Short answer: for one small panel on a 12V battery in warm weather, a PWM controller does the job and costs less. For almost anything else (cold mornings, cloudy days, a bit of shade on the roof, a bigger array, a long cable run) an MPPT controller gets noticeably more out of the same panels. If you're not sure, go MPPT.
What's the actual difference?
A PWM controller is basically a switch. It connects the panel straight to the battery, which drags the panel's voltage down to the battery's. The panel ends up working at whatever voltage the battery is at, not where it makes the most power.
An MPPT controller is a converter. It runs the panel at its best point (the maximum power point) and converts that power down to suit the battery. The panel side and the battery side are separate, so you can have a 12V battery on one side and panels wired in series at 36 V on the other.
Power is volts times amps, so dropping the voltage raises the current. Victron's example: 100 W coming in at 18 V becomes 100 W / 13 V = 7.7 A into a 13 V battery. A PWM controller can't make that trade; whatever amps the panel makes is what the battery gets.
How much more does MPPT really get?
Mostly it depends on how hot the panel is. Victron worked it through for a 100 W, 36-cell panel on a 12V battery (technical paper):
| Panel (cell) temperature | MPPT | PWM | Gap |
|---|---|---|---|
| 25 °C | 100 W | 81 W (13.5 V x 6 A) | PWM 19% less |
| 75 °C, battery at 13 V | 77.5 W | 77 W | Next to nothing |
Why the gap closes: panel voltage falls as it heats up, about 4.5% for every 10 °C in both power and voltage. A hot panel's best voltage sinks towards battery voltage, which is where PWM holds it anyway.
Victron's summary: PWM comes within about 10% of MPPT when the battery is between 13 and 15 V and the cells are between 45 °C and 75 °C. MPPT generally wins in cold to temperate climates; in subtropical to tropical climates the two are about the same. A low battery makes PWM worse again, because it drags the panel voltage down even further.
You'll also see Victron's SmartSolar manual claim 30% more harvest than PWM. That's a general product claim; the technical paper's 19%-to-nothing range shows how it moves with the weather, so use that one.
When is MPPT clearly the better choice?
Victron name four cases:
- Cold or very hot cells. Often below 45 °C, or above 75 °C.
- Cable savings. When running the panels at a higher voltage cuts cabling cost a lot.
- Low light. When output on overcast days and in winter matters.
- Partial shade. Shade lowers the panel's voltage, and MPPT handles that much better than PWM.
Every one of those is a normal day of touring. Winter trips, a gum tree over the roof, a week of cloud in Tassie.
Two more worth knowing:
- Bigger arrays. Once the array is more than a few hundred watts on a 12V battery, being able to wire panels in series and cut the current is a strong reason to go MPPT.
- Panels with no airflow behind them run very hot. With a fully insulated back, cell temperature can routinely go over 100 °C. At that point Victron say PWM can't fully charge at all, because the current drops to very low or zero before the battery reaches absorption voltage. Panels stuck flat to a roof or laid on a hot bonnet are worth thinking about here. Our fixed vs folding vs blanket solar guide covers mounting.
Why does higher panel voltage help MPPT but not PWM?
With MPPT, more cells in series raises the array voltage and the controller converts it all down. Victron recommend 72 cells (a 24 V array) or more for a 12V battery if you want harvest to hold up when the cells are very hot.
It helps the cable too. Double the array voltage and, for the same loss, the cable's cross-section can drop by a factor of four. Less copper, or less loss on a long run from the roof. The cable side is in our 12V cable sizing and voltage drop guide.
PWM can't use any of that. More cells in series actually hurts it at low temperature. A PWM controller wants the usual panel for the battery: 36 cells for 12 V, 72 for 24 V.
What panels can go on a PWM controller?
Check the controller's own limits. For Victron's BlueSolar PWM-Light 20A (and the rest of the PWM-Light 12/24V range):
- Maximum solar voltage: 28 V on a 12 V system, 55 V on a 24 V system.
- Panels: 36-cell panels for 12 V; for 24 V, 72-cell panels or two 36-cell panels in series. The manual is plain about it: "Use a 12V (36 cells) solar array for a 12V battery system."
So no stringing panels in series on a 12V PWM setup. That's an MPPT job.
For an MPPT controller, the model name gives you the limits. The SmartSolar 100/30 takes up to 100 V from the panels and charges at up to 30 A. We've fitted plenty of SmartSolar controllers.
So which should you buy?
- PWM for one small panel on a 12V battery, used mostly in warm weather, where the price difference matters more than the last few watts.
- MPPT for anything over a few hundred watts, any time you'll camp in cool weather or under trees, when the panels sit flat with no airflow, or when there's a long cable run to the battery.
- Not sure? MPPT. In Victron's figures it's never worse than PWM, only sometimes about the same.
How many watts you need is the next question: how much solar do you need takes it from there. If you're also charging from the vehicle, see DC-DC charger, solar controller or both?
Both kinds are grouped on the store as MPPT charge controllers and PWM charge controllers.