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Peak Sun Hours Across the United States for Solar Sizing

Updated 2026-10-21 · Published by Charland & Co. · Based on published specs; we have not tested the products discussed.

Quick answer: The continental US averages about 5.5 peak sun hours per day, with Arizona near 6.5 and Washington near 4. A 200 W solar panel in Arizona yields about 975 Wh per day (200 × 6.5 × 0.75), while the same panel in Washington produces about 600 Wh (200 × 4 × 0.75). Use local data for more accurate solar sizing.

Solar power generation depends on how much sunlight your location receives. This is measured as peak sun hours, the number of hours per day when the sun's intensity is at 1,000 watts per square meter. Knowing peak sun hours helps size solar panels and batteries for weekend cabins, RVs, or vans.

The continental US averages about 5.5 peak sun hours per day, but this varies by region. Arizona, for example, averages 6.5, while Washington averages 4. These differences mean the same panel produces more energy in Arizona than in Washington.

To calculate daily solar energy output, use:

Solar Wh per day = Panel W × Peak Sun Hours × 0.75

The 0.75 factor accounts for real-world losses like efficiency, temperature, and shading. A 200 W panel in Arizona would generate:

200 × 6.5 × 0.75 = 975 Wh per day

In Washington, the same panel would generate:

200 × 4 × 0.75 = 600 Wh per day

These numbers are based on NREL data and should be used as a starting point. Local conditions like tree cover, roof tilt, and panel orientation affect actual output.

What Peak Sun Hours Actually Measure

Peak sun hours measure sunlight intensity, not duration. A location might have 12 hours of daylight but only 5 peak sun hours if the sun is often blocked by clouds or buildings.

Peak sun hours are calculated by dividing total solar irradiance (in kWh/m²/day) by 1,000 W/m². This metric is essential for sizing solar systems because it tells you how much energy your panels can generate in a given location.

Regional Averages from Southwest to Northwest

Peak sun hours vary across the US. Here are some regional averages:

These averages are based on NREL data and should be used as a guide. Local conditions can affect the actual number of peak sun hours you receive.

Winter Versus Summer Differences

Peak sun hours change with the seasons. In the winter, they are generally lower than in the summer due to shorter days and lower sun angles. In Arizona, peak sun hours might drop from 6.5 in the summer to around 5.0 in the winter.

This variation means your solar system may need to be sized for worse-case conditions if you're using it year-round. If you're only using it in the summer, you can size it for higher peak sun hours. If you're using it in the winter, plan for lower output.

How Tilt and Orientation Change the Number

The tilt and orientation of your solar panels affect the number of peak sun hours you receive. Panels should face south in the northern hemisphere to capture the most sunlight. The tilt should be adjusted based on your latitude to maximize energy production.

For example, in Arizona, a panel tilted at 30 degrees (close to the latitude of Phoenix) will capture more sunlight than one tilted at 15 degrees. Panels facing east or west receive less sunlight than those facing south.

Using Local Data Instead of National Averages

While national averages are useful for general planning, they are not always accurate for your specific location. To get the most accurate estimate of peak sun hours, use local data from sources like the NREL Solar Radiation Data Manual or PVWatts Calculator.

These tools allow you to input your zip code or latitude and longitude for a more accurate estimate. This is especially important if you're in a rural area or near a large body of water, which can affect solar irradiance.

Plugging PSH into the Solar Formula

Once you have your peak sun hours, you can plug them into the solar formula to calculate your daily solar energy output. This is essential for sizing your solar panels and battery storage.

For example, if you need 1,500 Wh per day and your location has 5 peak sun hours, you can calculate the panel wattage needed as follows:

Panel W = (Daily Wh Needed) ÷ (Peak Sun Hours × 0.75)

Panel W = (1,500) ÷ (5 × 0.75) = 400 W

This means you would need a 400 W solar panel to generate 1,500 Wh per day in a location with 5 peak sun hours.

Planning for Low-PSH Weekends

If you're planning to use your solar system during the winter or in a low-PSH area, you need to plan for lower energy output. This means you may need to increase the size of your solar panels or add more battery storage to cover your energy needs.

For example, if you're in Washington and need 1,500 Wh per day, you would need a larger solar panel to compensate for the lower peak sun hours:

Panel W = (1,500) ÷ (4 × 0.75) = 500 W

This means you would need a 500 W solar panel to generate 1,500 Wh per day in a location with 4 peak sun hours.

Worked Example

Example 1: Arizona

You're planning a weekend trip to a cabin in Arizona, and you need 2,000 Wh per day of power. Your location has 6.5 peak sun hours.

Using the formula:

Panel W = (2,000) ÷ (6.5 × 0.75) = (2,000) ÷ (4.875) ≈ 409 W

So, you would need a 409 W solar panel to generate 2,000 Wh per day in Arizona.

Example 2: Washington

Now, imagine you're in Washington, and you also need 2,000 Wh per day of power. Your location has 4 peak sun hours.

Panel W = (2,000) ÷ (4 × 0.75) = (2,000) ÷ (3) ≈ 667 W

In this case, you would need a 667 W solar panel to generate 2,000 Wh per day in Washington.

These examples show how peak sun hours directly affect the solar panel size you need to meet your energy needs.

Step-by-Step How to Size Solar Panels Based on Peak Sun Hours

Here's a step-by-step guide to help you size your solar panels based on peak sun hours:

  1. Determine your daily energy needs in watt-hours (Wh). This includes all the devices you plan to power during your trip or stay.
  2. Find the peak sun hours for your location. Use NREL data or the PVWatts Calculator for accurate results.
  3. Apply the solar formula to calculate the required panel wattage:

Panel W = (Daily Wh Needed) ÷ (Peak Sun Hours × 0.75)

  1. Round up the result to the nearest whole number or to a standard panel size available in the market.
  2. Consider adding extra capacity if you're planning for low-PSH conditions, winter use, or unpredictable weather.

Common Mistakes to Avoid

When sizing solar panels based on peak sun hours, it's easy to make a few common mistakes. Here are some to avoid:

Who This Is Not For

This page is not for readers who need grid-tied systems, commercial solar installations, or residential solar systems. It is specifically for weekend and seasonal cabin, camper, and van owners who want to size portable power stations, batteries, and solar panels for off-grid use. If you're looking for large-scale solar installations, consult a professional solar installer.

FAQ

What is the US average peak sun hours?

The continental United States averages about 5.5 peak sun hours per day, according to NREL data.

How much less solar do I get in winter?

In the winter, peak sun hours are generally lower than in the summer due to shorter days and lower sun angles. For example, in Arizona, peak sun hours might drop from 6.5 in the summer to around 5.0 in the winter.

Should I use city data or state averages?

For the most accurate estimate of peak sun hours, use local data from your city or zip code. State averages are useful for general planning but may not reflect the conditions in your specific location.

Can I use the same solar panel size in different locations?

No, the same solar panel size will produce different amounts of energy in different locations due to variations in peak sun hours. You need to adjust the size based on your local conditions.

What if I don't know my peak sun hours?

Use the PVWatts Calculator or NREL Solar Radiation Data Manual to find the peak sun hours for your location. These tools allow you to input your zip code or latitude and longitude for accurate results.

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