Size a charge controller on open-circuit voltage on the coldest morning, not on watts. Victron’s datasheets say too much PV power is simply throttled — but too much voltage stops the controller charging and can damage it, outside its warranty, and panel voltage rises as temperature falls. Four 150 W panels in series read 89.2 V in the spec table and 100.1 V at 14°F, which is over the limit of every 100 V controller made.
Step 1 of 3 — the panels
What is on the roof?
Step 2 of 3 — the system
Battery bank and the coldest morning
The answer
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| Controller | Max PV volts | Max PV amps | Your array |
|---|
Before buying anything
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MPPT or PWM
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What takes it
Victron SmartSolar MPPT 75/15 Charge Controller
75 V · 15 A max PV short circuit
220 W at 12 V, 440 W at 24 V
Check price at Victron Energy(opens in a new tab)
Victron SmartSolar MPPT 100/20 Charge Controller
100 V · 20 A max PV short circuit
290 W at 12 V, 580 W at 24 V
Check price at Victron Energy(opens in a new tab)
Victron SmartSolar MPPT 100/30 Charge Controller
100 V · 35 A max PV short circuit
440 W at 12 V, 880 W at 24 V
Check price at Victron Energy(opens in a new tab)
Victron SmartSolar MPPT 100/50 Charge Controller
100 V · 60 A max PV short circuit
700 W at 12 V, 1400 W at 24 V
Check price at Victron Energy(opens in a new tab)
Victron SmartSolar MPPT 150/35 Charge Controller
150 V absolute maximum coldest conditions, 145 V start-up and operating maximum · 40 A max PV short circuit
500 W at 12 V, 1000 W at 24 V
Check price at Victron Energy(opens in a new tab)The figure that actually damages controllers
Panels are rated at Standard Test Conditions, which Victron defines as 1000 W/m² at 25°C. Almost nobody charges a battery at 25°C in the morning.
Open-circuit voltage has a negative temperature coefficient: as the panel gets colder, Voc goes up. Victron publishes −0.35% per °C across its whole monocrystalline range. So:
| Temperature | Voc multiplier | A 22.3 V panel reads |
|---|---|---|
| 40°F (4.4°C) | ×1.072 | 23.9 V |
| 32°F (0°C) | ×1.088 | 24.3 V |
| 20°F (−6.7°C) | ×1.111 | 24.8 V |
| 14°F (−10°C) | ×1.123 | 25.0 V |
| 0°F (−17.8°C) | ×1.150 | 25.6 V |
| −20°F (−28.9°C) | ×1.189 | 26.5 V |
Multiply by the number of panels in series and the problem appears fast. Four 150 W panels in series: 89.2 V at STC, 100.1 V at 14°F. A Victron 100/30 is rated to 100 V. You are 0.1 V over on the first cold morning.
Victron’s manual says what happens then. The charger “will cease charging, showing an overvoltage error #33”, and starts again only “when the PV voltage drops 5V below the rated maximum voltage” — so not until the panels warm up. It also “can sustain damage based on the extent of the PV voltage height”, and that damage “is not covered by warranty”. A tenth of a volt over is not a certain loss. It is still a configuration Victron tells you never to run.
Watts are the number that does not decide it
This is the part that makes every sizing guide wrong in the same way. They all give you the same formula — array watts ÷ battery volts × 1.25 — and Victron’s own datasheet footnotes say what each excess actually does:
1a) If more PV power is connected, the controller will limit input power.
- A PV array with a higher short circuit current may damage the controller.
Read those together. Too many watts: throttled. Too much short-circuit current: may damage. Too much open-circuit voltage: charging stops, and damage that grows with how far over you are, outside the warranty — and Victron puts the voltage limit in the product name. A 100/30 is 100 V maximum PV and 30 A charge current. The first number is the one the guides never check.
Oversizing an array in watts is a money problem, not a safety one. You lose the difference between your array and the controller’s nominal PV power, and nothing breaks.
How to read a controller’s name
Cold open-circuit voltage (Voc)
100 V maximum
Array short-circuit current (Isc)
35 A maximum
100/30
Converts array voltage and current into battery charging voltage and current.
The “30” belongs here.
It is a different limit from the array’s 35 A Isc ceiling.
Example limits are for this exact model. Extra panel watts can be clipped within Victron’s allowed limits; that does not permit excess Voc or Isc. PV input amps and battery charge amps are different quantities.
Conceptual energy path, with wiring and protection omitted. Victron 100/30 and 100/50 datasheet.
Victron’s model numbers are the specification, which makes them unusually easy to check: 100/30 is 100 V maximum PV and 30 A charge current. 75/15, 150/35 and so on read the same way.
The second number — the charge current — is the one the usual formula sizes, and it is also the one Victron expresses a second way on the datasheet, as nominal PV power: 440 W at 12 V for the 100/30, 700 W for the 100/50. Those are the same statement. A 30 A controller on a 12 V bank cannot pass much more than 440 W, so exceeding it is where the throttling happens.
This is why “round up to the next size” is not wrong so much as incomplete. Rounding up the current buys you headroom you may not need. Rounding up the voltage — 75 to 100, or 100 to 150 — is what stops a cold morning ending the controller, and the two do not move together: the 150/35 in our records has a lower current limit than the 100/50.
Series or parallel — and why rewiring is free
S means panels in series. P means strings in parallel. Each rectangle below is one identical panel.
4S · All in series
100.1 V cold Voc
8.69 A Isc at STC
Four times the voltage; one panel’s current.
2S2P · Two strings
50.1 V cold Voc
17.38 A Isc at STC
Twice the voltage and twice the current.
4P · All in parallel
25.0 V cold Voc
34.76 A Isc at STC
One panel’s voltage; four times the current.
Fixed example: four 150 W Victron SPM041501200 panels, 600 W total in every layout. Voc is corrected to 14°F (−10°C); Isc is the 25°C datasheet value, not operating current or a hot-panel maximum. These are the same panel data and cold-voltage formula used by the calculator above.
Conceptual connections only. Cable, connectors, fuses, disconnects and any required diodes must follow the exact panel, kit and controller instructions. Also check the controller’s minimum start-up voltage and battery voltage.
Series adds voltage. Parallel adds current. The panels do not care; only the controller does, and that makes wiring the cheapest fix available.
Those four 150 W panels:
- All four in series: 100.1 V at 14°F, 8.69 A. Over every 100 V controller.
- Two series × two strings: 50.1 V, 17.38 A. A 100/50 takes it whole.
- All four in parallel: 25.0 V, 34.76 A. Needs a controller rated past 34.76 A of short-circuit current.
Same panels, same 600 W, three completely different controller requirements. The calculator above says when a rewire rescues an array, and it says it before recommending anything, because moving two MC4 connectors costs nothing and a bigger controller does not.
The trade-off going to parallel is cable: four times the current needs conductors sized for it. Our 12V wire size calculator sizes the run from array to controller.
The current side, and the margin that is not a margin
Short-circuit current is the second limit, and on parallel arrays it is usually the binding one. Four panels in parallel at 8.69 A each is 34.76 A, against a Victron 100/30’s 35 A ceiling.
That clears it — by 0.24 A, or 0.7%.
This page treats that as no margin at all. Isc has a temperature coefficient too, +0.04% per °C by Victron’s own figure, so a panel hotter than 25°C draws more than the number being checked. A margin thinner than a few percent is inside the measurement, not outside it, and the calculator flags anything within 5% rather than calling it a pass.
Notice what that does to the recommendation: on a six-panel parallel array the 150/35 fails on current while the cheaper 100/50 passes. The dearest controller is not the answer, because its 40 A limit is lower than the 100/50’s 60 A. Sizing on price, or on the first number in the model name alone, gets this backwards.
One popular controller family is missing from the calculator for exactly this reason. Renogy publishes a 100 V open-circuit limit for its Rover Li controllers and a maximum solar input power, but no short-circuit current limit — so there is nothing to check this side against, and the tool leaves them out rather than pass them on a guess. Victron vs Renogy charge controllers compares the two on everything both makers do publish.
MPPT against PWM, in watts
PWM controllers are cheaper, and the usual explanation — “MPPT is up to 30% more efficient” — understates the gap badly for series arrays.
A PWM controller connects the array more or less directly to the battery, so the panels are dragged down to battery voltage. Victron’s white paper on the question works it out: a discharged 12 V battery at 13 V, plus about 0.5 V lost in the cable and controller, holds the panel at 13.5 V, and at that voltage a 12 V panel delivers close to its short-circuit current, because its current “will be nearly constant up to about 17 V”. Victron’s own example is “13.5 V x 6 A = 81 W”, 19% less than an MPPT controller gets from the same 100 W panel. On Victron’s 150 W panel, Isc is 8.69 A:
- Four in parallel on 12 V: 13.5 × 34.76 ≈ 469 W of 600 W. A 22% loss.
- Four in series on 12 V: 13.5 × 8.69 ≈ 117 W of 600 W. An 80% loss.
Those are at 25°C, like every figure on this page. In hot sun the gap narrows — Victron’s example panel gives 77 W on PWM against 77.5 W on MPPT at a 75°C cell — and in the cold it widens.
Series wiring and PWM are close to incompatible, because everything series wiring adds is voltage and PWM throws voltage away. If you have a PWM controller and a series string, that is the whole problem.
Already have the factory PWM controller?
Many solar-prepped RVs leave the factory with Go Power’s GP-PWM-30-UL. Its manual sets two limits: “the maximum allowable nameplate PV Panel rated Isc is 30A (30A x 1.25 = 37.5A), and the maximum voltage, Voc is 28VDC (28VDC x 1.25 = 35VDC)”, on a 12 V system only. The calculator checks your array against both, under the PWM result.
Two things follow. Most large panels cannot connect to it at all: Victron’s 285 W panel is 38.9 V open circuit, well past 28 V, and any two 12 V panels in series are too. And where the array does fit, the upgrade’s value is computable. Three of Victron’s 150 W panels in parallel fit at 22.3 V and 26.07 A. On the PWM controller they harvest about 352 W at 25°C, and on an MPPT controller about 441 W, so the swap recovers roughly 89 W. It recovers less in hot sun and more in the cold.
| Model | Max PV voltage | Max PV short circuit | Charge current | Nominal PV power | Battery | Buy |
|---|---|---|---|---|---|---|
| SmartSolar 75/15 | 75 V | 15 A | 15 A | 220 W at 12 V, 440 W at 24 V | 12/24 V auto select | Check price(opens in a new tab) |
| SmartSolar 100/20 | 100 V | 20 A | 20 A | 290 W at 12 V, 580 W at 24 V | 12/24/48 V auto select | Check price(opens in a new tab) |
| SmartSolar 100/30 | 100 V | 35 A | 30 A | 440 W at 12 V, 880 W at 24 V | 12/24 V auto select | Check price(opens in a new tab) |
| SmartSolar 100/50 | 100 V | 60 A | 50 A | 700 W at 12 V, 1400 W at 24 V | 12/24 V auto select | Check price(opens in a new tab) |
| SmartSolar 150/35 | 150 V absolute maximum coldest conditions, 145 V start-up and operating maximum | 40 A | 35 A | 500 W at 12 V, 1000 W at 24 V | 12/24/48 V auto select | Check price(opens in a new tab) |
Where this goes wrong
- Sizing on watts. It is the one excess the manufacturer says is handled.
- Comparing controllers on the watts a list quotes. Most ranking lists divide by 12, so a 30 A controller comes out at 360 W. Victron rates its 100/30 at 440 W on a 12 V battery and Renogy its Rover Li 30A at 400 W — the charge controller roundup sets seven side by side on their makers’ own figures.
- Reading Voc off the spec table. That figure is at 25°C. Your controller meets the cold one.
- Assuming the 1.25 factor covers it. It is applied to current. Voltage is a separate limit on a separate line.
- Treating a 1% current margin as a pass. Isc rises with heat; a margin that thin is inside the coefficient’s own swing.
- Buying a bigger controller to fix a voltage problem. Rewiring is free and often enough, and a “bigger” controller can have a lower current limit.
- Putting a PWM controller on a series string. On four panels in series that discards about 80% of the array.
- Putting 12 V panels in parallel on a 24 V battery. Victron’s chargers need the PV voltage at least 5 V above the battery’s before they start, and a 12 V panel makes its maximum power at about 18 V. Two in series is the minimum, and the calculator above refuses anything less.
- Forgetting the cable when going parallel. Four times the current needs the conductors to match — see the 12V wire size calculator — and from three strings up, each needs its own fuse: what size fuse between solar panel and charge controller.
- Sizing the controller before the array. This page takes the array as given. If you have not settled how many panels you need yet, how much solar an RV needs works it out from the month you actually camp in — which changes the array, and therefore this answer.
- Treating every 100 W panel as the same voltage. A CIGS thin-film panel such as BougeRV’s Yuma reads 30.5 V open circuit against about 22.5 V for a crystalline 100 W, which changes how many can go in series — flexible vs rigid RV solar panels covers the difference.
- Assuming a solar-prepped RV came with a controller. Side ports usually run straight to the battery for a kit that brings its own, and roof prep often stops at a space where one “may be installed” — check what your prep is wired to before sizing.
- Replacing a controller that is only reporting. A controller showing off, or zero amps, in sun is far more often looking at a full battery, a cold lithium BMS or a blown fuse than broken — four readings for a solar charger that will not charge come before any purchase.
One or two panels
SmartSolar 75/15
A 75 V ceiling is plenty for a single panel and nothing else, but it is also the limit three 12 V panels in series break on a cold morning — which is exactly the trap this page is about.
Most RV arrays
SmartSolar 100/50
The same 100 V ceiling as the 100/30 with a 60 A current limit, which is what a parallel array actually needs — and 700 W of nominal PV at 12 V, so it throttles far less often.
Long series strings
SmartSolar 150/35
The only controller here whose datasheet states its ceiling as a cold-weather figure — 150 V absolute maximum coldest conditions against 145 V start-up and operating maximum.
Questions worth answering
What size solar charge controller do I need?
Not the one the usual formula gives you. Array watts divided by battery volts sizes the controller's current, and Victron's datasheet says too much PV power is simply limited: "If more PV power is connected, the controller will limit input power." What can damage a controller is exceeding its maximum PV short circuit current or its maximum PV open circuit voltage, and Victron's manual says over-voltage damage is not covered by warranty. Size on those two.
Why does solar panel voltage go up when it gets cold?
Because panels are rated at 25°C and open-circuit voltage has a negative temperature coefficient — Victron publishes -0.35% per °C for its monocrystalline range. At 14°F, which is 35°C below the rating, Voc is 12.3% higher than the specification table says. A 22.3 V panel reads 25.02 V, and four in series reach 100.1 V against a table figure of 89.2 V.
Can I wire RV solar panels in series?
Yes, and it is often the better choice — it keeps current low, so thinner cable works. The catch is that series adds open-circuit voltage, and on a cold morning that is what takes a controller over its limit. Four 150 W panels in series exceed every 100 V controller at 14°F. In parallel the same four sit at 25 V and need a controller rated for four times the current instead.
Is MPPT worth it over PWM for an RV?
On the arithmetic, yes, and by more than most pages suggest. PWM holds the array at battery voltage plus about half a volt rather than tracking its maximum power point. Victron's own method puts that at 13.5 V on a 12 V battery, with the panels delivering close to their short-circuit current. On four 150 W Victron panels in parallel that is about 469 W of 600 W; wired in series it collapses to about 117 W, because panels in series add voltage that PWM immediately discards. The gap narrows in hot sun and widens in the cold.
What happens if my solar array is bigger than the controller?
In watts, nothing bad — Victron's own footnote says the controller limits input power. You waste panel, which is a money problem rather than a safety one. In voltage or short circuit current it is different: Victron says a PV array with a higher short circuit current "may damage the controller". Over the maximum PV open circuit voltage it stops charging, and Victron's manual says it "can sustain damage based on the extent of the PV voltage height" — damage the warranty does not cover.
Does the 1.25 safety factor cover the cold voltage?
No. The 1.25 multiplier in the usual formula is applied to current, and it comes from continuous-duty practice on the array's short circuit current. It does nothing about open-circuit voltage, which is a separate limit on a separate line of the datasheet, and it is the line the sizing guides leave out.