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
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.
Why it matters for sizing: 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.
What Is a Peak Sun Hour?
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 at which solar panels are rated.
The math:
1 peak sun hour = 1 hour × 1,000 W/m² = 1,000 Wh/m² = 1 kWh/m²
Because solar intensity rises from dawn, peaks at noon, and fades at dusk, a real day's irradiance curve is compressed into an equivalent rectangle. A location that receives 5,200 Wh/m² of total solar energy in a day has 5.2 peak sun hours — whether the sun was up for 14 hours or not.
Practical implication: A 300 W solar panel during 5 peak sun hours generates roughly 300 W × 5 h = 1,500 Wh (1.5 kWh) per day before system losses. That same panel in a 4 peak sun hour location generates only 1,200 Wh, a 20% reduction that compounds across an entire battery-dependent winter.
Peak Sun Hours vs. Solar Irradiance: The Same Number, Different Units
You will see both terms in technical tools. They are numerically identical:
| Metric | Unit | Example |
|---|---|---|
| Solar irradiance (GHI) | kWh/m²/day | 5.2 kWh/m²/day |
| Peak sun hours | h/day | 5.2 h/day |
| Peak sun hours | PSH | 5.2 PSH |
When the NREL PVWatts tool or the Global Solar Atlas shows "Global Horizontal Irradiance = 5.2 kWh/m²/day", that location gets 5.2 peak sun hours per day. The units differ; the value is the same.
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.
Table note: State-level figures are broad averages. For purchases and safety-critical sizing, confirm values with PVWatts for your exact address or zip.
How to Find Your Exact Peak Sun Hours by Zip Code
Annual state averages are starting points. Your actual value depends on:
- Latitude within the state (e.g., southern vs. northern California differ by 0.8 PSH)
- Panel tilt angle — a panel tilted to match local latitude captures more than a flat panel
- Panel azimuth — south-facing captures more than east- or west-facing
- Micro-climate — coastal fog, high-altitude clearness, and desert air all affect irradiance
Method 1: NREL PVWatts Calculator (Free, Most Accurate)
- Go to pvwatts.nrel.gov
- 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)
- Click Go to PVWatts Results
- Look at the Solar Radiation column — those monthly kWh/m²/day values are your monthly peak sun hours
Read the December row. That is your worst-month number for off-grid sizing.
Method 2: Global Solar Atlas (International, Visual)
- Go to globalsolaratlas.info
- Search your address
- Under Site Info, find Global Horizontal Irradiation in kWh/m²/day
- Switch from "Per year" to "Per day" for the daily average
Method 3: Manual Look-up from This Article
Use the state table above for a quick estimate. For a more precise number, narrow to your region using the city-level data below.
Peak Sun Hours for Major US Cities
| City | State | Avg Daily PSH | Worst Month (Dec/Jan) |
|---|---|---|---|
| Phoenix | AZ | 7.5 | ~5.5 |
| Tucson | AZ | 7.2 | ~5.2 |
| Las Vegas | NV | 6.7 | ~4.5 |
| Albuquerque | NM | 6.8 | ~5.3 |
| Denver | CO | 6.1 | ~4.6 |
| Salt Lake City | UT | 6.0 | ~3.9 |
| Los Angeles | CA | 5.9 | ~4.2 |
| San Diego | CA | 5.7 | ~4.0 |
| Dallas | TX | 5.8 | ~3.9 |
| Houston | TX | 5.5 | ~3.6 |
| Miami | FL | 6.0 | ~4.8 |
| Orlando | FL | 5.6 | ~4.2 |
| Atlanta | GA | 5.2 | ~3.4 |
| Charlotte | NC | 5.0 | ~3.3 |
| Nashville | TN | 4.9 | ~3.1 |
| Kansas City | MO | 5.0 | ~3.0 |
| Chicago | IL | 4.4 | ~2.3 |
| Indianapolis | IN | 4.3 | ~2.2 |
| Detroit | MI | 4.1 | ~1.9 |
| Columbus | OH | 4.2 | ~2.1 |
| Philadelphia | PA | 4.6 | ~2.7 |
| New York City | NY | 4.4 | ~2.5 |
| Boston | MA | 4.6 | ~2.6 |
| Portland | OR | 4.5 | ~1.8 |
| Seattle | WA | 3.8 | ~1.3 |
| Minneapolis | MN | 4.5 | ~2.2 |
| Boise | ID | 5.1 | ~2.8 |
| Billings | MT | 5.0 | ~2.6 |
Seasonal Variation: Why Annual Averages Are Misleading for Off-Grid Systems
The same location can swing dramatically between seasons. Phoenix, AZ example:
| Month | Avg Daily PSH |
|---|---|
| June | 8.5 |
| July | 7.8 |
| August | 7.5 |
| September | 7.2 |
| October | 6.5 |
| November | 5.8 |
| December | 5.5 |
| January | 5.7 |
| February | 6.3 |
| March | 7.0 |
| April | 7.8 |
| May | 8.2 |
| Annual Average | 7.2 |
Portland, OR example — a much starker swing:
| Month | Avg Daily PSH |
|---|---|
| June | 6.5 |
| July | 7.0 |
| August | 6.5 |
| September | 5.0 |
| October | 3.0 |
| November | 1.8 |
| December | 1.3 |
| January | 1.6 |
| February | 2.8 |
| March | 3.8 |
| April | 4.8 |
| May | 5.8 |
| Annual Average | 4.5 |
A Portland off-grid system sized on 4.5 PSH will be chronically power-starved every November through February. Sized on the correct 1.3 PSH December figure, the array is large enough to actually function year-round.
How to Use Peak Sun Hours in Off-Grid Solar Sizing
Step 1 — Calculate Daily Energy Need (Wh/day)
List every load, multiply wattage by daily hours of use, and sum:
Daily Load (Wh) = Σ (Appliance Watts × Hours/day)
Example for a small cabin:
- LED lighting: 50 W × 4 h = 200 Wh
- 12 V fridge: 45 W × 12 h = 540 Wh
- Laptop: 65 W × 3 h = 195 Wh
- Water pump: 120 W × 0.5 h = 60 Wh
- Total: 995 Wh/day
See How to Calculate Your Daily Energy Use (Wh) for Off-Grid Solar for a full load-audit walkthrough.
Step 2 — Derate for System Losses
Real systems lose energy to wiring resistance, inverter inefficiency (typically 90–93%), battery round-trip losses (LiFePO4 ≈ 95–97%), and temperature. Apply a 0.80 overall system efficiency factor as a conservative starting point:
Required Solar Generation (Wh/day) = Daily Load ÷ System Efficiency
= 995 ÷ 0.80 = 1,244 Wh/day
Step 3 — Divide by Worst-Month Peak Sun Hours
Solar Array Size (W) = Required Generation ÷ Worst-Month PSH
Using Portland, OR (December PSH = 1.3):
Array = 1,244 ÷ 1.3 = 957 W → round up to 1,000 W (four 250 W panels)
Using Phoenix, AZ (December PSH = 5.5):
Array = 1,244 ÷ 5.5 = 226 W → round up to 250 W (one 250 W panel)
The same load in the same state of charge, but nearly 4× more panels required in Portland vs. Phoenix. This is why knowing your actual zip code PSH is not optional — it is the most impactful variable in the entire design.
Step 4 — Enter Peak Sun Hours in the WattSizing Calculator
Open the WattSizing Calculator, enter your daily energy use (Wh/day), and set peak sun hours to the worst-month value you looked up (for example from PVWatts). The tool sizes recommended array power, battery energy, and related outputs from those inputs—it does not auto-look up irradiance by zip, so the number you enter should always come from PVWatts, Global Solar Atlas, or another trusted source.
Panel Tilt Angle and Its Effect on Peak Sun Hours
A flat (0° tilt) panel captures less annual energy than a tilted panel. Tilting to your latitude in degrees maximizes the annual total. Tilting slightly steeper (latitude + 10° to 15°) trades summer production for winter gain — the right tradeoff for off-grid systems that must survive their worst month.
| Location | Latitude | Flat Panel PSH | Latitude-Tilt PSH | Gain |
|---|---|---|---|---|
| Phoenix, AZ | 33° | 6.8 | 7.5 | +10% |
| Denver, CO | 40° | 5.5 | 6.1 | +11% |
| Chicago, IL | 42° | 3.9 | 4.4 | +13% |
| Seattle, WA | 47° | 3.3 | 3.8 | +15% |
This gain is most pronounced in winter months, which is exactly when off-grid systems need help most. Install at latitude tilt (or slightly steeper) whenever the mounting situation allows it.
GHI vs. DNI vs. GTI — Which Irradiance Figure to Use
Three irradiance metrics appear in solar data tools. They are not interchangeable:
| Metric | What It Measures | Use For |
|---|---|---|
| GHI (Global Horizontal Irradiance) | Total sunlight on a flat (0°) surface | Baseline comparison between locations |
| DNI (Direct Normal Irradiance) | Direct beam light hitting a surface perpendicular to the sun | Concentrating solar, tracking systems only |
| GTI (Global Tilted Irradiance) | Total sunlight on a tilted/oriented surface | Fixed-tilt off-grid panel sizing |
Use GHI for state-to-state comparisons and as a baseline. Use GTI at your specific tilt and azimuth for actual sizing — PVWatts computes this for you once you enter panel angle.
Many DIY sizing guides incorrectly use DNI, which overstates usable irradiance for fixed panels. When you use any calculator—including WattSizing—enter peak sun hours that match your mounting (typically from PVWatts with your tilt and azimuth filled in).
Common Mistakes When Using Peak Sun Hours
Mistake 1 — Using the annual average for an off-grid system. Off-grid systems must be self-sufficient every month. Design to the worst month, not the mean.
Mistake 2 — Forgetting system losses. Rated panel output × PSH gives theoretical energy. Multiply by 0.75–0.85 to get real-world delivered energy before sizing batteries or loads.
Mistake 3 — Using DNI instead of GHI or GTI. DNI overstates available energy for fixed flat panels by 15–30% in many climates.
Mistake 4 — Ignoring tilt. A panel lying flat on a low-pitch roof in Seattle may see only 3.0 PSH, but the same panel tilted to 47° (Seattle's latitude) sees 3.8 PSH — a 27% difference that significantly reduces the required panel count.
Mistake 5 — Treating PSH as daylight hours. A location with 14 hours of daylight in June and 2.8 PSH in December is not broken — the sun is present, but low in the sky and weak. Panel sizing must reflect energy delivered, not hours of sky brightness.
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). For grid-tied systems where the utility provides backup, the annual average is acceptable for payback calculations.
My state has a high PSH average but I live in a cloudy coastal city — which number do I use?
Use your city or zip code figure, not the state average. California's state average of 6.25 PSH is dominated by inland and desert areas. San Francisco averages about 4.9 PSH due to summer fog — a 21% difference from the state figure.
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 account for typical cloud cover patterns. Unusually cloudy years or seasons mean your battery bank autonomy (days of storage) becomes more important than your panel count.
Can I increase my effective peak sun hours?
Yes — through panel tilt angle, azimuth optimization, and removing shading. You cannot change your climate, but you can extract 10–25% more energy from the same climate by orienting panels correctly.
Trusted References
- NREL PVWatts Calculator — National Renewable Energy Laboratory
- Global Solar Atlas — World Bank / Solargis
- NREL Solar Resource Maps — GHI Data for the United States
Size Your System with Your Local PSH
Now that you have your peak sun hours figure, the next step is running the full sizing calculation — daily load audit, battery bank sizing (kWh and Ah), charge controller sizing, and inverter sizing. Pair this page with how many solar panels for off-grid, battery bank sizing, and days of autonomy. The WattSizing Calculator brings panel, battery, and related recommendations together once you enter daily use and peak sun hours.
