hours” or “solar radiation hours,” occur when the sun is at or near its highest position in the sky. That’s because sunlight can reach your solar panels using the most direct path possible for several hours before and after this zenith.

Across the United States, most areas will receive between four and six peak sun hours each day, usually falling somewhere between 10 a.m. and 4 p.m. As Ty Sauer, a development manager with global renewable energy company the RES Group, explained to us, peak sun hours are ”typically between midday and the afternoon when the sun is angled directly at the solar modules.”

How peak sun hours are measured

Peak sun hours are most often measured by looking at a location’s sun irradiance or intensity.

You don’t necessarily need to know all of the technical details of solar power production to benefit from solar panels, but if you’re curious, just know that the strength of the sunlight is recorded in terms of watts (or kilowatts) per square meter. One peak sun hour is equal to 1000 W/m² (or 1 kW/m²) of sunlight per hour.

Why peak sun hours matter

Peak sun hours are critical when designing a solar panel system because they are directly related to the amount of potential solar power that can be generated in any given location.

Simply put, with more peak sun hours each day, your system can generate a larger amount of electricity, offsetting more of your utility power consumption and saving you more money.

How to estimate your peak sun hours

There are a few different ways to estimate how many peak sunlight hours your property receives, depending on how accurate you want your estimate to be. We’ve included instructions for three different methods below.

Check the average peak sun hours in your state

Looking at the average peak sunlight hours in your state should give you a decent idea of your home’s solar potential, but keep in mind that estimates for larger states may be less relevant to your individual property since they have so much extra ground to cover.

While states that share a border with Canada fall on the lower end of the spectrum, states in the Southwest tend to receive the highest number of daily peak sun hours. For example, Arizona, New Mexico and California have the most solar potential in the continental U.S., while Alaska, Michigan and Washington receive some of the lowest amounts of peak sun hours in the country.

» MORE: Where your solar savings go the furthest

Check the average peak sun hours in a nearby city

If you live in one of North America’s major metropolitan areas, your property will likely receive between 3.5 and 7 hours of peak sunlight on any given day of the year. Here are the year-round daily peak sun hour averages for some of the United States’ most notoriously sunny (and not so sunny) cities.

Look up your property’s peak sun hours online

For a precise measurement of your property’s average peak sun hours, the National Renewable Energy Laboratory’s PVWatts calculator is a great, free resource with highly accurate solar irradiance data for both residential and commercial properties all across the United States.

To use this tool, simply enter your address (or city) into the calculator and click “Go.” After reviewing the resource data to confirm you’ve input the correct information, the “Results” tab will display your annual average solar radiation per day, as well as your peak sun hours month-to-month throughout the entire year.

For additional insight into how your solar panels may perform, you can also add your system’s total wattage to the table on the “System Info” tab of the PVWatts calculator. This will let you see your approximate monthly and annual kilowatt-hour (kWh) production in the “Results” tab.

» CALCULATE: How many kWh does a solar panel produce?

How many peak sun hours do you need to go solar?

There is no set number of peak sun hours necessary for you to benefit from a solar energy installation. However, you or your solar installer will need to consider your property’s solar potential when designing your system and deciding how large it needs to be.

Because peak sun hours vary, the exact same solar panel system can yield dramatically different results in different locations. For example, a six-kilowatt (kW) solar array will produce more energy in Phoenix than it will in Washington, D.C., which also means people in Phoenix can meet similar energy needs with smaller, cheaper systems.

Once you know how much energy your home uses and how many peak sun hours you can expect each day, you can calculate how large your solar energy system needs to be to achieve your goals.

» LEARN: How many solar panels do I need for my house?

To find out more about peak sun hours and other aspects of your solar installation, consider working with a reputable solar energy company. They should be able to answer any questions you have about peak sun hours, energy savings and the process of going solar before you commit to a purchase, and many even offer site assessments to help you understand what you can expect from your system once it’s installed.

(If you want to see how accurate these estimates can be, check out this review from Jack, a ConsumerAffairs reviewer from Arizona, who calculated that SunPower’s peak-sun-hour estimate was only about 8% off after a year of production.)