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TENMARS · Solar Power Meter, 2000 W/m² Range Handheld Solar Irradiance Meter with Dual Units, MAX/MIN Tracking, Data Hold, Manual Zeroing, Digital Sun Meter for Solar Panel Evaluation and Site Testing
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A handheld solar irradiance meter reading 0 to 1999 watts per square metre, or the same span expressed as 0 to 634 BTU per square foot per hour, with accuracy given as plus or minus 10 watts per square metre, a selectable 199.9 or 1999 scale, maximum and minimum tracking, data hold and manual zeroing. One accuracy figure covers the whole range, and that single figure behaves very differently at the two ends of it: about one percent of full sun, and about ten percent of a dull morning.
One accuracy figure across a range that spans ten to one
An accuracy quoted as an absolute amount rather than as a percentage of reading always has this property, and working it out is the first thing to do with any instrument specified that way.
Standard test conditions for a photovoltaic module are defined at 1,000 watts per square metre, which is a convenient anchor. At that level, plus or minus 10 watts per square metre is one percent, which is a good figure for a handheld instrument and the listing is entitled to be pleased with it.
Now move down the scale:
- At 1,000 watts per square metre the allowance is 1 percent.
- At 500 it is 2 percent.
- At 200 it is 5 percent.
- At 100, which is an overcast winter morning, it is 10 percent.
Nothing is wrong with that specification. It simply means the instrument is a different proposition at the bottom of its range than at the top, and the readings worth taking with it are the bright ones. Checking a roof on a clear day is well inside its competence. Comparing two readings taken under thin cloud, where the whole difference might be 30 watts per square metre, is not.
One note on the range itself. The product name offers a 2000 W/m2 headline while the bullet states 0 to 1999. That is a rounded headline against a real display limit rather than a contradiction. Take the bullet as the specification, because a three and a half digit display tops out at 1999 counts and that is where the number comes from.
Both unit scales convert correctly, which is a real credit
This is the kind of check that usually finds a problem and here does not.
One watt per square metre is about 0.317 BTU per square foot per hour. Multiply the 1999 watts per square metre top of scale by that and you get about 634, which is exactly the BTU figure in the bullet. Do the same with the accuracy and 10 watts per square metre becomes about 3.2 BTU per square foot per hour, against the 3 stated.
Two independent conversions that both land where they should is evidence that the specification was lifted from a real datasheet rather than assembled by a copywriter. It is a small thing and it is worth saying, because the opposite is common enough that we check it on every page.
It also means the dual unit feature is genuinely useful rather than decorative. Irradiance in watts per square metre is the language of photovoltaic work. BTU per square foot per hour is the language of solar heat gain through glazing and of heating and cooling load calculations. One sensor reading serves both, and the conversion inside the instrument is right.
Manual zeroing is a disclosure, not a missing automation
The fifth bullet describes an easy zero adjustment control and tells you to use it before measurement to establish the correct starting point.
It would be easy to read that as a missing feature, a meter that cannot zero itself. It is better read as an admission most listings in this category avoid making.
A light sensor and its amplifier produce a small output when no light reaches them. That offset is not stable: it drifts with temperature and over the life of the instrument. Every instrument of this type has one. An instrument with a manual zero tells you about it and hands you the means to remove it. An instrument that says nothing still has the offset, and you carry it into every reading you take.
In practice this is one extra action: cover the sensor, zero, uncover, measure. Doing it at the start of each set of readings, and again if the instrument has been sitting in the sun or has just come out of a cold van, is the difference between the stated accuracy and something worse than it.
Cosine response is the specification that is not here
If one figure could be added to this listing it should be the cosine error.
The physics is straightforward. A flat surface intercepts a beam of light in proportion to the cosine of the angle between the beam and the surface normal. Hold a sensor square to the sun and it sees the full beam. Tilt it by 60 degrees and the same beam spreads over twice the area, so the irradiance on that surface halves. A sensor behaving correctly follows that relation exactly, which is what makes its reading meaningful in the plane of a tilted array.
Real sensors deviate, and the deviation grows at steep angles because of reflection at the window and the geometry of the diffuser above the cell. Instruments that have been characterised publish it as a cosine error over a stated range of angles.
Without it there is no way to separate sensor behaviour from scene behaviour when you measure in the array plane in winter, or early and late in a day, which are exactly the conditions where the deviation would be largest. The same gap applies to spectral response, which determines how the sensor weights different wavelengths and therefore how well a reading under cloud compares with one in direct sun. Neither figure appears, nor does the sensor type, nor any instrument classification. For what those figures do, see choosing a solar irradiance meter.
The table, and the four absences that matter most
| Specification | In the listing |
|---|---|
| Range, metric | 0 to 1999 watts per square metre |
| Range, imperial | 0 to 634 BTU per square foot per hour |
| Accuracy, metric | Plus or minus 10 watts per square metre |
| Accuracy, imperial | Plus or minus 3 BTU per square foot per hour |
| Scale selection | Manual, 199.9 or 1999 |
| Resolution | Not stated in the listing bullets, implied by the scale digits |
| Hold functions | Maximum and minimum tracking, data hold |
| Zeroing | Manual, by a control on the instrument |
| Cosine error | Not stated in the listing bullets |
| Spectral response | Not stated in the listing bullets |
| Sensor type | Not stated in the listing bullets |
| Instrument classification | Not stated in the listing bullets |
| Temperature coefficient | Not stated in the listing bullets |
| Operating temperature range | Not stated in the listing bullets |
| Cell temperature probe or input | Not stated in the listing bullets |
| Data logging or output | Not stated in the listing bullets |
| Battery type and life | Not stated in the listing bullets |
| Ingress protection rating | Not stated in the listing bullets |
The missing cell temperature input is the absence that shapes how you use the instrument. Comparing a measured array output against a datasheet needs two corrections, one for irradiance and one for cell temperature, because module output falls measurably as the cells warm. This meter supplies the irradiance half, which is the half you cannot sensibly estimate, and leaves the temperature half to a second instrument. That is a normal division of labour and worth knowing in advance, as we set out in irradiance against panel output.
Who should own one, and what it cannot finish alone
This suits a site surveyor or an installer who needs to know how much light is arriving, in bright conditions, at a plane they can hold the sensor in. The maximum and minimum tracking is well matched to that, because standing with a meter watching a display through passing cloud is tedious and the instrument will remember the extremes for you. The scale switch earns its place on shading studies where the numbers are small. Correct unit conversions and an honest zeroing instruction suggest a specification you can take at face value.
It is not a reference instrument. With no classification, no cosine error, no spectral response and no sensor type, a reading from it is a good field figure rather than a characterised one. It is also not a complete performance tool on its own, because the cell temperature it needs to be paired with is not something it measures. Anyone weighing it against other options should start with our roundup of solar irradiance meters and look specifically for which of them publish a cosine figure.
Recommended Tools
Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.
Frequently Asked Questions
Is plus or minus 10 watts per square metre a good accuracy figure?
It depends entirely on what you point it at. Standard test conditions for a photovoltaic module are defined at 1,000 watts per square metre, so near full sun the allowance is about one percent, which is respectable for a handheld instrument. On a dull morning reading 150 watts per square metre the same allowance is nearly seven percent, and at 100 it is ten percent. An accuracy quoted as an absolute amount always behaves this way, which is why the useful readings from a meter like this are the bright ones.
Do the two unit ranges agree with each other?
They do, and it is worth checking because listings often do not. One watt per square metre is about 0.317 BTU per square foot per hour, so 1999 watts per square metre converts to about 634, which is the figure in the bullet. The accuracy converts too: 10 watts per square metre is about 3.2 BTU per square foot per hour against the 3 stated. Two independently correct conversions suggest the specification came from a real datasheet rather than from someone writing copy.
What does the 199.9 scale change?
The step size of the display rather than the tolerance. The listing describes manual selection of the 199.9 or 1999 scale and explains it as direct control over the displayed resolution. A display whose top reading is 199.9 has a tenth of a unit as its last digit, so low light readings move in finer steps on that scale. The accuracy figure is quoted once for the instrument, so a finer step does not make a weak light reading more accurate, it only stops the display from rounding it away.
Why does the listing ask you to zero it by hand?
Because a light sensor and the amplifier behind it produce a small output when no light reaches them, and that offset drifts with temperature and with age. Nulling it against a covered sensor before a set of readings removes it. This is a disclosure rather than a shortcoming. A meter that never mentions zeroing still has the offset, it simply does not tell you about it or give you a way to remove it.
What specification would you most want added?
Cosine response. An irradiance sensor should respond to a beam arriving at an angle in proportion to the cosine of that angle, because that is how much energy actually lands on a flat surface. Real sensors deviate, increasingly so at steep angles, and instruments that have been characterised publish the deviation as a cosine error over a stated range of angles. Without it there is no way to know how much of a reading taken on a low winter sun or in the plane of a tilted array is sensor behaviour. Spectral response, sensor type and any instrument classification are also absent.
Can I use this to check whether an array is underperforming?
You can take the irradiance half of that calculation, and it is the harder half to guess. What you also need is the cell temperature, because module output falls as the cells warm, and this meter does not measure it. Nothing in the bullets describes a temperature probe or an input for one. So the meter gives you an irradiance figure to correct against and leaves the temperature to a second instrument before a field measurement can be compared with a datasheet.
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