How to Size a Solar Charge Controller and Lay Out Your PV Strings

Size an MPPT charge controller or all-in-one inverter: cold-weather Voc math, panels in series, charge current, and string layout for multiple MPPTs.

System Sizing

Every MPPT charge controller and all-in-one inverter has a few numbers that decide how many solar panels it can take: a maximum PV input voltage, an operating voltage window, and a maximum current. Get the voltage wrong on a cold morning and you can damage the controller. Get the current wrong and you leave power on the table.

This guide walks through the math step by step, including how to lay out strings on an all-in-one inverter with more than one MPPT. You’ll need your panel’s datasheet, your controller or inverter’s spec sheet, and the record low temperature where you live. If you haven’t sized the array itself yet, start with how many solar panels you need.

Read the Controller’s Rating

Most charge controllers put their limits right in the name. A Victron SmartSolar 150/35 accepts up to 150V from the array and pushes up to 35A into the battery, and a 250/100 takes 250V and delivers 100A. All-in-one inverters list the same limits on their spec sheet, usually as max PV input voltage, MPPT operating range, and max input current per MPPT.

Those numbers do different jobs:

  • Max PV input voltage is a hard ceiling. Going over it, even for a few minutes on a cold morning, can permanently damage the controller.
  • MPPT operating range is the window where the controller tracks efficiently. If the array voltage falls below it, harvest drops off.
  • Max charge current or max input current is the most current the unit will use. Controllers limit what they push into the battery, and all-in-ones limit what each MPPT accepts. Extra array power is simply clipped, but the array’s short-circuit current (Isc) still has to stay within the unit’s rating.

Correct Voc for Cold Weather

Panel voltage is rated at 25°C (77°F). As cells get colder, voltage rises, so a string that reads fine on a spring afternoon can go over the limit at sunrise on the coldest day of the year. That’s why you size against open-circuit voltage (Voc) at your record low, not your average winter low.

The math uses your panel’s temperature coefficient of Voc, listed on the datasheet in %/°C. It’s a negative number, usually between -0.24 and -0.30 for modern panels.

Cold Voc = Voc × [1 + (record low in °C − 25) × coefficient ÷ 100]

Here’s an example with a 445W panel (Voc 49.5V, Vmp 41.6V, Isc 11.3A, Imp 10.7A, Voc coefficient -0.27%/°C) at a record low of -20°C (-4°F):

  • Temperature difference: -20 − 25 = -45°C
  • Correction factor: 1 + (-45 × -0.27 ÷ 100) = 1.1215
  • Cold Voc: 49.5V × 1.1215 = 55.5V per panel

If your datasheet lists the coefficient in mV/°C instead, divide it by the panel’s Voc in millivolts and multiply by 100. For example, -134 mV/°C on a 49.5V panel works out to about -0.27%/°C.

For your record low, use your area’s extreme minimum design temperature. ASHRAE publishes these, the National Electrical Code (NEC 690.7) references them, and some local codes specify their own number, so check yours.

Try it with your own panel. The calculator below starts with the example panel on a 500V all-in-one that tracks from 120V. Swap in your datasheet numbers and your record low.

String Voltage Calculator

Check a solar string against a charge controller's or inverter's voltage limits.

V

Open-circuit voltage at STC, from the datasheet

%/°C

From the datasheet, e.g. -0.27%/°C

°F

The record low for your area, not the average winter low

panels

Per string, on one MPPT input

V

From the charge controller or inverter spec sheet

In full sun, cells run about 30°C (54°F) hotter than the air. If a string is too short, its operating voltage can sag below the bottom of the MPPT window on a hot afternoon.

V

Max-power voltage at STC, from the datasheet

%/°C

Or the Pmax coefficient, a close stand-in

°F

About your record high air temperature + 54°F

V

Bottom of the MPPT operating range on the spec sheet

Near limit, 9 panels in series at -4°F

Cold Voc per panel
55.5V at -4°F
Cold Voc per string
499.6V (9 panels)
Headroom
0.4V (0.1%) under 500V
Max panels in series
9 (499.6V)
  • 9 panels fit, but anything over 8 leaves less than 3% headroom, so 8 (444.1V) is the safer choice.

Estimates from datasheet values. Confirm the final layout against your equipment manuals and local code, or with a qualified professional.

Count Panels in Series

Divide the max PV input voltage by your cold Voc and round down. That’s the most panels you can put in one string. With the 55.5V example panel on a 500V all-in-one, 500 ÷ 55.5 = 9.0. Nine panels would land at 499.6V, right at the limit, so 8 panels (444V) is the safer choice. Change the max PV input in the calculator above to see the count for a 150V or 250V controller.

Leave a little headroom. The record low is a design number, and a few volts of margin covers rounding and panel-to-panel variation.

Surge Protection Won’t Save You From Cold Voc

A surge protective device (SPD) is worth adding to any array with long wire runs, because it clamps the short, sharp spikes from nearby lightning before they reach your controller. What it can’t do is protect against cold-weather overvoltage. A cold morning raises your string voltage for hours, not microseconds, and an SPD isn’t built to absorb a continuous overvoltage. If your string goes over the controller’s limit, the SPD won’t stop it.

Cold Voc also matters when you pick the SPD itself. Every SPD has a maximum continuous operating voltage, and it needs to sit above the cold Voc of the strings it protects. If the array runs above that rating, the SPD starts conducting on its own and can overheat or fail. The MidNite Solar surge protective device comes in versions for different system voltages, up to 600V DC, so size it from the same cold Voc number you used for your controller.

Check the Hot-Weather Minimum

Voltage drops in the heat, and cells run about 25 to 30°C (45 to 55°F) hotter than the air in full sun. If a string is too short, its operating voltage (Vmp) can sag below the bottom of the MPPT window on a hot afternoon.

Estimate hot Vmp with the panel’s Vmp coefficient, or use the Pmax coefficient as a close stand-in (usually about -0.30 to -0.35%/°C). For the example panel with 70°C cells on a 104°F (40°C) day:

41.6V × [1 + (70 − 25) × -0.35 ÷ 100] = 35.0V per panel

High-voltage controllers make this check matter. The MidNite Barcelona and Hawke’s Bay track from 185V, so this panel needs at least 6 in series (210V hot) to stay in the window, and the 600V limit caps the string at 10 (555V cold). For a 12V, 24V, or 48V battery controller, the array just needs to stay comfortably above battery voltage to keep charging. For an all-in-one, keep hot Vmp above the bottom of the MPPT operating range on the spec sheet.

Size the Charge Current

For a battery charge controller, divide array watts by battery voltage to estimate charge current. 1,200W on a 24V bank is about 50A, and 3,000W on a 48V bank is about 63A. Pick a controller rated at or above that number, or accept a little clipping on the brightest days. Bigger arrays can be split across two controllers charging the same bank.

Then check the array’s combined Isc against the controller’s max PV short-circuit current, especially if you’re running strings in parallel.

Laying Out Strings on an All-in-One With Multiple MPPTs

Many all-in-one inverters have two or three MPPTs, and each one tracks its own group of panels independently. Each MPPT has its own voltage and current limits, and the inverter usually has an overall max PV power on top of that. A few rules make the layout easy:

  • One orientation per MPPT. Put panels that face the same direction, at the same tilt, with the same shade on the same MPPT. An east roof and a west roof belong on separate MPPTs so each can track its own peak.
  • Matching strings on each MPPT. Strings in parallel share one voltage, so every string on an MPPT needs the same number and model of panels.
  • Current adds up in parallel. Two strings of the example panel put 22.6A of Isc and 21.4A of operating current on one MPPT. Combined Isc has to stay within the MPPT’s short-circuit rating, and anything above its max input current is clipped.
  • No need to balance. One MPPT can carry a big array and another a small one, as long as each stays within its limits.
  • Stay under the total. Add up every MPPT and compare it to the inverter’s max PV power. Many all-in-ones accept more PV than their AC output rating, which stretches harvest into the morning and evening.

Example layout. Say an all-in-one has two MPPTs, each rated 500V max with a 120V to 450V operating window, 25A max input current, and 30A max short-circuit current. With the example panel and a -20°C record low:

  • MPPT 1, south roof: one string of 8 panels. That’s 444V cold, about 280V hot, and 3,560W.
  • MPPT 2, west roof: two strings of 6 in parallel. Each string is 333V cold and about 210V hot, the pair adds up to 22.6A of Isc, and the total is 5,340W.
  • A third string on MPPT 2 would push Isc to 33.9A, over the 30A rating, so it would need its own MPPT.
  • Total array: 8,900W. Compare that to the inverter’s max PV power before you buy.

Paralleling Strings and Shade

Paralleling strings on one MPPT is a common way to fit more panels, and it works well when every string sees the same sun. Strings in parallel share one operating voltage, though, so if one string is shaded, the MPPT has to settle on a voltage that works for both, and shade on one string can cost power from the other. Bypass diodes help, but they don’t make the problem go away.

Before you parallel strings, think about where shade falls through the day. Our Solar Panel Shading Trainer shows how shade on just a few cells affects a panel and a string. If a group of panels gets afternoon shade from a tree or chimney, give it its own MPPT, or add Tigo optimizers to the shaded panels.

Running three or more strings in parallel usually calls for a fuse or breaker on each string, which a PV combiner box handles neatly.

Before You Buy

  • Cold Voc of every string stays under the max PV input voltage, with some headroom.
  • Any SPD on the array is rated above that same cold Voc.
  • Hot Vmp stays above the bottom of the MPPT operating range.
  • Combined Isc on each MPPT stays within its short-circuit rating.
  • Charge current, or total PV power on an all-in-one, fits the unit’s rating.
  • Strings on the same MPPT match, and each MPPT sees one orientation.

Your controller or inverter manual has the final word on its limits, and our support team is happy to help with sizing as you plan.