Solar Irradiance Calculator — Find Your Location’s Peak Sun Hours and Annual Solar Yield

A solar irradiance calculator converts your location’s regional solar data into usable peak sun hours (PSH) — the standardized metric that determines how much electricity your solar panels actually produce. Select your US region, adjust for local weather conditions, choose your mounting type, and set your system derate factor. The calculator returns your effective daily PSH, total annual irradiance in kWh/m², specific yield per kilowatt of panels installed, and a month-by-month seasonal production chart showing exactly how your solar resource varies across the year.

How to Use the Solar Irradiance Calculator

Step 1 — Name your project and select your baseline region.

Enter a project name for the PDF export, then choose your geographic region from the dropdown. For US users, five options cover the main solar resource zones: US Southwest (Arizona, New Mexico, Nevada) at 6.0 PSH baseline — the strongest solar resource in the country; US California and Texas at 5.5 PSH; US Midwest and Central states at 4.5 PSH; and US Northeast and Pacific Northwest at 4.0 PSH — the weakest continental US solar resource. Selecting a region automatically sets the hemisphere for seasonal curve calculations and populates the baseline daily peak sun hours.

Step 2 — Use Custom Input for precise locations.

If your location does not fit neatly into one of the preset regions, select Custom Manual Input at the bottom of the dropdown. Two additional fields appear: one for your exact daily irradiance in kWh/m²/day, which you can find from NREL’s PVWatts tool or NASA POWER database for your specific coordinates, and one for hemisphere selection which controls whether the seasonal monthly distribution peaks in June-July (Northern Hemisphere) or December-January (Southern Hemisphere).

Step 3 — Choose your local weather modifier. This dropdown adjusts the baseline PSH for your specific microclimate conditions. Select Highly Clear or Desert (+10%) for locations with consistently clear skies and minimal humidity — high desert regions of the Southwest qualify here. Select Average Historical Data (0%) for standard conditions matching NREL regional averages. Select Frequent Fog or Marine Layer (-10%) for coastal California, the Pacific Northwest, or any location with regular morning overcast. Select Heavy Rain or Cloudy Climate (-20%) for the rainiest US markets such as Seattle or Miami during hurricane season.

Step 4 — Select your panel tracking system.

Fixed Mount applies no multiplier and represents the standard residential rooftop installation. Single-Axis Tracker adds 20% to your effective PSH by rotating panels east to west throughout the day, capturing steeper morning and evening sun angles that a fixed array misses entirely. Dual-Axis Tracker adds 35% by also adjusting the panel tilt angle seasonally — the most productive configuration available, though limited to utility-scale and premium commercial ground-mount installations due to cost and mechanical complexity.

Step 5 — Set your system derate factor.

The derate factor slider accounts for all real-world efficiency losses between the solar irradiance hitting your panels and the usable AC electricity delivered inside your home. The default 85% is the NREL standard assumption covering inverter conversion loss (roughly 4%), wiring and connection losses (2%), soiling and dirt (2%), thermal derating on hot days (3%), and mismatch losses between panels (2%). Systems in dusty environments or with older string inverters should use 80–82%. Premium systems with microinverters, clean panels, and optimal mounting can reach 88–90%.

Step 6 — Read the results.

The Effective Daily PSH banner shows your site’s usable solar energy after all adjustments — this is the number your installer uses to size your system and project annual production. The Annual Solar Radiation card shows total kWh/m²/year and breaks down exactly how the three multipliers (tracking, weather, and derate) interact with your regional baseline. The System Yield card shows your specific yield — the kilowatt-hours of usable electricity produced per year for every kilowatt of panels you install. A well-sited 8 kW system in California with a specific yield of 1,400 kWh/kW produces approximately 11,200 kWh annually.

Step 7 — Analyze the monthly bar chart.

The twelve-bar chart shows how your effective PSH distributes across each month of the year. Hover over any bar to see the exact PSH figure for that month. Northern Hemisphere systems peak in June and July and dip in December and January — the difference between peak and trough can be 50–70% in northern states, which is critical information for sizing battery storage or managing winter energy bills for off-grid systems.

Step 8 — Export your report.

Click Export PDF to save a site-specific irradiance report named after your project. Useful for system sizing documentation, financing proposals, or comparing solar resource across multiple potential installation sites.

What Is a Peak Sun Hour? (The Key Concept)

A Peak Sun Hour does not mean an hour of daylight. It means one hour during which solar irradiance reaches exactly 1,000 watts per square meter — the standard reference intensity used to rate solar panels at STC. In practice, irradiance varies continuously throughout the day: weak in the early morning, building to peak at solar noon, then declining through the afternoon. A 14-hour summer day in Phoenix might only accumulate 6.5 peak sun hours because the morning and evening hours contribute only a fraction of 1,000 W/m² each.

The specific yield formula the calculator uses:

Specific Yield (kWh/kW/year) = Daily PSH × 365 × Derate Factor

Example for a Los Angeles rooftop system:

• Regional baseline: 5.5 PSH
• Weather modifier: 1.0 (average)
• Fixed mount: 1.0
• Derate factor: 85%
• Effective PSH = 5.5 × 1.0 × 1.0 = 5.5 hrs
• Specific yield = 5.5 × 365 × 0.85 = 1,708 kWh/kW/year
• An 8 kW system would produce approximately 13,664 kWh/year

Frequently Asked Questions

Q: What is solar irradiance and why does it matter for solar panels?

A: Solar irradiance is the power of sunlight arriving at a surface, measured in watts per square meter (W/m²). It matters for solar panels because panel power ratings — the wattage on the label — are measured at exactly 1,000 W/m² in laboratory conditions.

When actual irradiance is lower than 1,000 W/m², your panels produce proportionally less power. On an overcast day in Seattle where irradiance might reach only 200–300 W/m², a 400W panel produces only 80–120W.

Irradiance also varies by time of day, season, latitude, and atmospheric conditions including clouds, humidity, and air pollution. Understanding your site’s irradiance profile is the foundation of accurate solar production estimation.

Q: How many peak sun hours does my US state get per day?

A: Peak sun hours vary significantly across the United States. The Southwest leads the country: Phoenix averages 6.0–6.5 PSH/day, Las Vegas 5.8–6.2, and Albuquerque 6.0–6.3. California varies widely from 5.2 PSH in San Francisco (marine influence) to 6.0 PSH in Los Angeles and 6.5 in Palm Springs. Texas averages 4.9–5.5 PSH depending on latitude.

The Midwest runs 4.0–5.0 PSH: Chicago averages 4.2, Denver 5.0 (high altitude helps), Kansas City 4.8. The Northeast receives the least: New York City 4.1, Boston 4.0, Seattle 3.5, and Portland, Oregon 3.8. These figures represent annual daily averages — summer months can be 30–50% higher and winter months 30–50% lower than the annual mean.

Q: What is a good specific yield for a US solar system?

A: Specific yield — kilowatt-hours produced per year for every kilowatt of installed panel capacity — is the most useful metric for comparing solar sites and system designs. In the US Southwest, well-designed fixed systems achieve 1,700–2,100 kWh/kW/year. California and Texas average 1,500–1,800 kWh/kW/year. The Midwest typically produces 1,200–1,500 kWh/kW/year.

The Northeast and Pacific Northwest deliver 1,000–1,300 kWh/kW/year. A system achieving less than 1,000 kWh/kW/year likely has significant shading, suboptimal orientation, or unusually high system losses.

Your installer’s proposal should include a specific yield projection — if it looks significantly higher or lower than these ranges for your region, ask for an explanation.

Q: How does cloud cover and weather affect solar production?

A: Cloud cover is the single largest source of variation in solar production between regions and years. A thick cloud layer transmits only 10–25% of surface irradiance, while a light overcast might allow 50–70% through.

Regions with marine influence — coastal California, the Pacific Northwest, New England — experience regular morning overcast that can reduce effective PSH by 10–20% compared to inland locations at the same latitude.

Rain itself has minimal direct effect on production, but the cloud systems that accompany rain do. High-altitude locations benefit from reduced atmospheric scatter, which is why Denver with 5,000+ feet of elevation achieves similar PSH to coastal Southern California despite being much further north.

Q: What is a system derate factor and what losses does it include?

A: The derate factor is a single number that represents all the efficiency losses between solar irradiance hitting your panels and usable electricity delivered inside your home. NREL’s standard 85% derate includes inverter efficiency losses (typically 4%, since inverters operate at 94–98% efficiency), DC and AC wiring resistance losses (1–2%), soiling from dust and debris (1–2%), panel mismatch losses from manufacturing tolerances (1–2%), and thermal losses from panels operating above their 25°C STC reference temperature (2–4% in hot climates).

Systems with microinverters eliminate string mismatch losses and typically achieve 87–90% derate. Systems in dusty environments like Phoenix or agricultural areas should use 80–83% to account for heavier soiling. The derate factor does not include panel degradation, which is a separate multi-year effect.

Q: What is the difference between solar irradiance and solar insolation?

A: Irradiance is an instantaneous measurement of solar power at a given moment — expressed in watts per square meter (W/m²). Insolation is the accumulated energy over a time period — expressed in kilowatt-hours per square meter (kWh/m²). One hour of 1,000 W/m² irradiance equals 1 kWh/m² of insolation, which is exactly one peak sun hour.

Your panel’s power output at any instant depends on irradiance. Your panel’s total energy production over a day, month, or year depends on insolation — the accumulated area under the irradiance curve. This calculator outputs daily insolation as peak sun hours, which directly predicts your system’s daily kWh production when multiplied by your system size in kilowatts.

Q: Does a solar tracker significantly improve production for US homeowners?

A: Single-axis trackers improve production by 20–25% compared to fixed mounts in high-irradiance locations, and the improvement is smaller in cloudy climates where diffuse light dominates. The economic case for residential rooftop tracking is weak because most homes cannot physically install motorized trackers on a fixed roof — trackers are primarily used in commercial and utility ground-mount systems. For US homeowners, the more relevant comparison is fixed mount at optimal tilt versus a compromised roof orientation.

A south-facing roof at 30° tilt in Phoenix is a far better investment than a tracker-equipped east-facing roof. If you have ground space available, a single-axis ground-mount tracker can justify its additional cost in high-irradiance markets like the Southwest where the extra output generates meaningful additional savings.

Q: How do I find the exact peak sun hours for my specific US address?

A: The most accurate free tool for US locations is NREL’s PVWatts Calculator (pvwatts.nrel.gov), which uses the TMY3 (Typical Meteorological Year) dataset — hourly weather data averaged across 30 years of historical measurements for over 1,000 US weather stations. Enter your address, specify your system size and tilt, and PVWatts returns monthly and annual production estimates including effective PSH.

NASA POWER (power.larc.nasa.gov) provides satellite-derived irradiance data for any coordinates worldwide at 0.5-degree resolution — useful for locations between weather stations. The irradiance calculator on this page uses regional averages that are appropriate for system sizing estimates; for bankable financial projections used in loan applications or PPA agreements, a PVWatts or PVsyst simulation with site-specific data is the industry standard.