Global Tilted Irradiance vs GHI

What is GHI?

GHI is global horizontal irradiance, the solar irradiance received on a horizontal surface.

GHI is a solar-resource value tied to a flat horizontal plane. It is useful for climate datasets, solar maps, and site-level comparisons. A horizontal surface is the reference, not necessarily the installed module surface.

PVWatts uses weather data from datasets such as the National Solar Radiation Database for covered locations. The model then uses system inputs, including tilt and azimuth, to estimate panel-plane results. GHI is part of the resource context, while the panel plane still needs orientation.

What is the difference between global tilted irradiance and GHI?

Global tilted irradiance belongs to a tilted panel surface, while GHI belongs to a horizontal surface.

The difference is the receiving plane. A horizontal plane has 0 deg tilt. A solar panel often uses a nonzero tilt. PVWatts accepts tilt values from 0 deg to 90 deg and azimuth values from 0 deg to less than 360 deg.

The same location and hour can produce a different irradiance value on a horizontal surface and on a tilted panel plane. The difference depends on sun position, panel angle, direct beam, diffuse light, reflected light, season, and weather.

Why does a tilted plane differ from a horizontal plane?

A tilted plane differs from a horizontal plane because it faces a different part of the sky. A south-facing tilted panel in the Northern Hemisphere often faces the central sun path more directly than a flat horizontal surface during some periods. A vertical or poorly directed surface can receive less direct exposure.

The tilted plane also changes the diffuse and reflected light relationship. A steep panel sees a different sky dome and ground-reflection geometry than a flat surface. Performance models account for these geometry differences more carefully than a simple GHI reading.

Why does panel azimuth matter?

Panel azimuth matters because two surfaces with the same tilt can face different directions. A 30 deg east-facing plane and a 30 deg south-facing plane receive different direct sun timing. GHI cannot describe that directional difference because GHI is horizontal.

PVWatts separates tilt and azimuth for this reason. The panel plane requires both slope and direction. Global tilted irradiance depends on that complete surface definition.

How does PVWatts use plane-of-array irradiance?

PVWatts uses plane-of-array irradiance as a modeled output for the solar radiation received by the array plane.

PVWatts reports monthly plane-of-array irradiance in kWh/m2 and hourly plane-of-array irradiance in W/m2 when hourly output is selected. It also reports AC and DC output fields, which connect the solar resource to modeled system production.

Plane-of-array irradiance is useful because it sits between raw solar resource and energy output. It reflects the array surface before the full system output chain is interpreted through module, inverter, temperature, and loss assumptions.

How does sun position change GTI vs GHI?

Sun position changes GTI versus GHI because solar zenith and solar azimuth determine how sunlight reaches each surface.

NREL's Solar Position Algorithm calculates solar zenith and solar azimuth for solar radiation applications. Solar zenith describes sun height from overhead. Solar azimuth describes direction along the horizon. A tilted panel responds to both values through incidence angle.

At some times, a tilted surface aligns better with direct sunlight than a horizontal surface. At other times, a horizontal surface can receive more favorable geometry. Season and time of day change the comparison. That is why monthly and hourly outputs reveal more than one annual average.

How does shade affect GTI and GHI?

Shade affects global tilted irradiance and GHI differently because shade depends on the actual surface and obstruction geometry.

A roof-mounted panel can be shaded by a chimney even when the broader horizontal site resource looks strong. A flat horizontal sensor and a tilted panel row do not necessarily share the same obstruction view. DOE Energy Saver identifies sunlight reaching the site as a planning factor.

Shade also depends on solar elevation and azimuth. Morning shade, afternoon shade, and winter shade affect different panel planes differently. A GHI value does not replace a roof-plane shade check.

How do direct, diffuse, and reflected light change the comparison?

Direct, diffuse, and reflected light change the GTI versus GHI comparison because tilted and horizontal surfaces receive each component differently.

Direct light depends strongly on surface alignment with the sun. A tilted panel can face direct sun more closely during some seasons and less closely during others. Diffuse light comes from the sky dome, so surface tilt changes how much sky the panel sees.

Reflected light depends on ground or nearby surface reflectance. PVWatts includes albedo as a ground reflectance input. A tilted surface can receive reflected light differently from a horizontal surface. The GTI/GHI relationship is therefore a surface-geometry relationship, not one fixed multiplier.

How does season change GTI vs GHI?

Season changes GTI versus GHI because sun height and day length change across the year.

Winter sun is lower in many mid-latitude locations. A tilted equator-facing panel can receive a different share of winter direct light than a horizontal surface. Summer sun is higher, so the horizontal and tilted surfaces can compare differently than in winter.

Monthly plane-of-array irradiance is useful for this reason. PVWatts reports monthly POA irradiance, which lets a user compare seasonal panel-plane resource instead of relying on one annual GHI value.

How do soiling and albedo fit the comparison?

Soiling and albedo modify the irradiance relationship after the receiving plane is defined.

PVWatts includes soiling as a reduction in incident solar irradiance on the module surface and albedo as ground reflectance. Soiling reduces the radiation that reaches the active module surface. Albedo contributes reflected-light context, especially when ground reflectance is relevant.

GHI alone does not explain soiling on a tilted module. A low-tilt panel can collect debris differently than a steep panel. Global tilted irradiance or plane-of-array analysis is closer to the actual module surface.

When is GHI useful for solar panel angle planning?

GHI is useful for understanding the horizontal site resource and comparing broad locations.

GHI helps answer whether a location has strong solar resource on a horizontal reference plane. It is useful in weather files, solar maps, and climate comparisons. It is less useful for deciding the final panel angle by itself.

Panel angle planning needs the next layer. Tilt, azimuth, array type, shade, and mount geometry convert the broad resource into a panel-plane resource. A site with strong GHI still needs a usable array surface.

When is global tilted irradiance more useful?

Global tilted irradiance is more useful when the question is how much sunlight reaches a specific solar panel surface.

GTI or plane-of-array irradiance belongs to the actual module plane. It answers a more specific question than GHI: how much irradiance reaches this tilted surface at this direction and location.

That specificity makes it valuable for comparing angle choices. A 20 deg south-facing array, 35 deg south-facing array, and 20 deg west-facing array can be compared by modeled plane-of-array irradiance while keeping the site and system assumptions the same.

What mistakes distort GTI vs GHI decisions?

GTI vs GHI mistakes include using GHI as a panel-plane value, ignoring azimuth, and treating one annual value as a full angle decision.

GHI is horizontal. A solar panel is often tilted. A tilted panel also faces a direction. Using GHI as if it were panel-plane irradiance skips the core geometry of solar panel angle.

Another mistake is using one annual value for a seasonal problem. Monthly and hourly patterns matter when tilt, shade, and sun path change across the year. A winter shade issue or afternoon orientation issue can hide inside an annual average.

How do you use GTI and GHI together?

Use GHI for broad site context and global tilted irradiance for panel-angle comparison.

A clean workflow starts with location and weather data. GHI describes the horizontal resource context. Tilt and azimuth define the panel plane. Plane-of-array irradiance or GTI describes the resource on that panel plane. Modeled AC output then adds module, temperature, inverter, and loss assumptions.

This sequence keeps each entity in the right place. GHI describes the site plane. GTI or plane-of-array irradiance describes the module plane. Output modeling describes the system.

Use one tool after this page: Check Solar Angle Loss.

Source Notes

• C001-C003: NREL PVWatts documents tilt, azimuth, and array type inputs.
• C004: NREL Solar Position Algorithm documents solar zenith, solar azimuth, and incidence-angle calculations.
• C005: NREL PVWatts documents weather data, plane-of-array irradiance, albedo, soiling, and output fields.
• C009: DOE Energy Saver identifies sunlight reaching the site as a planning factor.