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MPPT vs PWM charge controllers for vans and RVs
Every solar panel needs a charge controller between it and the battery. There are two main types. Here is how they differ, using the numbers from a published engineering comparison.
A solar charge controller stops a panel from overcharging the battery and manages the charging stages. The two common types are PWM (pulse width modulation) and MPPT (maximum power point tracking). PWM controllers are simpler and usually cheaper. MPPT controllers are more complex and can harvest more energy in many conditions.
The clearest public comparison we found is a white paper from Victron Energy, a maker of both types: "Which solar charge controller: PWM or MPPT?" The figures below come from it. Victron sells both kinds, so it is a manufacturer source; the physics it uses is standard.
How each one works
A solar panel produces the most power at one particular voltage, its "maximum power point." For a typical 12 V panel with 36 cells, Victron says that is around 18 V at 25°C cell temperature.
- PWM connects the panel almost directly to the battery. That forces the panel to run at roughly the battery's voltage, not at its best voltage.
- MPPT lets the panel run at its best voltage and converts the extra voltage into extra charging current.
The numbers from Victron's example
Victron uses a 100 W, 36-cell panel with a maximum power voltage of 18 V and current of 5.56 A, charging a 12 V battery sitting at 13 V.
| Cell temperature | MPPT harvest | PWM harvest | What it means |
|---|---|---|---|
| 25°C (77°F), standard test conditions | 100 W (7.7 A at 13 V) | 81 W (6 A at 13.5 V on the panel) | PWM gets 19% less |
| 75°C (167°F) | 77.5 W | about 77 W | Almost no difference |
| 100°C (212°F) | Both struggle; the panel's best voltage falls to 11.7 V | Need a higher-voltage panel | |
Source: Victron Energy white paper. Cell temperature is the temperature of the solar cells themselves, which runs well above air temperature in sun.
Why does heat matter so much? Panel voltage drops as cells get hotter. Victron's paper says output power and voltage both fall by about 4.5% for every 10°C rise. In heat, the panel's best voltage sinks toward the battery voltage, so PWM loses less by comparison. Overall, the paper finds PWM performs within about 10% of MPPT between roughly 45°C and 75°C cell temperature at battery voltages of 13 to 15 V.
Where MPPT pulls ahead
- Cold, bright days. Cold panels have a higher voltage, which PWM cannot use. Victron recommends MPPT for cold to temperate climates.
- Cloudy days and winter. At low light, Victron says an MPPT controller paired with a higher-voltage array "will perform far better" than PWM.
- Partial shade. Shade from roof vents or trees lowers panel voltage. Victron says MPPT has a clear advantage in partial shade.
- Higher-voltage panels. MPPT can charge a 12 V battery from panels wired in series or from larger "24 V" panels. PWM cannot use that extra voltage.
- Thinner cables. Doubling the array voltage halves the current. Victron notes that for the same cable loss, wire cross-section can drop by a factor of four.
Where PWM still makes sense
- Small, simple setups. A single 12 V panel on a hot, sunny roof may perform close to an MPPT setup, as the 75°C row shows.
- Budget. PWM controllers are usually cheaper. If your panel's voltage is matched to the battery and your climate is warm, the energy gap may be small.
- Maintaining a battery. A small panel that only keeps a battery topped up does not need the extra harvest. See solar trickle chargers.
Pairing panels with an MPPT controller
Victron's advice is to give an MPPT controller an array voltage several volts higher than the highest battery voltage. Its examples:
- 12 V battery: 72 cells in series (a "24 V" array) or more
- 24 V battery: 108 cells or more
- 48 V battery: 216 cells or more
Every MPPT controller has a maximum input voltage. Panel voltage rises in the cold, so check the panel's open-circuit voltage at your coldest temperature against the controller's limit. The controller's manual or the maker's sizing tool will show how.
Lithium batteries
Whichever type you choose, it needs a charge profile that suits your battery. Lithium (LiFePO4) batteries usually need settings from the battery maker. Battle Born publishes a guide to programming a Victron SmartSolar MPPT controller for its batteries, and our LiFePO4 charging basics page explains the stages.
Quick decision guide
| Your situation | Leaning |
|---|---|
| Cold or temperate climate, winter use | MPPT |
| Roof vents or racks shade part of the panels | MPPT |
| Panels wired in series, or larger 60/72-cell panels | MPPT (PWM cannot use the extra voltage) |
| One small 12 V panel, hot climate, tight budget | PWM can be close |
| Small panel just maintaining a parked vehicle's battery | PWM or a built-in controller is usually enough |
Next steps: size your panels with the van solar calculator, decide where they go with rooftop vs portable panels, and see how it all connects in camper van electrical systems.
Figures come from Victron's published example. Real results depend on your panels, controller, climate and wiring.