Understanding Your Solar Analysis Results
SolarScope returns several solar irradiance metrics that quantify the solar resource at your location. Understanding these metrics is essential for interpreting analysis results and making informed decisions about solar system sizing, orientation, and expected performance.
This guide explains each metric in plain language, with typical values for reference and guidance on what good versus poor solar resources look like.
Global Horizontal Irradiance (GHI)
GHI is the total solar radiation received on a horizontal surface. It combines two components: Direct Normal Irradiance (DNI) and Diffuse Horizontal Irradiance (DHI). GHI is the primary metric used for evaluating locations for flat or low-tilt solar installations.
Typical GHI values:
- Excellent resource (US Southwest): 5.5–7.0 kWh/m²/day
- Good resource (US Southeast/Mountain West): 4.5–5.5 kWh/m²/day
- Moderate resource (US Midwest/Northeast): 3.5–4.5 kWh/m²/day
- Lower resource (Pacific Northwest, Alaska): 2.5–3.5 kWh/m²/day
Direct Normal Irradiance (DNI)
DNI measures direct sunlight received on a surface perpendicular to the sun's rays. It's the most relevant metric for concentrating solar power (CSP) systems and tracking systems. High DNI indicates a clear-sky environment with minimal diffuse scattering.
Locations with high DNI are ideal for single-axis or dual-axis solar trackers, which follow the sun's path to maximize direct beam capture. The US Southwest (Arizona, Nevada, New Mexico) has some of the world's highest DNI.
Diffuse Horizontal Irradiance (DHI)
DHI is the portion of solar radiation that has been scattered by the atmosphere and arrives from all directions. It's the dominant component on overcast days. Coastal and humid locations tend to have higher DHI ratios because water vapor and clouds scatter more sunlight.
Fixed-tilt panels still capture DHI effectively, making solar viable in cloudy climates. Seattle, WA, for example, generates more solar energy than Germany, which has a thriving solar industry.
Peak Sun Hours
Peak sun hours (PSH) is a normalized measure of daily solar irradiance, expressed as the equivalent number of hours of full standard solar irradiance (1,000 W/m²). For example, 5 PSH means the location receives the equivalent of 5 hours of full sun per day on average.
How to use PSH for system sizing:
Estimated daily production (kWh) = System size (kW) × PSH × System efficiency (%)
A 10 kW system with 5.5 PSH and 80% overall efficiency would produce approximately:
10 × 5.5 × 0.80 = 44 kWh/day
Capacity Factor
Capacity factor (CF) is the ratio of actual energy output to the maximum possible output if the system ran at full rated power 24/7. It's expressed as a percentage.
For solar PV systems, capacity factors typically range from 10–25%. Lower latitudes with high solar resources achieve higher capacity factors. Fixed-tilt systems have lower capacity factors than tracking systems.
Temperature Effects
Solar panels lose efficiency as temperatures rise. The temperature coefficient of power (Pmax) for typical monocrystalline panels is approximately -0.35% to -0.45% per degree Celsius above 25°C (STC). Hot climates with high irradiance often see reduced summer performance despite more sunlight.
SolarScope accounts for temperature corrections using NASA POWER surface temperature data, providing realistic production estimates that reflect actual operating conditions at your site.