MPPT vs PWM Solar Charge Controllers: Which One Do You Need?

Choose an MPPT solar charge controller for larger or expandable arrays, panels wired in series, long cable runs or cold and changing weather. A PWM controller suits a small, simple system where the panel voltage closely matches the battery’s. Renogy, which makes both types, says neither is always better: the right one fits your panels and battery.

What a solar charge controller does

A solar charge controller sits between the solar panels and a battery bank. It stops charging when the battery is full and runs the charge stages that suit the battery’s chemistry, such as lead acid or lithium. Without one, panels wired straight to a battery can overcharge it.

You only need a separate controller for a panel and battery system you build yourself, such as one for a van, an RV, a shed or a cabin. A portable power station has a controller built in, so its panels plug straight in. Our guide to how to charge a solar generator covers that route.

How MPPT and PWM charge controllers differ

A PWM (pulse width modulation) controller works like a valve between panel and battery. It switches the charging current on and off in rapid pulses instead of converting a higher panel voltage, so Renogy says the panel and battery voltages need to be closely matched.

An MPPT (maximum power point tracking) controller tracks the point where the panels make the most usable power, then converts that voltage and current into a charge the battery can take. Renogy lists cold, cloudy or changing weather among the cases where MPPT is often the better choice. Victron rates its SmartSolar MPPT 100/30 at a maximum efficiency of 98%.

When a PWM charge controller is enough

A PWM controller makes sense for a single 12 V panel on a 12 V battery, such as a trickle charge for a boat, a gate opener or a small shed light. Renogy’s Wanderer Li 30A, a common example, is a PWM controller optimized for 12 V systems, and its manual says panel voltage must not exceed 25 V. That low limit is why a PWM controller pairs with 12 V nominal panels wired in parallel, not in series.

How to size an MPPT charge controller

Sizing an MPPT charge controller takes two checks: current and voltage.

Current. Renogy’s sizing guide divides the array’s watts by the battery’s charging voltage, not its nominal voltage. With a 12 V battery charging at about 14.4 V, 400 W of panels works out to about 28 A, so a 30 A MPPT controller is the next size, provided its PV power and voltage limits also suit the array. The same 400 W on a 24 V battery charging at 28.8 V needs about 14 A.

Voltage. The panels’ open-circuit voltage must stay under the controller’s maximum PV voltage, and that voltage rises in cold weather. Victron’s SmartSolar MPPT 100/30, for example, takes up to 100 V from the panels. Its datasheet lists 440 W of panels on a 12 V battery and 880 W on 24 V, and it limits input if you connect more.

Wiring order for a solar charge controller

Connect the battery to the controller first, then the solar panels. Victron’s installation manual for its larger SmartSolar models says this order lets the controller detect the system voltage automatically, and that the wrong order can disable or damage it. Renogy’s Wanderer Li manual also says never to connect the panels before the battery. To shut down, Victron’s manual disconnects the panels first and the battery last.

Battery wiring, fuses and wire sizes for a fixed system follow the controller’s manual and local electrical code. Have a licensed electrician check any system tied into a building.

Most controllers also let you pick the battery type so the charge voltages suit it. Renogy’s Wanderer Li lists sealed, gel, flooded and lithium options, and its lithium mode is set for 12.8 V LiFePO4 batteries by default, so other lithium types need the battery maker’s settings. Set the type before the first charge, following the controller’s manual.

Which charge controller fits your setup

• One 100 W panel keeping a 12 V battery topped up: a small PWM controller.
• Two to four panels on a van or RV: an MPPT sized with the formula above. Our comparison of LiFePO4 vs NMC batteries explains how the two lithium chemistries differ in cycle life. RV solar panel kits works through daily watt-hours and kit sizes.
• A cabin or shed with panels on a long cable run: MPPT, so you can wire panels in series at a higher voltage.
• Charging a power station: no controller needed. Match panels to the station instead.

For the battery side, see the 12V LiFePO4 100Ah battery guide, or start from one of the solar panel kits that bundle a controller with panels. For balcony systems that feed the home through a microinverter instead of a battery, see plug-in balcony solar. More panel guides are in the solar panels section.