Without an irradiance figure, a panel output reading means nothing: the same module produces half as much under thin cloud as it does in full sun. This hub compares the meters that measure the light itself, on stated range, stated accuracy and the extra measurements a site survey needs.
Measure the Light
Then the Panel
Compare the Two
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Every instrument below reads solar irradiance. They divide into three groups: pocket meters that do nothing else, survey tools that add tilt, compass and temperature for siting an array, and test kits where an irradiance head is bundled with a clamp meter or a PV tester. They are compared on stated measuring range, published accuracy, sensor type and the additional measurements each one offers. We ran no comparisons against a reference pyranometer and publish no measurements of our own.
The instrument the rest of the Fluke solar range is built to work with.
Five instruments in one body: radiation, inclinometer, compass and two temperature channels.
The clearest published specification on the page: range and accuracy both stated.
No pick matches that combination yet. Try a wider budget or a different measurement.
| Product | Type | Stated range | Stated accuracy | Extra measurements | Sensor stated | |
|---|---|---|---|---|---|---|
| Fluke IRR1 | Dedicated irradiance meter | Not stated | Not stated | Not stated | Not stated | View |
| Seaward Solar Survey 100 | Survey tool | Not stated | Not stated | Inclinometer, compass, 2 temperature channels | Not stated | View |
| Triplett SPLT400 | Survey tool | Not stated | Not stated | Temperature, tilt and direction | Not stated | View |
| TENMARS solar power meter | Dedicated irradiance meter | 0 to 1999 W per sq m, 0 to 634 BTU | Plus or minus 10 W per sq m | None stated | Not stated | View |
| TENMARS handheld irradiance meter | Dedicated irradiance meter | 0 to 1999 W per sq m, 0 to 634 BTU | Not stated | MAX and MIN hold, 199.9 or 1999 scale | Not stated | View |
| DANOPLUS digital solar meter | Dedicated irradiance meter | 0.1 to 1999.9 W per sq m | Drift under 1.5 percent per year | Dual units W per sq m and BTU | Not stated | View |
| AquaHorti AH-SOL | Dedicated irradiance meter | Not stated | Not stated | None stated | Industrial photodiode, made in Japan | View |
| Fluke 393FC clamp meter with irradiance meter | Clamp meter kit with irradiance head | Not stated | Not stated | CAT III 1500 V clamp, solar test leads | Not stated | View |
All values are quoted from the manufacturer listings at the time of research. Only the TENMARS B07HY8YVKQ publishes an accuracy figure on its listing, and the DANOPLUS publishes a drift figure rather than an accuracy figure; the two are not the same thing. Where a listing is silent the cell reads Not stated rather than carrying a value taken from a datasheet or a similar model.
Four things separate these instruments, and price is not one of them:
An unstated accuracy is not an accuracy. If the figure has to appear in a commissioning document, choose an instrument that publishes it or obtain it from the manufacturer before you buy.
Full midday sun at sea level is roughly 1000 W per square metre. A 2000 W per square metre range covers reflection and altitude with headroom; anything much lower will clip on a bright day.
Tilt and compass functions matter only when you are deciding where an array goes. For troubleshooting an existing array, they add cost and nothing else.
A performance ratio needs module temperature as well as irradiance. Instruments with two temperature channels let you capture ambient and module together instead of estimating one.
Full sun at sea level with the sensor square to the sun is close to 1000 W per square metre, which is the figure module ratings are defined at. Thin cloud typically halves it, heavy overcast can take it below 200, and a sensor tilted away from the sun reads lower than one facing it. The number only means something alongside the angle you held the meter at.
In the plane of the array, every time. The modules receive light at their own tilt and orientation, so a horizontal reading is a different quantity. Lay the sensor on the module glass or hold it parallel to it, and keep your own shadow and reflections off the sensor.
Because short-circuit current scales almost linearly with irradiance. A module producing 6 A at 1000 W per square metre will produce about 3 A at 500, and without the irradiance figure those two readings look like a healthy panel and a failed one. Irradiance turns an output measurement into a comparison you can defend.
No. A lux meter is weighted to human vision and measures only the visible band, while a photovoltaic module responds across a wider spectrum including the near infrared. Conversion factors between lux and W per square metre exist but depend on the light source and are unreliable outdoors. Use an instrument built for solar irradiance.
Ask the manufacturer for the specific model, because none of these listings states an interval. The DANOPLUS listing does publish a drift figure of under 1.5 percent per year, which indicates the kind of change to expect. If readings feed into commissioning documents or warranty claims, an annual calibration check against a reference is normal practice.
The IRR1 is the irradiance meter that a professional PV testing workflow is usually built around, which is why it heads this page despite a listing that publishes almost no numbers. Fluke describes it as covering the critical measurements needed for installing, testing, maintaining and reporting on solar panels and photovoltaic systems in one easy-to-use tool, and it is designed to hand its readings to the rest of the Fluke solar range rather than to be used in isolation.
That integration is the argument for it. A performance ratio calculation needs irradiance, module temperature and electrical output captured at the same moment, and an instrument family designed to do that together produces a defensible report. An irradiance figure written on a clipboard and typed in later is worth less.
Be clear about what the listing does not say. It states no measuring range, no accuracy figure, no sensor technology and no temperature measurement capability. Every one of those is published in the Fluke datasheet rather than on the listing, and a buyer who needs an accuracy percentage for a commissioning document should get it from Fluke before ordering. Judged on published listing detail alone, the TENMARS meters below are better documented.
Siting an array asks four questions at once: how much light reaches this roof, at what pitch, facing which way, and how hot does the surface get. The Solar Survey 100 answers all four. Seaward states a five in one tool combining solar radiation measurement, an inclinometer, a compass and two channels of temperature measurement, which is the exact set a surveyor writes down before specifying modules.
The inclinometer and compass are the parts that turn an irradiance reading into a design input. Knowing that a roof receives 820 W per square metre at this instant is interesting; knowing that it receives it at a 35 degree pitch facing 15 degrees west of south is what goes into the yield model. Doing it with one instrument rather than a meter, a phone app and a tape measure removes the transcription errors.
The two temperature channels suit an ambient and a module measurement taken together, which is what a temperature-corrected performance check needs. The listing does not state the measuring range, the accuracy, the sensor type or the logging capacity, so as with the Fluke the detailed specification lives in the manufacturer documentation rather than on the listing.
This is the meter to buy if you want to know what you are getting. TENMARS publishes both halves of the specification that the premium instruments leave on their datasheets: a range of 0 to 1999 W per square metre with a parallel 0 to 634 BTU scale, and an accuracy of plus or minus 10 W per square metre and plus or minus 3 BTU per square foot hour. Nothing else on this page states an accuracy figure on the listing.
Ten watts per square metre is roughly one percent of full sun, which is entirely adequate for the job most people need an irradiance meter for: establishing whether conditions are good enough for a meaningful panel test, and normalising one module reading against another taken ten minutes later. It is not a reference-grade figure and a laboratory would not accept it, but it is honest and it is published.
MAX and MIN tracking hold the extremes over a measurement period, which is useful when clouds are moving and you want to know the range the array saw. The dual unit scale suits mixed documentation. There is no tilt, no compass and no temperature channel, so for survey work the Seaward or the Triplett is the better instrument.
Short answer: the Fluke IRR1 is the instrument to buy if the rest of your solar kit is Fluke and the readings feed a commissioning report; the Seaward Solar Survey 100 is the one for site surveys because it adds an inclinometer, a compass and two temperature channels; and the TENMARS 2000 W per square metre meter is the value pick and, unusually, the only one on this page that publishes both its range and its accuracy on the listing.
A photovoltaic module is a light-driven current source. Its short-circuit current rises and falls almost in step with the light falling on it, which means any current or power reading taken without knowing the irradiance is a number with no denominator. Measure a string at 11am and again at 2pm and the two readings will differ by more than most faults do. This is why commissioning standards express performance as a ratio between what the array produced and what the light should have allowed, and why the irradiance meter is the first instrument out of the bag rather than an accessory. Our explainer on what solar irradiance is covers the physics, and irradiance vs panel output covers the relationship in practice.
Eight instruments, and exactly one publishes an accuracy figure on its listing. The TENMARS B07HY8YVKQ states plus or minus 10 W per square metre and plus or minus 3 BTU per square foot hour. The DANOPLUS states a drift of under 1.5 percent per year, which describes how the instrument ages rather than how close it reads today. The other six say nothing, including both Fluke instruments and the Seaward. This is not evidence that they are inaccurate; Fluke and Seaward both publish full specifications in their datasheets, and those instruments are widely used for exactly this work. It is evidence that the listing is the wrong place to shop for a figure you need to defend. If an accuracy percentage has to appear in a report, obtain it in writing from the manufacturer before ordering, and treat the table above as a guide to what is visible rather than a ranking of what is good.
The published accuracy of any of these instruments is smaller than the error you can introduce by holding it badly. Irradiance measured on a plane falls with the cosine of the angle between that plane and the incoming light, so tilting the sensor ten degrees away from the array plane loses a measurable slice of the reading, and holding it flat when the modules sit at 35 degrees produces a number that has nothing to do with the array. Lay the sensor on the module glass where the design allows it, keep your own shadow clear, avoid standing where a light-coloured wall or a neighbouring array reflects into the sensor, and take the reading and the panel measurement within the same minute. The survey instruments exist partly to remove this error, because an inclinometer tells you the plane you are actually measuring in.
Two instruments here, the Seaward Solar Survey 100 and the Triplett SPLT400, add tilt and orientation to the light measurement. That combination belongs to a specific job: standing on a roof that has no array yet and deciding whether one is worth installing and at what pitch and bearing. For that work the extra functions are the point, and carrying one instrument instead of a meter plus a phone compass removes a transcription step. For everything afterwards, commissioning, annual checks and chasing a fault in an installed system, they are dead weight, and a pocket meter that reads light accurately is the better buy. The Fluke 393FC kit is a third case again: an irradiance head bundled with a CAT III 1500 V clamp meter and solar test leads, for a technician who wants the electrical and light measurements from one purchase. Our how to choose a solar irradiance meter guide works through the decision in more detail.
The narrower guides that sit under this hub:
For a one-off check on a domestic array, the TENMARS is enough and states what it can do. For a contractor surveying roofs, the Seaward pays for itself in removed instruments and fewer noted-down errors. For a commissioning engineer whose readings end up in documents somebody else relies on, buy into an instrument family designed to work together, get the accuracy figure from the manufacturer in writing, and keep a calibration record.
Meters were compared on manufacturer listing text: stated measuring range and units, published accuracy or drift figures, sensor technology where named, additional measurements such as tilt, compass and temperature, and the stated purpose of the instrument. We did not compare any meter against a reference pyranometer, did not test any unit and publish no measurements of our own. Where a specification appears only in a datasheet and not on the listing, the table reads Not stated. Amazon data supplies images, availability and links only.
A solar insulation resistance tester that works on energised PV strings up to DC 1000 V, with test voltages of 125, 250, 500 and 1000 V, a range to 4000 megohms, automatic discharge, 1000 stored result sets and USB or Bluetooth transfer.
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A 12 volt battery, cranking and charging system tester covering 100 to 2000 CCA that reports state of health, state of charge, voltage, current and CCA, drawing its own power from the battery under test.
Read Guide→A handheld solar irradiance meter reading 0 to 1999 watts per square metre or 0 to 634 BTU per square foot per hour, with a selectable 199.9 or 1999 scale, maximum and minimum tracking, data hold and manual zeroing.
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