How this is calculated
Open-circuit voltage moves with cell temperature along a straight line, and every datasheet publishes the slope. In words: cold voltage equals rated voltage times one plus the coefficient as a fraction, times the degrees below the 25°C rating point.
Voc(T) = Voc(STC) × (1 + coeff⁄100 × (T − 25))
Both terms are negative, so they multiply into a rise. Worked for a 405 W module rated 47.4 V at −0.28 %/°C on a site whose design low is −20°C.
- Degrees below the rating point: −20 − 25 = −45°C.
- Voltage change: −0.28 % × −45 = +12.6 %.
- Cold open-circuit voltage: 47.4 V × 1.126 = 53.4 V per panel.
- Against a 500 V input: 500 ÷ 53.4 = 9.4, rounded down to nine in series; against 250 V, four; against 150 V, two.
The floor works in reverse. Vmp falls as the glass heats, and a tracker below its window stops harvesting. That panel’s 38.9 V Vmp at −0.36 %/°C lands at 33.3 V on a 65°C cell, so a tracker whose window starts at 150 V — the Sol-Ark below — needs five in series. Five to nine is the workable range on that unit.
The 149°F cell is a design assumption rather than a published rating. Canadian Solar rates that module at 108°F nominal operating temperature in 68°F air at four-fifths of full sun, so 149°F is what full sun on a still, hot afternoon adds to it. Raise it if your roof runs hotter.
The 65°C cell is a design assumption rather than a published rating. Canadian Solar rates that module at 42 ± 3°C nominal operating temperature in 20°C air at 800 W/m², so 65°C is what full sun on a still, hot afternoon adds to it. Raise it if your roof runs hotter.
Current is the third check. The National Electrical Code counts a source circuit at 125 % of rated short-circuit current, so two strings of a 10.98 A module arrive as 27.5 A.
Why the coldest morning is the design case
The ceiling is instantaneous, and the array only has to cross it once, so designing for a typical winter day misses it entirely.
Two conditions have to line up, and they line up often: cold glass and full sun. Open-circuit voltage is a no-load measurement, so it appears the instant light lands, before current flows and before the module warms itself. The worst case is the first clear minute after sunrise on the coldest morning of the decade.
Three of those modules in series measure 142 V at the 77°F rating point, safely under a 150 V controller. The same string crosses 150 V near 41°F and reaches 168 V at −40°F.
Three of those modules in series measure 142 V at the 25°C rating point, safely under a 150 V controller. The same string crosses 150 V near 5°C and reaches 168 V at −40°C.
Victron’s manual asks for an extra 10 % margin wherever night temperatures approach or fall below 10°C.
Controller ceilings and panel coefficients to start from
Every row comes from a published datasheet, and every row is an example to replace with your own. Click a heading to sort.
Read the third and fourth columns as two separate limits. The ceiling is what the input survives on the coldest morning; the window is where the tracker actually works. A Sol-Ark tracker accepts 500 V but only tracks to 425 V, so a string that clears the ceiling can still sit above the useful range at sunrise.
| Class | Example product | Ceiling for the cold morning | Tracking window | PV input current limit |
|---|---|---|---|---|
| 150 V | Victron SmartSolar MPPT 150/35 | 150 V absolute in the coldest conditions, 145 V start-up and operating | Battery voltage + 5 V to start, + 1 V to keep running — about 34 V on a 24 V bank, 63 V on a 48 V bank at the default absorption settings | 35 A short-circuit in the 2026 manual; the datasheet still prints 40 A, so size to 35 A |
| 250 V | Victron SmartSolar MPPT 250/70 | 250 V absolute in the coldest conditions, 245 V start-up and operating | Battery voltage + 5 V to start, + 1 V to keep running | 35 A short-circuit, and no more than 30 A through any one MC4 pair |
| 500 V | Sol-Ark 15K-2P-LV, three trackers | 500 V maximum input | 150–425 V rated, 125 V start-up | 44 A short-circuit per tracker, 26 A operating |
| 600 V | Morningstar TriStar MPPT 600V | 600 V absolute, but arrays are specified to 525 V Voc | 100 V to 525 V | 15 A operating, self-limiting; no short-circuit figure published |
| Example module | Voc | Vmp | Isc | Voc coeff. | Pmax coeff. |
|---|---|---|---|---|---|
| 36-cell “12V” 150 W (Victron BlueSolar) | 22.3 V | 18.2 V | 8.69 A | −0.35 %/°C | −0.45 %/°C |
| 60-cell 305 W (Victron BlueSolar) | 39.7 V | 32.5 V | 10.27 A | −0.35 %/°C | −0.45 %/°C |
| 72-cell 360 W (Victron BlueSolar) | 47.4 V | 38.4 V | 10.24 A | −0.35 %/°C | −0.45 %/°C |
| 144 half-cell 405 W (Canadian Solar HiKu CS3W-405P) | 47.4 V | 38.9 V | 10.98 A | −0.28 %/°C | −0.36 %/°C |
Datasheets rarely print a Vmp coefficient. Substituting the Pmax coefficient is standard, because Imp barely moves with temperature, so the power loss shows up as lost voltage.
Maximum panels in series by design temperature
Worked for the 405 W module, 47.4 V at −0.28 %/°C. The last column is 525 V rather than 600 V, because 525 V is the array voltage the 600 V controller above is actually specified for.
| Design low | Cold Voc per panel | 150 V | 250 V | 500 V | 525 V |
|---|---|---|---|---|---|
| −40°C / −40°F | 56.0 V (+18.2 %) | 2 | 4 | 8 | 9 |
| −30°C / −22°F | 54.7 V (+15.4 %) | 2 | 4 | 9 | 9 |
| −20°C / −4°F | 53.4 V (+12.6 %) | 2 | 4 | 9 | 9 |
| −10°C / 14°F | 52.1 V (+9.8 %) | 2 | 4 | 9 | 10 |
| 0°C / 32°F | 50.7 V (+7.0 %) | 2 | 4 | 9 | 10 |
| 10°C / 50°F | 49.4 V (+4.2 %) | 3 | 5 | 10 | 10 |
The bottom row is the trap. Three panels on a 150 V controller, commissioned on a mild autumn afternoon, report full output while they wait for a cold snap.
Parallel strings, string current and fusing
Once the series count is fixed, extra panels go on as identical parallel strings: voltage stops changing and current adds up.
Two limits sit at that input, and only one is on the front of the sheet. Short-circuit current is what the hardware survives. Maximum operating current is what it usefully takes.
The 500 V inverter above tolerates 44 A of short-circuit current per tracker but tracks 26 A, so a third string on one tracker is wasted copper.
- Two strings in parallel generally need no fuses: one healthy string cannot push more into a faulted one than its series fuse rating allows.
- Three or more do, one fuse per string, at or below the maximum series fuse rating on your own module sheet — 20 A on the Canadian Solar module above — in holders rated for the full string voltage in direct current.
- Size the combiner-to-controller cable for the summed current, in the wire size calculator.
What breaks when the ceiling is crossed
Nothing here is a wear-out mechanism. The ceiling is the reverse breakdown voltage of the input switching devices and the working voltage of the capacitors beside them. Past it, a part conducts when it should block, in microseconds, taking its neighbors with it.
Victron’s manual describes both sides of the line. A mild overshoot stops charging and raises an overvoltage error, clearing only once the voltage falls 5 V below the rated maximum. Further past it, the charger sustains damage that sits outside warranty.
That is why the type plate quotes the ceiling for the coldest conditions: a 150 V unit publishes 150 V absolute and 145 V operating, and those five volts absorb the sunrise.
MPPT and PWM change what a string can be
A PWM controller is a switch. It pulls the array down to roughly battery voltage, so the panel works below its own maximum-power point and everything above is thrown away. Series strings then only make sense in whole multiples of battery voltage, which is why PWM systems use 36-cell modules with a Vmp near 18 V.
An MPPT controller is a converter, which is what makes string sizing a real calculation. It steps a high-voltage string down efficiently, so the array runs at 150 V or 400 V while the bank sits at 48 V. The reward is thinner cable over a long run; the price is that the ceiling now matters. The full trade sits in the MPPT against PWM comparison.
Quick answers
How many solar panels can I put in series?
Controller maximum voltage divided by cold-corrected Voc, rounded down. A 47.4 V panel reaching 53.4 V at −20°C gives two on a 150 V controller, four on 250 V, nine on 500 V.
What is the formula for temperature-corrected Voc?
Rated Voc times one plus the coefficient over 100 times the temperature minus 25. Both terms go negative below the rating point, so voltage rises.
What happens if the array exceeds the limit?
A small overshoot stops charging with an overvoltage error until the voltage drops 5 V below the rating. A larger one damages the input stage, outside warranty.
How do I find the minimum in series?
Divide the bottom of the tracking window by Vmp corrected to a 65°C cell, then round up. A 38.9 V Vmp panel falls to 33.3 V, so a 150 V floor needs five.
Do parallel strings need fuses?
Two normally do not. Three or more do, one per string, at or below the maximum series fuse rating printed on the module sheet — 20 A on the Canadian Solar 405 W module above.
Sources
Controller limits from the Victron SmartSolar MPPT 150/35 & 150/45 and 250/60 & 250/70 datasheets and the Victron MPPT solar charger manual (the overvoltage, warranty and 10 % cold-margin wording is that manual’s). Hybrid inverter figures from the Sol-Ark 15K-2P-LV datasheet, revision 7, July 2026. The 600 V class from the Morningstar TriStar MPPT 600V datasheet and product specifications. Module data from the Victron BlueSolar monocrystalline and Canadian Solar HiKu CS3W-P datasheets, cell physics from PVCDROM at pveducation.org, and the 125 % source-circuit rule from National Electrical Code 690.8(A)(1)(1), 2017 edition. Figures checked September 2026; read the revision date on your own sheets.
Go deeper
String sizing sits between two other decisions. Fix array size, bank and system voltage first with the off-grid solar sizing calculator, then read the paperwork with the solar panel guide. The wiring choice behind the count is in series against parallel.