Your peak sun hours number is the single most location-specific input in any solar sizing calculation. Get it wrong and your panels are either undersized — leaving batteries empty every cloudy week — or needlessly oversized, wasting thousands of dollars. This guide gives you a lookup table for every state, explains what the number actually means, and shows you exactly where it enters the sizing formula.
For the core definition and a shorter overview, see Peak Sun Hours Explained. For a full off-grid system estimate once you have your PSH, use the WattSizing Calculator.
Quick Answer: What Are Peak Sun Hours?
Most of the continental US receives between 3.5 and 6.5 peak sun hours per day on an annual average. The desert Southwest (Arizona, Nevada, New Mexico) tops out around 6.5–7.5. The Pacific Northwest and northern Great Lakes drop to 3.0–4.2. Coastal and mid-Atlantic states typically land around 4.2–4.7.
A peak sun hour is not the number of hours of daylight. It is the number of equivalent hours in a day when solar irradiance averages 1,000 W/m² (the standard test condition for solar panels). Every watt-hour your load needs must be replenished within your available sun window. A system designed for Phoenix needs roughly 40% fewer panels than an identical load system in Seattle.
The National Solar Context: Who Measures This Data?
In the United States, solar irradiance data is rigorously tracked and modeled by the National Renewable Energy Laboratory (NREL), a division of the Department of Energy. They maintain the National Solar Radiation Database (NSRDB), which uses satellite imagery and ground stations to calculate how much sunlight hits any given square meter of the country.
This data is the gold standard for both residential and utility-scale solar planning. When you look up a zip code in NREL's PVWatts calculator, you are accessing decades of historical weather and irradiance data. This ensures that your system sizing accounts for regional cloud cover, atmospheric thickness, and seasonal variations, rather than just raw latitude.
Peak Sun Hours by State — Full Reference Table
Annual averages based on Global Horizontal Irradiance (GHI) data from NREL. Use the worst-month column when sizing an off-grid system that must survive winter.
| State | Avg Daily PSH | Avg Monthly PSH | Avg Annual PSH | Typical Worst Month |
|---|---|---|---|---|
| Alabama | 4.95 | 148 | 1,807 | Dec: ~3.2 |
| Alaska | 2.10 | 63 | 767 | Nov–Jan: ~0.5–1.0 |
| Arizona | 7.50 | 225 | 2,738 | Dec: ~5.5 |
| Arkansas | 4.95 | 148 | 1,807 | Dec: ~3.2 |
| California | 6.25 | 188 | 2,281 | Dec: ~3.8 (coast) |
| Colorado | 6.00 | 180 | 2,190 | Dec: ~4.5 |
| Connecticut | 4.70 | 141 | 1,716 | Dec: ~2.5 |
| Delaware | 4.70 | 141 | 1,716 | Dec: ~2.7 |
| Florida | 5.45 | 164 | 1,989 | Dec: ~4.0 |
| Georgia | 4.95 | 148 | 1,807 | Dec: ~3.1 |
| Hawaii | 5.75 | 173 | 2,099 | Dec: ~4.5 |
| Idaho | 5.00 | 150 | 1,825 | Dec: ~2.8 |
| Illinois | 4.55 | 137 | 1,661 | Dec: ~2.4 |
| Indiana | 4.20 | 126 | 1,533 | Dec: ~2.1 |
| Iowa | 4.20 | 126 | 1,533 | Dec: ~2.3 |
| Kansas | 5.70 | 171 | 2,081 | Dec: ~3.8 |
| Kentucky | 4.45 | 134 | 1,624 | Dec: ~2.5 |
| Louisiana | 4.95 | 148 | 1,807 | Dec: ~3.3 |
| Maine | 4.20 | 126 | 1,533 | Dec: ~2.0 |
| Maryland | 4.70 | 141 | 1,716 | Dec: ~2.7 |
| Massachusetts | 4.70 | 141 | 1,716 | Dec: ~2.5 |
| Michigan | 4.20 | 126 | 1,533 | Dec: ~1.8 |
| Minnesota | 4.40 | 132 | 1,606 | Dec: ~2.0 |
| Mississippi | 4.95 | 148 | 1,807 | Dec: ~3.2 |
| Missouri | 4.95 | 148 | 1,807 | Dec: ~2.9 |
| Montana | 4.95 | 148 | 1,807 | Dec: ~2.5 |
| Nebraska | 5.45 | 164 | 1,989 | Dec: ~3.5 |
| Nevada | 6.75 | 203 | 2,464 | Dec: ~4.5 |
| New Hampshire | 4.25 | 128 | 1,551 | Dec: ~2.0 |
| New Jersey | 4.25 | 128 | 1,551 | Dec: ~2.6 |
| New Mexico | 6.75 | 203 | 2,464 | Dec: ~5.2 |
| New York | 4.25 | 128 | 1,551 | Dec: ~2.0 |
| North Carolina | 4.95 | 148 | 1,807 | Dec: ~3.2 |
| North Dakota | 4.95 | 148 | 1,807 | Dec: ~2.4 |
| Ohio | 4.20 | 126 | 1,533 | Dec: ~2.0 |
| Oklahoma | 5.70 | 171 | 2,081 | Dec: ~3.6 |
| Oregon | 4.95 | 148 | 1,807 | Dec: ~1.8 |
| Pennsylvania | 4.45 | 134 | 1,624 | Dec: ~2.3 |
| Rhode Island | 4.70 | 141 | 1,716 | Dec: ~2.5 |
| South Carolina | 4.95 | 148 | 1,807 | Dec: ~3.3 |
| South Dakota | 5.20 | 156 | 1,898 | Dec: ~3.0 |
| Tennessee | 4.70 | 141 | 1,716 | Dec: ~2.9 |
| Texas | 5.75 | 173 | 2,099 | Dec: ~3.8 (north) |
| Utah | 6.20 | 186 | 2,263 | Dec: ~4.2 |
| Vermont | 4.20 | 126 | 1,533 | Dec: ~1.9 |
| Virginia | 4.45 | 134 | 1,624 | Dec: ~2.8 |
| Washington | 3.75 | 113 | 1,369 | Dec: ~1.2 |
| West Virginia | 4.20 | 126 | 1,533 | Dec: ~2.1 |
| Wisconsin | 4.45 | 134 | 1,624 | Dec: ~2.0 |
| Wyoming | 5.45 | 164 | 1,989 | Dec: ~3.8 |
Off-grid sizing rule: Always size your array and battery bank using the worst-month figure, not the annual average. An annual average of 5.0 PSH may conceal a December value of 2.8 PSH — that is your design constraint.
Crucial Factors Often Overlooked in Solar Sizing
When planning a solar array, many basic guides tell you to look up your state's average and divide your daily usage by that number. But this simplified approach misses several critical realities of off-grid and hybrid solar design:
1. The DNI vs. GHI vs. GTI Trap
Many DIYers accidentally use Direct Normal Irradiance (DNI) instead of Global Horizontal Irradiance (GHI) or Global Tilted Irradiance (GTI). DNI measures only the direct beam of sunlight, which is highly relevant for tracking solar arrays but overstates the usable energy for fixed, flat panels by 15–30% in many climates. Always use GHI for baseline comparisons, and GTI (which PVWatts calculates based on your tilt) for actual sizing.
2. Micro-Climates Matter More Than State Averages
California has a state average of 6.25 PSH, but if you live in coastal San Francisco, summer fog drops your actual average to around 4.9 PSH. Similarly, elevation changes in Colorado or Oregon can drastically alter your local irradiance compared to a city just 50 miles away. You must use your exact zip code.
3. The Winter Tilt Advantage
A panel lying flat on a low-pitch roof in Seattle may see only 3.0 PSH. But if you tilt that same panel to 47° (Seattle's latitude), it sees 3.8 PSH. That is a 27% increase in winter production, which can mean the difference between keeping the lights on and running a gas generator all December.
Worked Cost & Sizing Example (Illustrative)
Let’s look at a realistic, step-by-step sizing calculation for an off-grid cabin.
Scenario:
- Location: Portland, Oregon (Worst-month December PSH = 1.3)
- Daily Load: 2,500 Wh/day (LEDs, fridge, laptop, water pump, small TV)
- System Efficiency: 80% (accounting for inverter, battery, and wiring losses)
The Math:
- Calculate Required Generation:
2,500 Wh/day ÷ 0.80 efficiency = 3,125 Wh/day - Calculate Required Array Size:
3,125 Wh/day ÷ 1.3 PSH = 2,403 Watts - Select Panels: To get 2,403W, you might buy six 400W panels (2,400W total).
- Illustrative Cost: At roughly $0.80 per watt for DIY solar panels, the array alone would cost about $1,920. (Note: This excludes batteries, charge controllers, inverters, and mounting hardware, which typically make up the bulk of an off-grid system's cost).
If this exact same cabin were in Phoenix, AZ (December PSH = 5.5), the required array would be just 568 Watts (two 300W panels), costing around $480.
How to Look Up Your Exact Zip Code
Annual state averages are starting points. To get the exact number for your address, follow these steps using the industry-standard tool:
- Go to the NREL PVWatts Calculator.
- Enter your zip code or street address and click Go.
- Confirm the pin location on the map, then click Go to System Info.
- Set your panel tilt (use your latitude in degrees as a starting point) and azimuth (180° = south-facing for the Northern Hemisphere).
- Click Go to PVWatts Results.
- Look at the Solar Radiation column. Those monthly
kWh/m²/dayvalues are your monthly peak sun hours. - Find the lowest number (usually December). This is the crucial figure you must use for off-grid sizing.
Frequently Asked Questions (FAQs)
How do I find peak sun hours for my specific zip code?
Use the NREL PVWatts calculator at pvwatts.nrel.gov and enter your zip code. Read the Solar Radiation column on the results page. Each monthly value in kWh/m²/day equals your peak sun hours for that month.
Should I use the annual average or the worst-month figure for sizing?
For off-grid systems, always use the worst-month figure (typically December or January). If you size an off-grid system based on the annual average, you will not generate enough power during the winter months. For grid-tied systems where the utility provides backup power, the annual average is acceptable for calculating financial payback.
My state has a high PSH average but I live in a cloudy coastal city — which number do I use?
Always use your specific city or zip code figure, not the state average. State averages are heavily skewed by large, sunny inland areas. Coastal fog, mountain shadows, and local weather patterns can reduce your actual PSH by 20% or more compared to the state average.
Does cloud cover permanently reduce peak sun hours?
Cloud cover reduces instantaneous irradiance below 1,000 W/m², lowering the effective PSH for that day. Climate averages (like those from NREL) already account for typical historical cloud cover patterns in your area. However, unusually cloudy weeks mean your battery bank's "days of autonomy" (storage capacity) becomes critical.
Can I increase my effective peak sun hours?
Yes, you can maximize your effective PSH by optimizing your panel tilt angle, ensuring a true south-facing azimuth (in the Northern Hemisphere), and removing any physical shading from trees or chimneys. While you cannot change the weather, proper physical orientation can extract 10–25% more energy from the sunlight you do receive.
What is the difference between daylight hours and peak sun hours?
Daylight hours simply measure the time between sunrise and sunset. Peak sun hours measure the intensity of that sunlight, compressed into equivalent hours of 1,000 W/m² irradiance. A location might have 10 hours of weak winter daylight, but only accumulate 2.5 peak sun hours of usable solar energy.
Sources & Further Reading
- National Renewable Energy Laboratory (NREL) - PVWatts Calculator
- U.S. Department of Energy - Planning a Home Solar Electric System
- Global Solar Atlas — World Bank / Solargis
Internal Links & Next Steps
Now that you have your peak sun hours figure, the next step is running the full sizing calculation.
- Use the WattSizing Calculator to bring panel, battery, and related recommendations together.
- Learn How Many Solar Panels You Need for Off-Grid.
- Understand Battery Bank Sizing and Days of Autonomy.
