Solar Panel Stringing and Wiring: Series, Parallel, MPPT

A string that is one panel too long can push an inverter past its voltage limit on the first cold morning. This guide shows the checks that catch it: cold and hot voltage, current at every input, and a record each reviewer can follow.

Stringing links the roof layout to the electrical equipment. This guide is for installers and designers who plan strings. It is a planning guide. It does not tell you how to work on live wires.

Last updated: November 2026

1. What are series, parallel and MPPT?

In a series string, the same current flows through every panel, and the panel voltages add. In parallel, strings share one voltage, and their currents add.

Take ten identical panels. Assume each works at 37.2 V and 11.47 A. These are teaching numbers, not a real product.

The arithmetic: 10 × 37.2 = 372 V. 2 × 11.47 = 22.94 A. 3 × 11.47 = 34.41 A. Real systems lose some power, and heat changes the numbers.

A string is not an MPPT. An MPPT is the part of the inverter that finds the best working point for the strings on it. Two connectors on an inverter can feed one MPPT. SMA’s STP50-40 technical data shows this. It lists 6 independent MPP inputs and 2 strings per MPP input. Read your inverter’s manual to see how its inputs map to MPPTs.

The NREL System Advisor Model (SAM) help page uses the same ideas: modules in a string, strings in parallel, and subarrays (groups of strings with the same setup).

2. Which limits must you collect first?

Collect these for the panel: exact model, datasheet revision, Voc, Vmp, Isc, Imp, and the temperature coefficients. Voc is open-circuit voltage, with no load. Vmp and Imp are voltage and current at maximum power. Isc is short-circuit current. All of these are listed at STC (standard test conditions: 25°C cell temperature and 1,000 W/m² of light).

Also collect the panel’s maximum system voltage, its series fuse rating and the connector type.

For the inverter, record:

• The maximum DC input voltage.
• The MPPT voltage range.
• The start-up voltage and the minimum operating voltage.
• The maximum current per input, and the short-circuit current it can accept.
• The number of strings each input allows.

These limits are different things. Do not swap one for another. SMA’s STP50-40 lists them separately, as the chart shows:

The limits: maximum input voltage 1,000 V, MPP voltage range 500 V to 800 V, minimum input voltage 150 V, initial input voltage 188 V, 20 A usable current per input and 30 A short-circuit current per input. These numbers belong to that model. Use the manual for your own inverter and region.

SMA’s DC connection rules for that inverter also say:

• All panels on an input should be the same type, and aligned and tilted the same way.
• Each string on an input needs the same number of panels.
• On the coldest day on record, the array’s open-circuit voltage must never exceed the inverter’s maximum input voltage.
• With one string on an MPPT and AFCI (arc-fault protection) on, the input current must not exceed 16 A.

3. How do you check voltage at hot and cold temperatures?

Cold weather raises voltage. Heat lowers it. So you check both ends.

• Cold Voc must stay below the inverter’s maximum DC voltage.
• Hot Vmp must stay above the bottom of the MPPT range.
• Start-up voltage must also be met.

For a coefficient given in %/°C, a simple correction is:

V(T) = V(STC) × [1 + (coefficient ÷ 100) × (T − 25)]

T is the temperature in °C. If the coefficient is already a fraction per degree, do not divide by 100 again. Use the coefficient for the voltage you are correcting. The Pmax coefficient is not the Vmp coefficient.

Which temperature you use depends on the maker’s rules. Canadian Solar’s installation manual (revision 3.1) corrects Voc with the module’s Voc coefficient and the lowest expected ambient temperature at the site. Follow the rule in your own equipment documents.

Worked example for voltage

These inputs are teaching numbers. They do not describe a real 440 W panel or an approved system.

Maximum panels per string (cold limit).

• Cold Voc = 37.20 × [1 − 0.0027 × (−10 − 25)] = 37.20 × 1.0945 = 40.7154 V.
• 600 ÷ 40.7154 = 14.74, so 14 panels at most.
• Check: 14 panels give 570.0156 V. 15 panels give 610.731 V, which is over 600 V.

Minimum panels per string (hot limit).

• Hot Vmp = 31.10 × [1 − 0.0034 × (63 − 25)] = 31.10 × 0.8708 = 27.08188 V.
• 150 ÷ 27.08188 = 5.54, so 6 panels at least.
• Check: 5 panels give 135.4094 V, which is under 150 V. 6 panels give 162.49128 V.

Upper MPPT check. At −10°C, Vmp = 31.10 × 1.119 = 34.8009 V. Fourteen panels give 487.2126 V. That is under 550 V.

So the screen gives 6 to 14 panels per string. This is a voltage screen only. It leaves out start-up voltage, current, tolerances, wiring losses and the maker’s own rules. Use unrounded numbers. If a result lands right on a limit, check the margin your design method requires.

For terms, see open-circuit voltage and temperature coefficient.

4. How do you check current and count every module?

A series string does not add current. Parallel strings do. Keep Imp, Isc and the design current as three separate numbers.

SMA Australia’s AS/NZS 5033:2021 page shows one method. AS/NZS 5033:2021 is the Australian and New Zealand standard for PV array wiring. For arrays with no DC optimizers, SMA gives this formula: maximum string current = 1.25 × K_I × Isc of the module. K_I is generally 1, except for bifacial modules. This is the rule for that standard. Do not copy it to other countries.

SMA’s own example uses a module with an Isc of 11.32 A:

• Maximum string current = 1.25 × 1 × 11.32 = 14.15 A.
• Two strings in parallel on one input = 2 × 14.15 = 28.30 A.
• The inverter’s short-circuit current rating on that input is 30 A (STP50-41), so 28.30 A passes.

Build a table of every input and the current at the point where strings join. Also check the panel’s reverse-current and fuse limits. Do not rely only on the inverter’s current rating.

Count the modules on their own. Ten strings of 17 panels make 170 panels, not 172. Each MPPT must meet its own limits. A total for the whole inverter can hide one overloaded input.

5. What if modules, directions or shade differ?

Use the same panel type and the same string length where you can. That keeps the review simple. SMA’s rules above ask for panels of one type, aligned and tilted the same, with the same count in each string.

A wattage match does not prove two panels work well together. Compare the real current and voltage curves, and ask the maker. A separate MPPT can let a different group run on its own. That group still needs enough voltage to start and run.

Shade-aware grouping

1. List the shaded panels and the hours they are shaded.
2. Draft other string layouts that keep shaded panels together.
3. Compare the energy of each layout using the same assumptions.

Do not put three shaded panels alone on one MPPT before you check that such a short string can start. More MPPTs do not make any string length valid.

Bypass diodes (switches that let current go around a shaded group of cells) limit the damage, but they do not erase it. In NREL’s 2009 field test, one fully shaded cell cut its panel’s output by one third. Read how shading affects solar panels and the shading analysis guide for the full picture. For panel circuits, see half-cut vs full-cell solar panels.

6. What belongs on the wiring checklist?

• Cable. Size it for the current, the install conditions and the voltage drop, using the rule that applies to your project. Canadian Solar’s manual says to use dedicated solar cable. It names PV wire rated 90°C wet in North America. Elsewhere it names at least 4 mm² (12 AWG), rated 90°C wet.
• Connectors. Use connectors that match the cable and the panel maker’s rules. Canadian Solar’s manual says connectors must only join the circuit and never switch it on or off.
• Protection. Plan bonding, grounding, fault protection, rapid shutdown and labels as their own review.
• Local rules. Where a U.S. code such as the NEC (National Electrical Code) applies, ask the local building authority which edition it has adopted. Cities and states adopt model codes through their own ordinance or law, as the ICC code adoption page shows. Other countries have their own rules.

Panels make DC power in light, even when the inverter is off. Canadian Solar’s manual warns that PV connectors pass direct current when exposed to sunlight. Only qualified people should install and test. This guide gives no live-wiring steps.

7. How do you hand off a stringing record?

Give each string, input and MPPT an ID that links to the roof drawing. Keep the document revisions and the temperature and current assumptions.

When a layout changes, redo the checks. The panel layout design guide covers the layout handoff. Before release, run the 9 solar stringing checks. For auto-stringing output, use the auto-stringing review checklist. For design theory, read the solar string design guide.

When you compare solar design software, ask it to reproduce a calculation you know. Ask how it assigns strings to inputs and how it tracks revisions. Treat software output as evidence for review. It does not replace engineering approval or a permit.

Want to see these checks inside a design workflow? Book a demo and bring one project with its datasheets.

Frequently Asked Questions

Should solar panels be wired in series or parallel?

Use the layout your equipment and design allow. In series, voltages add. In parallel, currents add at one voltage. Check cold and hot voltage, current, start-up voltage and the inverter’s input rules before you choose.

How many solar panels can you put on one string?

Divide the inverter’s maximum DC voltage by the panel’s cold Voc, then round down. In the worked example, 600 V ÷ 40.7154 V gives 14 panels. This is only a first screen. You must also check the MPPT range, start-up voltage, current and the maker’s rules.

Can you mix different wattage solar panels on the same string?

It is safer not to. SMA’s rules for its STP50-40 inverter ask for panels of the same type, aligned and tilted the same way, with the same number in each string. Wattage alone does not show whether two panels work well together. Compare their current and voltage curves and ask the maker.

What happens if one solar panel in a string is shaded?

The string makes less power. In NREL’s field test, one fully shaded cell cut its panel’s output by one third, because a bypass diode cut that group of cells out of the circuit. The loss depends on the shade, the panel circuit and the inverter. Module-level devices can recover part of the loss, but not all of it.

How do you calculate the maximum panels per string for an inverter?

First correct the panel’s Voc for the coldest expected temperature. Then divide the inverter’s maximum DC voltage by that number and round down. Check the full string voltage against every limit. Then check the MPPT range, start-up voltage and current.