One String Low Output: Why PV1 and PV2 Show Different Readings

This page is about one string or input reading lower than another. If a whole input reads zero, see inverter not taking solar power. If the whole system is low, see why your solar system is producing less than expected.

One string low output: what it means

Panels are wired in series into a string. Each string goes to an inverter input, often labelled PV1, PV2 and so on, and each input is controlled by an MPPT (maximum power point tracker). Some inverters have one string per MPPT; others put two strings in parallel on one MPPT. Our series vs parallel guide explains the wiring.

Two readings matter:

• String voltage depends mainly on how many panels are in series and how hot they are.
• String current depends mainly on how much sunlight reaches the panels.

So when comparing strings, ask: is the difference in voltage or in current?

When PV1 and PV2 readings differ normally

There is no single percentage that marks a fault. Comparisons are only fair between strings of the same design, measured at the same moment in steady sun.

Quick-check table: string current difference

Safety: DC can be dangerous. Panels produce DC voltage whenever light falls on them, even with the inverter off. Do not unplug MC4 connectors, open junction boxes, combiner boxes or the inverter, or walk on panels. If you see a melted connector, smell burning or see smoke, switch off in the manufacturer's sequence your installer showed you, keep away and call a qualified engineer.

Safe owner checks

1. Pick the right moment. Compare readings around midday on a clear, sunny day, not at sunrise, sunset or in passing cloud.
2. Read both inputs at the same time. On the inverter screen or in the app, note voltage, current and power for each PV input. Screens and apps vary in what they show. Our monitoring app guide shows where to look.
3. Check whether the strings match. From your quotation or layout drawing, confirm how many panels are on each input, which roof faces they are on and whether two strings share one MPPT.
4. Look for shade and dirt. From the stairs or the roof edge, look for dust lines, bird droppings, leaves, a new water tank, dish antenna or a neighbour's new floor. See solar panel cleaning and birds under solar panels.
5. Look for visible damage without touching. Cracked glass, brown patches or a scorched junction box are signs of a panel fault. See hot spots and cracks.
6. Clean safely, then compare again on a similar clear day.
7. Record the pattern. Is the gap there all day, only at certain hours, or growing over weeks? Screenshots of a few days are more useful than one reading.

A simple example (illustrative numbers only)

Suppose two strings have the same panels, the same number of panels and face the same way. At noon on a clear day, PV1 shows 9.5 A and PV2 shows 7.5 A, with similar voltages. PV2 is about 20% lower. With clean panels and no shade, that gap is worth investigating. If PV2 instead showed similar current but noticeably lower voltage, the engineer would first suspect a bypass diode or a short panel. These are round numbers for illustration, not readings from a real system.

What an engineer will check

• Open-circuit voltage and current of each string, measured safely with the right instruments and compared with the panel datasheet and the other string.
• An I-V curve trace, where available, which shows steps caused by shading or bypass diodes.
• Thermal imaging in good sun to find hot cells, hot connectors or a hot junction box. See bypass diodes and junction boxes.
• Every connection on the string, including MC4 connectors, for heat or poor crimping. A burnt connector is covered in MC4 connector burnt.
• String fuses and DC breakers on larger systems; see DC combiner boxes and string fuses.
• Strings sharing one MPPT. Solis notes, for example, that strings with different panel counts or different models on one MPPT can push current back into the weaker string, and that shaded strings are better on their own MPPT.

How to prevent one string running low

• Put each roof face or orientation on its own MPPT.
• Keep strings on the same MPPT identical in panel model, count and direction.
• Plan around shade at design stage, especially winter shade from water tanks and parapets.
• Use matched, properly crimped connectors and support the cables so they do not hang or rub.
• Clean regularly in dusty months and after dust storms.
• Check string readings in the app once a month on a clear day, so a new gap is spotted early.

For Solar Works customers

Our engineers design strings from the actual panel and inverter datasheets and record the string layout in your documents. If you see one string running low on a system we installed, WhatsApp 0336 666 4321 with the inverter model, a photo of the inverter screen and screenshots of both PV inputs at midday. Our 1-year service warranty applies under the applicable project terms, and complaints are handled by our own engineering staff. See how after-sales service works.

We can also test strings on systems installed by other companies, as a chargeable visit in Punjab.

Frequently asked questions

Why is one string low output on my inverter?

If both strings are built the same way, the usual causes are dirt, a new shadow, a damaged panel, a bypass diode problem or a poor connection. If the strings have different panel counts or face different directions, a difference is normal.

PV1 PV2 different: is that a fault?

They may have different numbers of panels, face different directions, or get shade at different times. Compare them only at midday on a clear day, and only if they are built the same way.

MPPT one string low: what does it mean?

Two strings in parallel on one MPPT should be identical. If one has fewer panels, a different model or more shade, the stronger string can push current into the weaker one. Ask an engineer to check the pairing.

How much string current difference is normal?

There is no fixed number. Small differences are common between matched strings. A clear, steady gap on sunny days with clean panels and no shade should be checked.

Is a low string dangerous?

Not always, but it can be. A low string caused by a loose or burnt connector can overheat. If a reading jumps around or you see melting or smell burning, switch off in the correct order and call an engineer.

Can I swap the PV1 and PV2 cables to test the inverter?

No. That means unplugging DC connectors under load, which can cause an arc and injury. Leave DC wiring tests to a qualified engineer.

Sources

1. ICS Schneider Messtechnik, "Comparing PV string currents: detecting partial shading, bypass diode issues and connection faults": https://www.ics-schneider.de/pv-strangstrom-vergleichen-teilverschattung-bypassdioden-und-anschlussfehler-erkennen/?lang=en
2. Solis, "Inverter underproduction / no production: causes and solutions for negative current and power": https://usservice.solisinverters.com/support/solutions/articles/73000652957-inverter-underproduction-no-production-causes-and-solutions-for-negative-current-and-power-under-t