Irradiance Estimator
Estimate peak sun hours by US state using NREL NSRDB data. Calculates tilt and azimuth corrections, monthly PSH breakdown, annual production, and required system size — free, no signup.
Solar Irradiance Estimator
Select your state, set tilt and azimuth, and get effective peak sun hours with tilt/azimuth correction factors, monthly breakdown, annual production estimate, and required system size.
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What This Irradiance Estimator Covers
State-level solar resource data from NREL NSRDB, with tilt and azimuth correction, monthly PSH breakdown, and production or system sizing output. No signup required.
Peak Sun Hours by State
Annual average horizontal PSH for all 50 US states, sourced from NREL NSRDB 1998–2022 typical meteorological year data. Includes month-by-month breakdown with min/max months highlighted.
Tilt & Azimuth Corrections
Adjusts horizontal PSH for panel tilt (0–90°) and orientation (16-point compass). Shows correction multipliers and percentage gain/loss vs flat mount and true south. Calculates optimal tilt from state latitude.
Production & System Sizing
Enter system size (kW) to get daily and annual production estimates. Or enter an annual kWh goal to get the required system capacity. Applies a configurable derate factor (default 0.78).
Key Features
Built for solar designers who need a fast, reliable irradiance reference without pulling up PVWatts for every preliminary estimate.
NREL NSRDB 1998–2022 TMY Data
All 50 states use typical meteorological year averages from NREL's National Solar Radiation Database - the same source underlying PVWatts and most utility-scale solar modeling tools.
Auto-Calculated Optimal Tilt
Displays the optimal tilt angle for each state using the formula (latitude × 0.87 + 3.1) and pre-fills the tilt input accordingly - one less thing to look up.
Solar Resource Quality Badge
Color-coded resource rating: Excellent (≥6.0 PSH), Very Good (≥5.0), Good (≥4.0), Average (≥3.5), Below Average. Useful for quick project feasibility checks.
Monthly PSH Chart
Animated bar chart showing PSH for all 12 months. Highlights the worst month (critical for off-grid autonomy sizing) and best month. Updates instantly when state, tilt, or azimuth changes.
16-Direction Azimuth Selection
Select from N, NNE, NE … S … NW or enter a custom azimuth degree. Shows azimuth loss percentage for non-south orientations - east/west panels lose 13–20% annually vs true south.
Configurable Derate Factor
Adjust the system derate from 0.60 to 1.00. Default 0.78 matches industry standard (inverter, wiring, temperature, soiling losses combined). PVWatts default is 0.86 - useful for comparison.
How to Use This Calculator
From state selection to production estimate in under a minute.
Select your state
Choose any of the 50 US states (plus DC) from the dropdown. The tool immediately loads the state's annual horizontal PSH, month-by-month data, latitude, and pre-fills the optimal tilt angle. The solar resource quality badge appears instantly.
Set tilt and azimuth
Adjust the tilt slider or enter a value (0° = flat, 90° = vertical). The tool shows the tilt correction multiplier and percentage gain vs flat. Select panel orientation from the 16-direction compass or enter degrees - azimuth loss is shown for any non-south orientation. An SVG diagram updates in real time to visualize the angle and direction.
Read effective PSH
The results panel shows three PSH values: horizontal (state baseline), tilt-corrected, and effective (tilt + azimuth combined). The effective PSH is what you use for production modeling and system sizing.
Optionally estimate production or required system size
Expand the advanced section and enter your system size (kW) to see daily and annual production. Or enter the customer's annual kWh goal to get the required system capacity. Adjust the derate factor if you have project-specific loss data.
Peak Sun Hours by State (Annual Average)
State-level averages from NREL NSRDB 1998–2022 TMY data. Values are horizontal PSH before tilt or azimuth correction.
| State | Annual PSH | Resource Quality | State | Annual PSH | Resource Quality |
|---|---|---|---|---|---|
| Arizona | 6.5 | Excellent | Montana | 4.5 | Good |
| Nevada | 6.4 | Excellent | Kansas | 4.9 | Good |
| New Mexico | 6.3 | Excellent | Nebraska | 4.8 | Good |
| Hawaii | 6.0 | Excellent | Illinois | 4.2 | Good |
| California | 5.5 | Very Good | Indiana | 4.0 | Good |
| Utah | 5.5 | Very Good | Ohio | 3.9 | Average |
| Colorado | 5.4 | Very Good | Michigan | 3.9 | Average |
| Texas | 5.2 | Very Good | New York | 3.9 | Average |
| Florida | 5.0 | Very Good | Oregon | 4.0 | Good |
| Georgia | 4.7 | Good | Washington | 3.8 | Average |
Horizontal PSH before tilt/azimuth correction. Use the tool to apply correction factors for your specific panel configuration. Alaska is included in the tool (varies 2.5–3.5 PSH by location).
Calculation Methodology
Transparent formulas and correction tables so you can verify outputs and explain the numbers to clients or AHJs.
Optimal Tilt Angle
A widely-used empirical approximation. For California (lat ≈ 36°): optimal tilt = 36 × 0.87 + 3.1 = 34.4° ≈ 35°. Maximizes annual energy capture for a fixed-tilt south-facing array.
Tilt Correction Factor
Tilt Factor = interpolate(tilt_table, tilt_degrees)
Tilt-Corrected PSH = Horizontal PSH × Tilt Factor
The tool uses a 15-point lookup table (0°–90°) with linear interpolation. Maximum gain of ~16.5% occurs at 30–35° tilt. A flat (0°) mount loses that gain; a vertical (90°) mount captures only ~65% of horizontal irradiance.
Azimuth Correction Factor
Deviation = |Azimuth − 180°| (degrees from true south)
Azimuth Factor = interpolate(azimuth_table, deviation)
Effective PSH = Tilt-Corrected PSH × Azimuth Factor
True south (180°) = 1.00× factor. East or west (90° deviation) = ~0.83× factor (17% loss). North-facing (180° deviation) = ~0.47× factor (53% loss). The table mirrors for east and west - a 90° east array and a 90° west array have the same annual energy, though different production profiles.
Production & System Sizing
Daily Production (kWh) = System kW × Effective PSH × Derate Factor
Annual Production (kWh) = Daily Production × 365
Required System (kW) = Annual kWh Goal ÷ (Effective PSH × Derate × 365)
Derate factor (default 0.78) captures combined real-world losses: inverter efficiency (~96%), DC wiring (~98%), AC wiring (~99%), soiling (~95%), temperature (~89%), system availability (~99%). PVWatts uses 0.86 as its default - adjusting to 0.78 gives a more conservative sizing estimate appropriate for most residential installs.
How Tilt & Azimuth Affect Annual Energy
Relative annual energy output compared to optimal tilt + true south for a mid-latitude US location. Values change by state - use the calculator for site-specific results.
Tilt Angle Impact (South-Facing)
| Tilt Angle | Correction Factor | vs Flat |
|---|---|---|
| 0° (flat) | 1.000× | baseline |
| 10° | 1.045× | +4.5% |
| 20° | 1.105× | +10.5% |
| 30° | 1.150× | +15.0% |
| 35° (typical optimal) | 1.165× | +16.5% |
| 45° | 1.130× | +13.0% |
| 60° | 0.970× | −3.0% |
| 90° (vertical) | 0.650× | −35.0% |
Azimuth Orientation Impact (Optimal Tilt)
| Orientation | Correction Factor | vs South |
|---|---|---|
| South (180°) | 1.000× | optimal |
| SSE / SSW (157.5° / 202.5°) | 0.990× | −1.0% |
| SE / SW (135° / 225°) | 0.960× | −4.0% |
| ESE / WSW (112.5° / 247.5°) | 0.910× | −9.0% |
| East / West (90° / 270°) | 0.830× | −17.0% |
| NE / NW (45° / 315°) | 0.650× | −35.0% |
| North (0°) | 0.470× | −53.0% |
Who Uses This Tool
From first client call to permit package - irradiance data comes up at every stage.
Solar Designers - Preliminary Sizing
Before pulling up PVWatts for a full simulation, use this tool to get a reliable PSH estimate in seconds. Enter the system size to check whether the roof can produce the customer's kWh target, or enter the kWh goal to find the required array size. Adjust tilt and azimuth for the actual roof pitch and direction.
Sales Teams - First Call Estimates
Get a credible system size estimate during the first customer conversation without needing a site visit. Select the state, use the default optimal tilt, and apply the kWh goal to produce a preliminary system size that holds up as a real-world ballpark. The resource quality badge gives an instant "solar viable" read for the location.
Off-Grid System Designers - Worst Month
Off-grid systems are sized for the worst solar month, not the annual average. The monthly PSH chart highlights the minimum month for each state. Size the array to meet load in that month and you'll have excess capacity the rest of the year - the right approach for off-grid autonomy.
Students & Educators
The tilt and azimuth correction tables - and the formulas behind them - are directly visible in the tool output. Use it to teach the relationship between latitude, panel tilt, and annual energy yield. The instant feedback when changing tilt or azimuth makes the sensitivity analysis intuitive.
Pro Tips
Use 0.78 derate for conservative residential proposals
The default 0.78 derate gives a conservative production estimate that accounts for real-world soiling, temperature derating, and wiring losses typically seen on residential installs. PVWatts' 0.86 default is more optimistic and suits newer, well-maintained systems. When quoting production to customers, the conservative figure protects against over-promise.
East-west split arrays: use east and west separately, then sum
For an east-west split roof, run the tool twice - once for each orientation - and sum the daily/annual production. Each half will show the 17% azimuth loss vs south, but combined they smooth the daily production curve, reducing peak demand charges and improving self-consumption for systems with batteries.
State-level data is an average - local conditions vary
This tool uses state-level averages from NREL NSRDB. Local microclimate, elevation, coastal fog, or urban heat island effects can move PSH 10–20% from the state average. For permit-level accuracy, use NREL PVWatts with site-specific coordinates. This tool is appropriate for preliminary design, proposals, and customer education.
Match your pitch in degrees, not a roof pitch ratio
Roof pitch is typically given as rise:run (e.g., 4:12 or 6:12). Convert to degrees: arctan(rise/run). A 4:12 pitch = 18.4°. A 6:12 pitch = 26.6°. A 7:12 pitch = 30°. Most residential roofs fall between 15° and 35° - within the range where tilt gain is 10–16% over flat mount.
Frequently Asked Questions
What is a peak sun hour (PSH)?
How does this compare to PVWatts?
What tilt angle should I use for a flat roof?
How much does azimuth orientation really matter?
What does the derate factor include?
Which US states have the best solar resource?
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