How to Use a Solar Irradiance Meter for PV Testing

How to Use a Solar Irradiance Meter for PV Testing – Step-by-Step Guide Malaysia

How do you correctly use a solar irradiance meter when testing a photovoltaic system?

The basic measurement is simple, but sensor orientation, shading, timing and changing weather conditions can significantly affect the usefulness of the reading.

For PV installers and maintenance technicians, a solar irradiance measurement should ideally be recorded together with the relevant electrical and environmental conditions.

The VICI LX-107 Solar Power Meter provides portable solar power measurement in W/m², together with MAX/MIN and Data Hold functions.

This guide explains a practical approach to using a handheld solar power meter during PV inspection.

Quick Answer: How to Measure Solar Irradiance

For a basic PV field measurement:

Select W/m² measurement.

Go to the PV array being inspected.

Position the sensor appropriately relative to the PV module plane.

Make sure nothing is shading the sensor.

Allow the reading to be observed.

Record the W/m² value.

Record the time and weather conditions.

Take related PV electrical measurements as close in time as practical.

Repeat the irradiance measurement if sunlight changes.

The objective is not simply to obtain a number.

The objective is to obtain a solar irradiance value that meaningfully represents the conditions during the PV test.

Step 1 – Understand What You Are Measuring

Solar irradiance is commonly expressed in:

W/m² – watts per square metre

It describes incident radiant power per unit area.

This is different from:

Lux

PV voltage

PV current

Module temperature

Electrical power

PAR / PPFD

Make sure W/m² is actually the parameter required for the job.

Step 2 – Check the Weather Conditions

Before taking the measurement, observe the environmental conditions.

Record whether the sky is:

Clear

Partly cloudy

Cloudy

Hazy

Rapidly changing

Why?

Because solar radiation can change quickly.

If a cloud passes over the site between your irradiance measurement and electrical measurement, the two results may not represent the same operating conditions.

Step 3 – Go to the Area Being Tested

Measure irradiance at a location relevant to the PV system being inspected.

For a large rooftop or solar farm, a reading taken far away from the problem area may not always represent exactly the same local conditions.

Portable solar power meters are useful because technicians can bring the instrument directly to the inspection location.

Step 4 – Position the Sensor Correctly

Sensor orientation is important.

If the purpose is to understand solar radiation incident on the PV module plane, position the sensor appropriately relative to that plane.

For example, if the modules are installed at an angle, simply holding the sensor horizontally may not represent exactly the same incident radiation as the module surface.

Use a consistent orientation appropriate to the measurement objective.

Step 5 – Avoid Shading the Sensor

This sounds obvious, but it is an easy field mistake.

The sensor can be shaded by:

Your body

Your hand

Your head

Tools

Test equipment

Rooftop structures

Nearby modules

Pipes

Air-conditioning equipment

Other objects

Even partial shading can affect the reading.

Before recording the measurement, check that the sensor has an unobstructed exposure appropriate to the test.

Step 6 – Read the Solar Irradiance in W/m²

Observe the displayed W/m² measurement.

For example:

Solar irradiance: ______ W/m²

Do not immediately interpret the number as “good” or “bad”.

The value should be considered together with:

Time

Weather

Module temperature

Electrical measurements

PV system condition

Step 7 – Use Data Hold When Needed

The VICI LX-107 includes a Data Hold function.

This can be useful when the instrument is positioned where it is inconvenient to write down the value immediately.

Take the measurement, hold the displayed value and record it safely.

Data Hold is a convenience function—it does not change the measurement itself.

Step 8 – Use MAX/MIN to Observe Variation

The LX-107 also provides MAX/MIN functionality.

This can be useful when solar radiation is fluctuating.

For example, under intermittent cloud conditions, the technician can observe how the measured solar radiation varies.

However:

Do not automatically use the maximum reading as the irradiance corresponding to a PV electrical measurement taken at another time.

For troubleshooting, timing matters.

Step 9 – Record the Measurement Time

Always consider recording the time.

Instead of writing:

Irradiance: 850 W/m²

write:

Time: 11:42 AM

Irradiance: 850 W/m²

Weather: Clear / Partly Cloudy / Hazy

This makes the measurement much more useful later.

Step 10 – Take PV Electrical Measurements

Depending on the maintenance procedure, technicians may also measure:

DC voltage

DC current

Other relevant electrical parameters

The important principle is:

Take the irradiance measurement and related PV measurements as close together in time as practical.

This is particularly important under changing cloud conditions.

Example PV Field Record

A practical field sheet may look like this:

Site: ____________________

Array / String: ____________________

Date: ____________________

Time: ____________________

Weather: ____________________

Solar irradiance: __________ W/m²

Module temperature: __________ °C

DC voltage: __________ V

DC current: __________ A

Visible shading: Yes / No

Module contamination: ____________________

Remarks: ____________________

This creates a much better technical record than simply writing:

“Solar panel output low.”

What If the Irradiance Reading Keeps Changing?

First check whether environmental conditions are changing.

Possible causes include:

Passing clouds

Haze variation

Sensor movement

Changing orientation

Accidental shading

Outdoor solar radiation is naturally variable.

If conditions are unstable, take measurements accordingly and document the conditions.

Should You Measure in the Morning or Afternoon?

There is no single clock time that is correct for every PV test.

The correct timing depends on the purpose of the inspection.

For troubleshooting, what matters is understanding the irradiance present during the corresponding electrical measurement.

If comparing measurements taken at different times, consider that solar position, temperature and weather may also have changed.

Can You Compare Today's Reading with Last Month?

You can record and compare data, but interpretation requires care.

A reading from today and a reading from last month may have been taken under different:

Solar conditions

Times

Temperatures

Weather

Sensor orientations

Module conditions

For meaningful trend comparison, use a consistent measurement procedure.

Common Solar Irradiance Measurement Mistakes

Mistake 1 – Using a Lux Meter

Lux and W/m² are different measurements.

For solar irradiance work, use an instrument designed for the required solar power measurement.

Mistake 2 – Holding the Sensor at Random Angles

Changing orientation can affect the reading.

Mistake 3 – Shading the Sensor

The technician can accidentally become part of the measurement problem.

Mistake 4 – Measuring Irradiance Long Before the PV Test

Weather may change between measurements.

Mistake 5 – Recording Only W/m²

Record time and environmental conditions too.

Mistake 6 – Assuming 1000 W/m² Means Full STC

1000 W/m² is one STC reference condition, but field measurement of approximately 1000 W/m² does not automatically reproduce all Standard Test Conditions.

Mistake 7 – Assuming the Irradiance Meter Diagnoses the Fault

It does not.

It measures solar radiation.

Low PV Output + Low Irradiance

Suppose PV output appears lower than expected.

You measure solar irradiance and find that available solar radiation is also relatively low under the current conditions.

This suggests environmental conditions should be considered before concluding that the PV equipment is faulty.

Further testing may still be required.

Low PV Output + Strong Irradiance

Now suppose solar irradiance is relatively strong, but system performance remains unexpectedly low.

Technicians may decide to investigate other factors such as:

Dirt or contamination

Partial shading

Module condition

Module temperature

Connectors

Cables

String differences

Inverter conditions

How to Use a Solar Irradiance Meter for PV Testing – Step-by-Step Guide Malaysia

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