What Is GHI (Global Horizontal Irradiance)?

Global Horizontal Irradiance (GHI) is the total solar radiation received on a perfectly horizontal surface — the most widely used measure of a location's solar resource.

GHI includes two components:
1. Direct beam radiation: The portion that comes straight from the sun, projected onto the horizontal surface
2. Diffuse radiation: Scattered sunlight from the sky, clouds, and atmosphere

Units: Kilowatt-hours per square meter per day (kWh/m²/day) as a daily average, or Watts per square meter (W/m²) at a specific moment.

Typical US annual average GHI values:
- Phoenix, AZ: 5.7–6.0 kWh/m²/day
- Los Angeles, CA: 5.5–5.8 kWh/m²/day
- Miami, FL: 5.1–5.4 kWh/m²/day
- Chicago, IL: 4.0–4.3 kWh/m²/day
- Seattle, WA: 3.1–3.4 kWh/m²/day
- Boston, MA: 4.2–4.5 kWh/m²/day

GHI is the primary input for estimating solar panel production from standard fixed-tilt systems (rooftop solar, flat-roof commercial). SolarScope's city pages display monthly and annual GHI data for 200+ US cities, sourced from NREL's NSRDB.

What Is DNI (Direct Normal Irradiance)?

Direct Normal Irradiance (DNI) measures only the solar radiation received directly from the sun's beam, measured perpendicular to the sun's rays (on a surface that always faces the sun). It excludes diffuse radiation.

Units: kWh/m²/day or W/m²

Typical US annual average DNI values:
- Tucson, AZ: 7.0–7.5 kWh/m²/day (among the world's best DNI)
- Las Vegas, NV: 6.8–7.2 kWh/m²/day
- Dallas, TX: 5.0–5.5 kWh/m²/day
- New York, NY: 3.5–4.0 kWh/m²/day
- Seattle, WA: 2.5–3.0 kWh/m²/day

DNI is highest on clear, sunny days in arid regions. It is the critical metric for:
- Concentrating Solar Power (CSP) systems: parabolic trough, power tower, and dish/Stirling systems that focus direct sunlight
- 2-axis tracking PV arrays that continuously orient toward the sun
- Heliostat fields in solar thermal power plants

For conventional fixed-tilt PV panels, GHI is more relevant than DNI. However, high DNI regions also tend to have high GHI — the US Southwest's outstanding solar resources are reflected in both metrics.

What Is DHI (Diffuse Horizontal Irradiance)?

Diffuse Horizontal Irradiance (DHI) measures the solar radiation received from the sky dome excluding the direct solar beam — light scattered by the atmosphere, clouds, aerosols, and particulates.

Units: kWh/m²/day or W/m²

The three irradiance components relate mathematically:

GHI = DHI + DNI × cos(Solar Zenith Angle)

In cloudy, high-latitude regions, DHI makes up a larger proportion of GHI than in clear, arid regions. Seattle, WA receives a higher proportion of its GHI from DHI than Phoenix, AZ, where clear skies and low humidity mean most GHI comes from the direct beam.

Practical significance: Even on overcast days, solar panels receive diffuse radiation and produce electricity — typically 10–25% of peak output. This is why solar performs meaningfully in Germany, the UK, and the Pacific Northwest despite frequent cloud cover. High DHI fractions also affect the optimal tilt angle for fixed-mount systems — flatter tilts capture more diffuse sky radiation.

Peak Sun Hours Explained

Peak Sun Hours (PSH) is a simplified metric that translates a location's daily solar energy into an equivalent number of hours of full-strength (1,000 W/m²) sunlight. It's defined as:

PSH = Daily GHI (kWh/m²/day) ÷ 1 kW/m²

Since solar panel wattage ratings use 1,000 W/m² as the reference condition, PSH directly gives you the number of hours a panel would operate at its rated wattage if all daily solar energy arrived at full intensity.

Example: Phoenix, AZ GHI = 5.8 kWh/m²/day → 5.8 peak sun hours

Production estimate using PSH:

Annual kWh = System Size (kW) × PSH/day × 365 × Performance Ratio (0.80)

For a 10 kW system in Phoenix with 5.8 PSH:

10 × 5.8 × 365 × 0.80 = 16,936 kWh/year

PSH varies significantly by month — Phoenix averages 7.0+ in June and only 4.5 in December. Monthly PSH data is available for all cities in our Solar Data portal.

Where Solar Irradiance Data Comes From

Solar irradiance data is derived from a combination of satellite imagery, ground-based instruments, and atmospheric modeling:

NREL NSRDB (National Solar Radiation Database) is the primary US solar data source. NREL processes satellite imagery from GOES weather satellites to derive surface irradiance at 4 km × 4 km spatial resolution for the contiguous US, Hawaii, and Puerto Rico. Updated annually with data from 1998–present. Available free via the NREL Developer API. SolarScope uses NSRDB as its primary irradiance source.

NASA POWER API (Prediction of Worldwide Energy Resource) provides 40+ years of daily solar and meteorological data at 0.5° × 0.5° spatial resolution globally. Its broader coverage makes it valuable for non-US locations and for cross-validation. SolarScope uses NASA POWER as a secondary data source.

TMY (Typical Meteorological Year) datasets represent a statistically typical year for a location based on 15–30 years of historical data. TMY3 and TMY2 datasets are widely used in simulation software (PVsyst, SAM) as the standard weather input for energy yield analysis.

Ground-based pyranometers measure actual irradiance at weather stations. While more accurate than satellite-derived data, they have sparse geographic coverage. Ground data is used to validate and calibrate the satellite-derived datasets.

Using Solar Data for Site Analysis

Solar analysis software translates irradiance data into energy production estimates through a systematic process:

  1. Fetch irradiance data (GHI, DNI, DHI) for the site coordinates from NREL or NASA
  2. Calculate plane-of-array (POA) irradiance accounting for panel tilt, azimuth, and tracking type using transposition models (Perez model, Hay-Davies model)
  3. Apply system losses using a performance ratio (typically 0.75–0.85) that accounts for inverter efficiency, temperature effects, wiring losses, soiling, and mismatch
  4. Estimate annual energy production in kWh from POA irradiance × system capacity × PR
  5. Calculate financial metrics using local electricity rates, ITC eligibility, net metering assumptions, and system cost

SolarScope's Site Studio automates all these steps. Enter any US location, specify system parameters, and receive NREL-backed production estimates with full financial analysis in under a minute. Explore monthly GHI, DNI, and PSH data for 200+ US cities to compare locations before your site visit.