What Is a PV String?

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A PV string is a group of solar panels connected in series, so their voltages add together and the group feeds one input of a charger or inverter. The term appears in the Victron SmartSolar manual, which warns about a series string with too few panels, and in Fluke’s solar guidance, which talks about PV modules and strings. This explainer uses those sources to explain the term, why the string voltage matters, and how to measure it without creating a hazard.

Quick answer

In series, panel voltages add and the current stays the same as one panel. The Victron manual describes an array problem as having an insufficient number of panels in a series string (SmartSolar manual, PDF page 53).
The open-circuit voltage of the array must stay below the charger’s maximum rated voltage. The manual advises a 10 percent margin where cold temperatures can raise the voltage (PDF page 58).
Fluke gives voltage ranges for PV work, from 30 to 40 V for a single module up to 1500 V for large arrays, and says the meter’s voltage rating must exceed the system maximum (Fluke Learn, opened 2026-10-27).
Measuring a string can be dangerous. Only a qualified technician should handle voltages above 50 V (PDF page 53).

What a string is

A solar module is a group of cells. When modules are wired in series, the positive of one is joined to the negative of the next, and the chain has a single positive and a single negative end. The chain’s voltage is the sum of its modules’ voltages, and its current is limited by the weakest module. Our explainer uses the word string in that sense. The Victron manual uses it in the same way when it refers to panels in a series string (PDF page 53). The sources we opened do not give a formal engineering definition of a string, so the description above is a plain reading of how the manual uses the term.

Strings are then connected in parallel or fed to a charger input. The manual does not give a full wiring method for every array type, so check the charger’s own limits before you connect a string.

Why the string voltage matters

String voltage has two limits in the Victron manual. The first is the lower limit that lets the charger start. The manual says the PV voltage must be 5 V higher than the battery voltage before charging starts, and it lists low sunlight and a wrong panel count among the causes of a low PV voltage (PDF page 53). A string with too few panels may never reach that level even in good light.

The second limit is the upper one. The manual says that if the PV voltage exceeds the charger’s rated maximum, the charger stops charging and shows an overvoltage error. It advises checking that the open-circuit voltage of the array is lower than the charger’s maximum rated voltage. For cold climates, where the open-circuit voltage can rise above its rated value near or below 10 degrees C, the manual suggests keeping an extra 10 percent margin (PDF page 58).

A simple calculation shows why this matters. If each panel has a given open-circuit voltage, the string’s open-circuit voltage is the number of panels times that figure. The manual’s limit applies to the array as a whole. The sources do not give a formula for cold correction, so this article does not provide one.

Measuring a string safely

The manual’s advice on PV measurement starts with the charger display or the app. If those are not available, it says to use a multimeter in DC mode at the charger’s PV terminals (PDF page 53). A string can carry a dangerous voltage even when it is not producing power, because the panels generate voltage whenever they are exposed to light. The manual’s warning applies: voltages above 50 V are generally considered dangerous, and only a qualified technician should handle them.

Fluke’s solar guidance adds two points. First, the voltage range of a PV system can span from 30 to 40 V for a single module up to 1500 V for large arrays, and the tool must be rated above the maximum voltage in the application. Second, Fluke says to look for a CAT rating on the tool itself (Fluke Learn, opened 2026-10-27). For rooftop residential systems, Fluke says CAT III 600 V tools will suffice, and for commercial systems CAT III 1000 V is appropriate. Fluke’s wording is a guideline. Check the meter’s own rating and your installer’s specification before use.

This article does not cover current measurement in a PV circuit. Our sources do not give a safe method for that, so none is described here.

Where a multimeter fits

A multimeter with the right CAT rating and a DC voltage setting can read the string’s open-circuit voltage and compare it with the charger’s rated maximum. For the meter features that matter for this work, see our guide to multimeters for solar panels. For a basic method of reading DC voltage with any meter, our step-by-step multimeter guide gives the sequence.

Common mistakes the manual names

Too few panels in a series string, which can keep the PV voltage too low to charge (PDF page 53).
Reverse PV polarity, where the positive and negative PV wires are swapped at the charger (PDF page 53).
An open-circuit voltage that is too close to the charger’s rated maximum in cold weather (PDF page 58).

How we chose these sources

We used the Victron SmartSolar MPPT manual, revision 13 dated 10/2026, which names the series string in its troubleshooting and voltage-limit sections. We added the Fluke Learn article on solar tools because Fluke is a meter manufacturer whose guidance on voltage ratings applies to this measurement. We opened both on 2026-10-27 and read them as text.

What we could not verify

The sources we opened do not give a textbook definition of PV string. The description above reflects how the Victron manual uses the term.
The sources do not give a maximum number of panels per string for any specific charger, so no number is given. Use the charger’s own specification.
The sources do not give the cold-weather correction formula, only the 10 percent margin advice.
We did not verify Fluke’s voltage range or CAT guidance against a second Fluke document.

FAQ

Is a string the same as an array?

Not exactly. An array is the whole group of panels. A string is one series chain within the array. The Victron manual refers to the array and to panels in a series string as separate items (PDF page 53).

Why is my string voltage too low to charge?

The manual lists too few panels in the series string, shade, dirt, wrong orientation and wiring faults among the causes (PDF page 53).

What voltage can a string reach?

The Victron manual says some models accept PV voltages up to 250 V DC. Fluke’s range runs up to 1500 V for large arrays. Check the charger label and the array design.

Can I use any multimeter on a PV string?

Choose a meter with a CAT rating and a voltage rating above the system maximum, as Fluke advises. Check the meter’s own markings.

Bottom line

A PV string is a series chain of panels whose voltages add together. Its open-circuit voltage must stay below the charger’s maximum rating, and too few panels in a string can stop charging. Measure only with a meter rated for the voltage, and leave anything above 50 V to a qualified technician.

Related guides

Sources

Victron Energy, MPPT solar charger manual (SmartSolar MPPT 150/70 up to 250/100 VE.Can), rev 13 – 10/2026, PDF page 53 (PV and battery voltage check; PV array troubleshooting). https://www.victronenergy.com/upload/documents/Manual_SmartSolar_MPPT_150-70_up_to_250-100_VE.Can/29694-MPPT_solar_charger_manual-pdf-en.pdf (opened 2026-10-27).
Same manual, PDF page 58 (section 8.5.5, PV voltage too high). Same URL (opened 2026-10-27).
Fluke Corporation, “The right solar tools based on CAT and voltage ratings”, Fluke Learn. https://www.fluke.com/en-us/learn/blog/renewable-energy/the-right-solar-tools-based-on-cat-and-voltage-ratings (opened 2026-10-27; the page shows no revision date).

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