Understanding Solar Irradiance: DNI, DHI, and GHI Made Simple

We all know that solar panels absorb sunlight and produce electricity. But do you know that solar radiation has three components of light, and these are:

• DNI
• DHI
• GHI

And which is/are responsible for the solar panels to produce electricity?

In this post, we will understand each of them and then determine which component (s) make solar panels capable of producing electricity.

Solar Radiation

Solar radiation is the energy that we receive from the Sun. It is measured in Watts/m².

It has 3 components, namely:

• DNI: Direct Normal Irradiance
• DHI: Diffused Horizontal Irradiance
• GHI: Global Horizontal Irradiance

Direct Normal Irradiance

The sunlight that comes straight from the sun and falls on a normal surface. It is the amount of solar radiation received per unit area on a surface normal to the incoming rays.

Childhood Experiment

While writing about DNI, I recollected my childhood memories of utilizing DNI using our simple lens to burn the piece of paper. We used to keep the lens (convex lens) surface normal to the incoming rays, and on the other side, we kept the paper piece in a way that it is positioned at the focal point of the lens. The incoming rays converge at the focal point, the sunlight intensity increases, and burns the piece of paper.

So, unknowingly, we were using the DNI component of the solar radiation.

So intelligent of us!

Can you tell me one practical commercial application of DNI?

It is Concentrated Solar Power (CSP). This technology uses mirrors and lenses to focus the DNI component of the solar radiation into a small area (called the focus) to produce heat. This heat can be used to produce steam from water, which in turn can be used to run a turbine to generate electricity.

How to measure DNI?

The relative position of the Sun and the Earth changes from sunrise to sunset. To get the DNI on the surface, I need to keep that surface normal to the incoming rays and to always keep it normal to the rays. I need to adjust it every time, and manually adjusting it continuously is impractical.

This is done by a device called the solar tracker. It checks the direction of the incoming rays and tilts the surface in that direction to get the DNI.

And if I somehow manage to mount an instrument that can measure the value of DNI from sunrise to sunset. I can get the DNI for that day.

It is done by a Pyrheliometer.

Pyrheliometer

A pyrheliometer is a precision instrument specifically designed to measure direct solar radiation (DNI) coming from the direction of the sun. The acceptance angle of a standard pyrheliometer is about 5°.

How does it work?

• It has a blackened metal surface inside a long tube that points directly at the sun.
• Sunlight enters through the front opening and hits this surface.
• The black surface absorbs the sunlight and heats up.
• A thermopile sensor measures this temperature rise.
• The sensor converts heat into a small electric voltage, which tells us how strong the sunlight is.

Diffused Horizontal Irradiance

Not all the sunrays come straight to the Earth’s surface. Approximately 20–25% of the incoming solar irradiance gets scattered by dust, air, water molecules, and clouds in Earth’s atmosphere.

After scattering, these rays have no particular direction, or they reach the Earth’s surface from all directions. The DHI does not include direct beam from the Sun.

The DHI is highest on a cloudy day. On a cloudy day, you may not see the sun, but the brightness is still there; it is due to the diffused sunlight in the atmosphere, which is DHI.

It is measured on a horizontal surface.

Global Horizontal Irradiance

It is the total amount of solar radiation falling on a horizontal surface, and its cumulative value is called Solar Insolation.

It includes:

• Direct sunlight (DNI × cos (zenith angle))
• Scattered sunlight (DHI)

Formula:
GHI = DHI + DNI × cosꞵ
(ꞵ = angle between sun and vertical)

How do we measure the GHI?

The GHI is measured using an instrument called Pyranometer.

• The glass dome (protective glass) lets all the radiation (direct and scattered) to enter through it.
• These radiations are being absorbed by the black body (black body has the property to absorb all the incoming radiations).
• Upon absorption, the black body heats up, and the thermopile connected to it converts heat into equivalent voltage
• This voltage is measured across the terminals.

Main Question: Which component(s) do solar panels take?

Well, the solar panels take it all.

The solar panels convert all incoming sunlight (DNI + DHI + reflected) into electricity, as long as light reaches the surface of the solar panel.

Reason

A solar cell converts any photon from any angle that hits the surface into electrical current, as long as the light is strong enough to break the bond. There is no requirement that light must be direct. Even the scattered photons generate current.

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Why keep a solar panel in the direction of Sun?

Ifthe solar panels take it all, then why is there a need to keep a solar panel in the sun’s direction?

We can give it any tilt; still, it will take the radiation to generate current. Yes, this is true, but the intensity of incoming radiation falling on wrongly tilted panel reduces, and hence the output current from the solar panel is reduced.

Let us understand this way:

When I keep the solar panel in the direction of the sunrays. The sunrays strike on 2 points A and B (assuming 2 parallel rays strike the solar panel surface for simplicity and easy understanding).

• Measure the distance AB
• Now keep the solar panel flat on the ground without displacing its end which touching the ground (just make a hinge of the end that is touching the ground).
• The same rays will now be striking at points C and D. Measure CD
• You will notice that CD > AB
• It means that the same number of photons are now farther apart than they were when they hit the solar panel at AB.

You will get more clarity with the two-dimensional view because intensity is a cross-sectional area phenomenon.

Two-dimensional view

It’s the same example but with a two-dimensional view. Assuming n number of photons are falling on area ABEF when the solar panel is facing the sunlight.

• The sunlight intensity in that case would be (n/ar. ABEF).
• Now, keep the solar panel flat on the ground.
• Now, the same number of photons is striking a larger area of CDGH. Here the intensity is (n/ar. CDGH)
• You will appreciate that (n/ar. CDGH) < (n/ar. ABEF). The sunlight intensity on a solar panel is reduced when it is kept away from the direct sunlight.

Let’s quantify it with an example

Assuming on a particular day at noon:

• DNI = 800 W/m²
• DHI = 200 W/m²

Case 1: When the solar panel is placed flat on the ground (not normal to the direct sun rays), making an angle of 45⁰ (zenith angle) with the vertical line.

The total radiation on the solar panel would be:

DNI cos(zenith angle) + DHI {1+cos(tilt)}/2

= 800 cos 45⁰ + 200{1+cos0⁰}/2

= 800 * 0.7071 + 200{2}/2

(Cos45⁰ = 0.7071 and cos0⁰=1)

= 565.68 + 200

Solar intensity on  the panel surface = 765.68 W/m²

Case 2: When the solar panel is placed facing the sunlight.

In this case:

Zenith angle = 0⁰ and the tilt is 45⁰

Putting the values in the same formula, we get:

Solar intensity = DNI cos0o + DHI(1+cos 45o)/2

= 800 x 1 + 200 (1+0.7071)/2

Solar Intensity = 800 + 341.42/2

= 800 + 170.71 = 970.71 W/m², which is more than when the solar panel is directed away from the direct sunlight.

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Summary

Finally, we can say that solar panels take all radiation (direct and diffuse + reflected in the case of bi-facial solar panels). As long as the photons falling on the panel’s surface have enough energy to break the bond and generate an electron-hole pair in the depletion region, the solar panel will produce current.

No matter in what direction the photons are coming from.

However, by keeping the solar panels in the direction of the sunlight, the sunlight intensity falling on their surface increases, causing them to generate more electrons for forming the current.

FAQ

Q1. What is DNI?
Ans: DNI (Direct Normal Irradiance) is the sunlight that travels in a straight line directly from the sun to a surface that faces the sun at 90°.
👉 Used in solar concentrators like CSP, solar cookers, and tracking mirrors.

Q2. What is DHI?
Ans: DHI (Diffuse Horizontal Irradiance) is the scattered sunlight that comes from all directions due to clouds, particles, and the atmosphere.
👉 Even when the sun is behind clouds, DHI is present.

Q3. What is GHI?
Ans: GHI (Global Horizontal Irradiance) is the total sunlight received on a horizontal surface.
It is calculated as:
GHI = DHI + DNI × cos(θ)
(Where θ = angle of sun from vertical)

Q4. Which one is useful for rooftop solar panels?
Ans: GHI is most useful for rooftop solar design, as it gives the total solar energy falling on a flat or fixed panel.

Q5. Why should we care about these terms?
Ans: They help in: i) Estimating solar output, ii) Choosing the right solar panel angle iii) Solar site assessment.

Q6. Can solar panels use both DNI and DHI?
Ans: Yes! Regular rooftop panels (like poly/mono panels) use both DNI and DHI.
But CSP plants (solar towers, dishes) use only DNI.

Q7. Which is higher: DNI or DHI?
Ans: On clear sunny days, DNI is higher, while on cloudy days, DHI becomes dominant.

PRO Tip!
✅ Use GHI for rooftop solar panel estimation
✅ Use DNI for solar concentrators
✅ Always check local values to improve solar feasibility