Solar Charge Controllers: PWM vs MPPT and How to Size One

A solar charge controller sits between your panels and your battery and decides how much of the panel’s output actually arrives. There are two kinds, PWM and MPPT, and the difference is measurable in watts. Sizing one correctly comes down to a single relationship most product listings get wrong. Here is both, from published datasheets.

The sizing rule, from Victron’s own table

Charge controllers are sold by amps, and the amps are on the battery side, not the panel side. This is the source of nearly every sizing mistake. Victron’s SmartSolar MPPT datasheet publishes the resulting wattage limits directly:

Model	Rated charge current	Max PV watts at 12V	Max PV watts at 24V
MPPT 75/10	10 A	145 W	290 W
MPPT 75/15	15 A	220 W	440 W
MPPT 100/15	15 A	220 W	440 W
MPPT 100/20-48V	20 A	290 W	580 W

Work backwards and the rule falls out. Victron’s absorption charge voltage is 14.4V for a 12V battery and 28.8V for a 24V battery. Multiply rated amps by that:

• 10 A x 14.4 V = 144 W, and Victron lists 145 W
• 15 A x 14.4 V = 216 W, and Victron lists 220 W
• 20 A x 14.4 V = 288 W, and Victron lists 290 W
• 15 A x 28.8 V = 432 W, and Victron lists 440 W

Nominal PV watts is roughly rated charge amps multiplied by battery charging voltage. Not by panel voltage. If you size off a panel’s 18.6V Vmp instead, a 15A controller looks like it should take 279W of panel, and you will be a third over.

Notice the second column too. The same 15A controller handles 220W on a 12V bank and 440W on a 24V bank. Doubling system voltage doubles what a given controller can deliver, which is one of several reasons larger systems move to 24V or 48V.

What PWM actually costs you

A PWM controller is essentially a fast switch. When it is closed, the panel is connected almost directly to the battery, which drags panel voltage down to whatever the battery is sitting at. The panel’s current stays roughly at its maximum, but the voltage does not.

Run that against a real panel. Rich Solar’s MEGA 100 publishes Vmp 18.6V and Imp 5.38A, which multiply to its 100W rating. Connect it through PWM to a battery at 13.5V and the panel operates near 13.5V while still passing roughly 5.38A. That is about 73W from a 100W panel, an arithmetic loss of roughly 27%.

An MPPT controller converts instead of connecting. It lets the panel sit at 18.6V, takes the full 100W, and steps the voltage down while stepping the current up, so more amps reach the battery. Victron’s datasheet claims an ultra-fast MPPT will “improve energy harvest by up to 30 % compared to PWM charge controllers.” Treat that as what it is, a claim from a company that sells MPPT controllers, though it lands in the same place the panel arithmetic does.

Two honest qualifications on the MPPT advantage:

It shrinks as the battery fills. The gap between panel Vmp and battery voltage is what MPPT recovers. A battery at 14.4V in absorption is much closer to the panel’s 18.6V than a battery at 12.2V, so the deficit PWM suffers is smaller at the end of a charge cycle than at the start.

It is smallest exactly where systems are smallest. On a single panel maintaining a battery that is rarely deeply discharged, the difference in delivered energy may not be worth the price gap. PWM is not a scam. It is a simpler device with a real cost in harvest, and whether that cost matters depends on whether roof space or budget is your binding constraint.

The two specs that kill controllers

Maximum PV open circuit voltage. Victron rates the 75/xx models at 75V and the 100/xx models at 100V. This is a hard ceiling, and the number to check it against is not the panel’s printed Voc. Panel voltage rises as temperature falls, and Victron publishes the coefficient at -0.35% per degree C for Voc. Apply that to the MEGA 100’s 22.8V: at a cell temperature of -10C, 35 degrees below the 25C rating point, Voc works out to roughly 25.6V. Two in series is about 51V, fine on a 75V controller. Three is about 77V, which is over. The panel that was safe in the parking lot in July fails the check in January.

Maximum PV short circuit current. Victron lists 13A, 15A, 15A, and 20A for the four models above and states plainly: “A PV array with a higher short circuit current may damage the controller.” Panels wired in parallel add current, so this limit binds on parallel arrays the way the voltage limit binds on series ones.

Wattage, by contrast, is the forgiving limit. Victron notes that if more PV power is connected, the controller simply limits input power. Oversizing an array against the wattage rating is a design choice with a real rationale, since you rarely see full rated output anyway. Oversizing against voltage or current is a way to buy a new controller.

Small numbers that add up

Startup threshold. Victron’s datasheet specifies that PV voltage must exceed battery voltage plus 5V for the controller to start, and battery voltage plus 1V to keep running. On a 12V system that is about 18V to wake up. This is part of why panels are built with Vmp well above battery voltage.

Self-consumption. The SmartSolar units draw 10mA at 12V with the load output off, and 19mA with it on. Over 24 hours, 10mA is 0.24Ah. That is genuinely negligible, which is worth saying because so much else in an off-grid system is not.

Peak efficiency: 98%. That is the conversion efficiency of the MPPT stage itself, and it is high enough that it is not where your losses are.

Full output up to 40C. Operating range is -30C to +60C, but rated output only holds to 40C. Like an inverter, a controller in a hot sealed compartment is working somewhere below its label.

Standards on the sheet: EN/IEC 62109-1, UL 1741, CSA C22.2. A controller listing recognized safety standards has been through third-party evaluation. Many marketplace controllers list none.

Buying advice, including the part that does not help

The controller is the component where a cheap purchase costs you continuously rather than once. A PWM unit sold as MPPT does not announce itself, and we have not tested any specific product’s claims, so we are not going to name brands we cannot verify. There is a check you can run yourself: a real MPPT controller displays array voltage well above battery voltage while charging. If a controller reports the panel sitting at battery voltage in good sun, it is behaving like PWM regardless of what the box says.

Match the controller to the array you will eventually have, not the one you are starting with. Adding a panel later is a common upgrade, and controllers are not.

For the panel side of the calculation, see solar panels. For the load side, see inverters. To run your own array numbers, use the RV solar calculator, and for complete packaged systems see the solar kit comparison.