MPPT vs. PWM charge controllers
A PWM controller connects the solar panel almost directly to the battery, so the panel is forced down to battery voltage and any extra voltage goes unused. An MPPT controller runs the panel at its best voltage and converts the surplus into extra charging current. How much that matters depends on how far the panel's voltage is above the battery's.
If you charge a portable power station, the choice is usually made for you: most have an MPPT controller built in. Check the "solar input" section of the spec sheet. The choice matters most when you build your own system around a separate battery, such as in a van, RV, boat or shed.
How a PWM controller works
PWM stands for pulse-width modulation. The controller is essentially a fast electronic switch between the panel and the battery. While the battery needs charge, the switch stays closed. As the battery fills up, the controller switches on and off rapidly to hold the voltage at the right level.
While the switch is closed, the panel and battery are joined, so they sit at the same voltage, the battery's. A solar panel's current changes little as its voltage drops below its maximum power point, so the panel keeps pushing about its rated current. The power delivered is therefore roughly:
How an MPPT controller works
MPPT stands for maximum power point tracking. Every panel has one voltage at which it produces the most power, listed on the spec sheet as Vmp (with the matching current, Imp). The MPPT controller keeps adjusting to find that point as sunlight and temperature change. It then uses a DC-DC converter to step the voltage down to what the battery needs. Since power in roughly equals power out, lower voltage means more current:
The converter loses a few percent as heat. Check the controller's datasheet for its efficiency figure.
Worked examples
These use example spec-sheet values. Read the real ones off the label on the back of your panel.
A "12 V" 100 W panel on a 12 V battery
Say the panel lists Vmp = 18 V and Imp = 5.56 A (18 × 5.56 ≈ 100 W). A 12 V lithium battery sits at around 13–14 V while charging, so take 13.5 V.
- PWM: 13.5 V × 5.56 A ≈ 75 W. About a quarter of the panel's power is never used.
- MPPT, assuming 95% converter efficiency: 100 W × 0.95 ≈ 95 W, which is about 7 A into the battery (95 ÷ 13.5).
So MPPT gives roughly a quarter more in this example, under the same sun. For a single small panel, that difference may cost less to make up with a slightly bigger panel than with a pricier controller.
A 400 W house-style panel on a 12 V battery
Larger panels sold for rooftops often have a Vmp in the 30s. Say Vmp = 34 V and Imp = 11.8 A (about 400 W).
- PWM: 13.5 V × 11.8 A ≈ 159 W, well under half the panel's rating.
- MPPT at 95%: 400 × 0.95 ≈ 380 W, about 28 A into the battery. The controller must be rated for that output current.
Here PWM wastes most of the panel. High-voltage panels and series strings need MPPT.
Compare them for your panels
These are full-sun, rated-condition figures, to compare the two controller types. Real output is lower for both. To estimate a real day's charging, use the solar charging time calculator.
Temperature changes the gap
A panel's voltage rises when it's cold and falls when it's hot. On a cold, bright day, Vmp climbs further above the battery voltage. An MPPT controller turns that extra voltage into extra current, while a PWM controller can't use it. On a hot day, Vmp drops closer to battery voltage and the gap narrows. So MPPT gains the most in winter, which is often when you're shortest on solar. Remember that cold also raises the open-circuit voltage, which must stay under the controller's maximum input voltage. Our series vs. parallel guide has a calculator for that check.
Which one should you use?
| Situation | Better fit | Why |
|---|---|---|
| One or two small "12 V" panels on a 12 V battery | PWM can be fine | Panel voltage is only a few volts above the battery, so less is lost, and PWM controllers cost less. |
| Panels with Vmp well above the battery voltage (e.g. 30 V+ panels on 12 V) | MPPT | PWM throws away most of the panel's power. |
| Panels wired in series | MPPT | Only MPPT can use the higher string voltage. Series wiring also lowers current, so thinner cable works on long runs. |
| Cold climates or winter use | MPPT | Cold raises panel voltage, which only MPPT can use. |
| Long cable run from panels to batteries | MPPT with a higher-voltage array | Higher voltage means less current and less voltage drop. See the wire size calculator. |
| Portable power station | Built in | Most have MPPT inside. Match your panels to its solar input voltage and watt limits. |
Frequently asked questions
Is MPPT always 30% better than PWM?
No. The gain depends on how far the panel voltage is above the battery voltage, and on temperature. With a matched 12 V panel on a hot day, the difference can be small. With a high-voltage panel on a 12 V battery, MPPT can deliver more than twice as much. Run your own panel's numbers in the calculator above.
Can I use a PWM controller with a 24 V battery?
Yes, if the panels' Vmp is comfortably above the 24 V battery's charging voltage. That usually means panels made for 24 V systems, or two "12 V" panels in series. The same rule applies: anything above battery voltage is wasted.
How do I size an MPPT controller?
Two ratings matter. The output current must cover the array watts divided by the battery voltage, so 400 W on a 12 V battery needs about 400 ÷ 13.5 ≈ 30 A. The maximum input voltage must stay above the array's open-circuit voltage on the coldest morning. Follow the manufacturer's sizing tool where they provide one.
Does my power station have MPPT?
Most current models do, and they usually say so in the solar input spec. The spec also gives the accepted voltage range and maximum watts. Stay inside both.