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A string reads lower than the sum of its panel figures for three reasons that stack. First, panels in series carry the same current, so the weakest panel or the most shaded cell sets the current for the whole string, and a 10 percent current loss in one panel becomes a 10 percent loss across every panel behind it. Second, nameplate power is measured at a defined irradiance and cell temperature, and a hot roof in strong sun produces less voltage per panel than the flash test did, so the sum falls even when every panel is healthy. Third, the number on the meter is a snapshot of a moving quantity, and a handheld irradiance meter such as the TENMARS solar power meter tells you what the light was doing at the moment you sampled, not what the string produced over the day. An I-V curve tracer such as the FLIR PV48 or a PV analyzer such as the PROVA 218 shows you the shape of the loss, which is the only way to separate mismatch from heat from soiling.
The series circuit is why one weak panel taxes the whole string
Panels wired in series share one current path. The current that flows is set by the panel that can pass the least, not by the average. If nine panels can each pass 10 A and one can pass only 8 A because a leaf, a chimney shadow or a failing bypass diode is limiting it, the whole string runs at 8 A. The nine healthy panels do not make up the difference. Their voltage stays roughly the same, so the string power falls by roughly the fraction of current lost, applied to every panel.
This is why the sum of panel nameplate currents is not the string current you will measure. It is also why a string with one shaded panel can read far below the sum even when every panel tests healthy on its own. The bypass diode limits the damage by letting current skip the shaded subgroup, but it does so by removing that subgroup’s voltage from the string, so you lose voltage instead of current. Either way, the string total drops.
Mismatch also appears between panels that are not faulty. Manufacturing tolerance means two panels of the same model can differ slightly in current at the same irradiance. In a long series string those small differences compound, because the lowest current panel sets the operating point for all of them. The effect is small per panel and larger per string.
Nameplate power assumes a light level and a cell temperature the roof rarely delivers
A panel’s rated power is measured under a defined set of conditions: a stated irradiance and a stated cell temperature. Those conditions are a laboratory reference, not a roof at midday. Two things move when the panel is installed. Irradiance on a tilted plane changes through the day and across the array, and cell temperature rises well above the air temperature because the panel absorbs sunlight it cannot convert.
Voltage falls as cells get hotter. That is a property of the semiconductor junction, and it is why a string on a hot afternoon can read below the sum of its panel figures while the same string on a cool bright morning reads closer to them. The current is driven mainly by irradiance, so a slightly hazy sky reduces current across the whole string, and every panel contributes less at once.
So the sum of the panel figures is a reference at one operating point. The string reading is a different operating point. Comparing them directly is comparing two different conditions, not two different quantities. A handheld meter such as the TENMARS solar power meter lets you record the irradiance at the moment you take the string reading, which is the minimum you need to make the comparison meaningful. Without that, you are dividing one number by another number measured under unknown light.
What an I-V curve shows that a single multimeter reading cannot
A multimeter gives you one point on the panel or string characteristic. An I-V curve tracer sweeps the operating point and plots current against voltage, so you see the whole shape. That shape separates the causes of a low reading. A curve that is the right shape but shifted down suggests low irradiance or soiling across the whole array. A curve with a stepped or flattened shoulder suggests mismatch or a bypass diode conducting. A curve whose open-circuit voltage is low suggests heat or a bad connection in the string.
The listings for this class of instrument are specific about what they measure and vague about what they do not. The FLIR PV48 listing states an I-V curve for panels up to 800 W and temperature measurement, and the PROVA 218 listing states an 85 V, 8.5 A range with 1 mV and 1 mA resolution and a manual single point I-V test. Those figures tell you the size of panel or string section the instrument can sweep. They do not tell you the accuracy of the power calculation, which the listing does not publish. A buyer comparing the FLIR PV48 with the PROVA 218 should read the voltage and current ceilings first, because an instrument that cannot reach the string’s open-circuit voltage cannot trace it.
The arithmetic of a low reading, worked through
Suppose a string of ten panels, each rated at a current and voltage you can read from its label. The sum of the panel figures is ten times the per-panel power. Now suppose one panel is limited to 90 percent of the string current by a small shadow, and cell temperature has reduced each panel’s voltage by a few percent. The string current falls to the limited panel’s current, and the string voltage falls by the temperature effect across all ten panels. The measured string power is the product of those two reduced numbers, so the loss is larger than either effect alone.
This is why the gap between the sum and the reading is usually bigger than any single cause would explain. The effects multiply rather than add. A careful technician records irradiance, cell or ambient temperature, and the string current and voltage at the same moment, then compares against the panel label corrected for those conditions. The correction is standard practice, and it is the only way to decide whether the shortfall is normal or a fault.
The table below sets out what each common cause does to current and voltage, so you can match the pattern to the instrument reading you have.
| Cause | Effect on string current | Effect on string voltage | What the I-V curve looks like |
|---|---|---|---|
| Uniform low irradiance | Falls across the whole string | Falls slightly | Same shape, scaled down |
| Heat | Nearly unchanged | Falls per panel | Open-circuit voltage moves left |
| One shaded panel | Set by the shaded panel | Bypass diode removes a subgroup | Step or notch in the shoulder |
| Soiling across the array | Falls proportionally | Nearly unchanged | Same shape, reduced current |
| Bad connection or cable loss | Falls under load | Falls under load | Curve collapses near the knee |
The listing does not publish accuracy figures for the power calculation on either tracer, so treat the curve shape as the reliable output and the absolute power number as indicative until you have checked it against a known reference.
Where the instrument stops being the answer
No meter fixes a mismatch. If the string reads low because one panel is shaded at the same hour every day, the answer is a different string layout, a different mounting position or a DC optimiser, not a better tracer. If the shortfall is heat, the answer is ventilation behind the array, not a more accurate instrument. The instrument’s job is to tell you which of these you have, and it can only do that if you measure irradiance and temperature at the same time as the string.
It is also worth saying plainly what a handheld irradiance meter does not do. It does not measure panel temperature, it does not sweep the I-V curve, and it does not log the string over a day unless the listing says it does. The TENMARS listing describes a manual range selection and a zero adjustment, not a logging function. If you need a day-long record, that is a different instrument class, and the comparison of solar string testers sets out which ones publish that capability.
For a buyer starting from scratch, the practical order is to establish the irradiance and temperature at the moment of measurement, then trace the curve if the shortfall is unexplained, then investigate the wiring. The guide to choosing a multimeter for solar panels covers the first step, and the string tester comparison covers the second.
The sum is a reference point, not a promise
A string that reads below the sum of its panels is usually behaving correctly under conditions the nameplate does not describe. The work is to measure the conditions alongside the string, so the comparison is fair. Once you have irradiance, temperature and an I-V curve at the same moment, the shortfall either explains itself or points to a specific fault. Until then, the gap is arithmetic, not evidence.
Frequently Asked Questions
Does a lower string reading always mean a faulty panel?
No. Heat, uniform low irradiance and soiling all reduce the string reading without any panel being faulty. A fault is more likely when the shortfall is larger than the conditions explain, or when the I-V curve has a step or a collapsed knee.
Can I compare the string reading directly with the sum of the panel labels?
Only after correcting for irradiance and cell temperature. The labels are measured at defined reference conditions. A direct comparison mixes two different operating points and will usually show a shortfall even on a healthy array.
What does an I-V curve tracer add that a multimeter does not?
A multimeter gives one operating point. A tracer sweeps the whole characteristic, so you can see whether the loss is a scaled-down curve, a voltage shift or a step caused by mismatch. That shape is what identifies the cause.
Do the listed tracers publish their power accuracy?
Not stated in the listing bullets. The FLIR PV48 listing states an 800 W per panel curve capability, and the PROVA 218 listing states an 85 V, 8.5 A range with 1 mV and 1 mA resolution. Neither publishes a power accuracy figure, so treat the curve shape as the dependable output.
Will a handheld solar meter tell me why the string is low?
It tells you the irradiance at the moment you measure, which is one of the two conditions you need to correct the comparison. It does not measure panel temperature or trace the curve, so it explains part of the gap, not all of it.
Is a low reading more likely in summer or winter?
The causes differ. In hot weather, cell temperature reduces voltage per panel. In low sun or haze, current falls across the whole string. Both can produce a shortfall against the sum, and the I-V curve shape distinguishes them.