A solar charge controller sits between your panels and your battery. It stops the panels overcharging the battery and, depending on the type, decides how much of the panel's power actually reaches it. In a caravan, a cabin, a gate-motor or camera setup, or any small 12V or 24V system, it is the part that decides whether your panels earn their keep.
There are two kinds. A PWM controller is simple and cheap, but it pulls the panel down to battery voltage and throws away the difference. An MPPT controller converts the panel's extra voltage into extra charging current. This guide explains when each makes sense, how to size an MPPT controller so a cold winter morning doesn't destroy it, and what changes with lithium batteries.
Key takeaways
A PWM controller connects the panel more or less straight to the battery. A 100W panel then delivers roughly battery voltage × the panel's current, about 75W on a 12V battery.
An MPPT controller lets the panel work at its best voltage and converts the surplus into current. Victron says this can harvest up to 30% more than PWM.
PWM only makes sense when the panel is built for the battery: 36-cell panels on 12V, 72-cell panels (or two 36-cell in series) on 24V.
Size an MPPT controller on two numbers: the array's open-circuit voltage on the coldest morning must stay under the controller's maximum, and its charge current must suit your array's watts.
Lithium batteries need a controller with a proper lithium profile. Hybrid inverters already contain MPPT trackers, so a home system usually doesn't need a separate controller.
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What a charge controller does
A solar panel is not a battery charger. Its voltage rises and falls with sunlight and temperature, and left connected straight to a battery it would keep pushing current in after the battery is full. The charge controller manages that. It:
charges in stages, typically bulk, absorption and float, so the battery reaches full charge and then sits at a lower holding voltage;
stops overcharging once the battery is full;
blocks reverse current, so the battery doesn't drain back through the panel at night; and
often has a load output that switches off small DC loads, such as lights, before the battery runs flat.
Victron's BlueSolar PWM-Light datasheet shows the basics. Its 12V settings are a 14.2V absorption voltage, a 13.8V float voltage and a load output that disconnects at 11.2V. Those three stages are fixed on that model, "not programmable".
PWM vs MPPT: the difference in one example
Every panel has a voltage at which it produces the most power, called the maximum power point (Vmp). For small 12V panels it sits well above battery voltage, so the panel can still charge a battery on a hot day or in weak light.
Take the Cinco 100W panel from our solar panel wattage guide. Its specification gives a Vmp of 18.0V at 5.56A.
On a PWM controller, the panel is switched onto the battery and runs at roughly battery voltage. At a charging voltage of about 13.5V, it delivers about 13.5V × 5.56A, or roughly 75W, at best. The rest of the panel's rated power is never used.
On an MPPT controller, the panel runs near its 18V sweet spot, and the controller steps the voltage down and the current up to suit the battery. Victron's SmartSolar MPPT 100/30 and 100/50 datasheet lists a maximum conversion efficiency of 98%. At standard test conditions, that leaves about 98W of the panel's 100W for the battery, before cable losses.
PWM controller75 W
MPPT controller98 W
Best-case charging power from a 100W panel (Vmp 18.0V, 5.56A) on a 12V battery at standard test conditions. SolarGuide calculation.
That is a best case for both. On a hot roof the panel's Vmp falls, which narrows the gap, and on a cold, bright day it rises, which widens it. Victron says an ultra-fast MPPT "will improve energy harvest by up to 30% compared to PWM charge controllers", especially when cloud makes light levels change constantly. EPEver's Tracer AN G3 manual gives 20% to 30%, and adds that the figure "may fluctuate". Treat those as manufacturer claims for good conditions, not a guaranteed gain.
The gap grows quickly with bigger panels. Cinco's 200W panel runs at 33.55V and 5.625A. On a 12V PWM controller it would deliver about 13.5V × 5.625A, roughly 76W, barely more than the 100W panel. In practice it won't connect at all: its open-circuit voltage is 43.53V, and Victron's PWM-Light accepts a maximum of 28V on a 12V system.
When PWM still makes sense
PWM isn't wrong, it is just narrow. Victron's PWM datasheets spell out the rule in a footnote: "For 12V use 36 cell solar panels. For 24V use 72 cell solar panels or 2x 36 cell in series."
PWM is a reasonable choice when:
you have one or two 36-cell, 12V-class panels on a 12V battery, such as a 100W to 160W panel for a caravan, trickle-charging or a small light;
the budget is very tight and the panel is small, so the watts lost are only a few; or
the system is a simple lighting or standby setup where the controller's load output and timer matter more than the last watts.
Choose MPPT when:
the panel's Vmp is well above battery voltage, such as 60-cell, 72-cell or home-sized panels;
you want to wire panels in series to cut cable losses on a long run;
the array is above a few hundred watts, where the lost energy adds up every day; or
you run lithium batteries and want a controller with a full lithium profile and BMS support.
Battery voltage matters too. A 24V or 48V battery suits larger arrays because the same power flows at lower current. Our guide to 12V vs 24V vs 48V battery systems explains when to move up.
How to size an MPPT controller
MPPT controllers are usually named by two numbers: the maximum PV voltage and the maximum charge current. A Victron "100/30" accepts up to 100V from the panels and charges at up to 30A. Size it in this order.
1. Open-circuit voltage on the coldest morning
This is the number that breaks controllers. A panel's open-circuit voltage (Voc) is the voltage it produces with nothing drawing current, and it rises as the panel gets colder. Victron's SmartSolar MPPT 100/30 manual puts it bluntly: "The maximum open circuit PV voltage can not exceed 100V," and "at ambient temperatures below 25°C, the Voc will be higher."
To check a design:
Find the panel's Voc and its Voc temperature coefficient on the datasheet.
Pick the coldest morning the site sees. Panels are coldest at sunrise, before they warm up.
Work out the rise: degrees below 25°C × the coefficient.
Multiply by the number of panels in series and compare with the controller's maximum.
Worked example (SolarGuide calculation). Cinco's 200W panel has a Voc of 43.53V and a Voc coefficient of -0.27% per °C. On a frosty -5°C morning, an assumption you should replace with your own site's minimum, the panel is 30°C below standard test conditions. Its Voc rises by 30 × 0.27% = 8.1%, to about 47.1V.
Panels in series
Voc at 25°C
Voc at -5°C
On a 100V controller
1
43.5V
47.1V
Fine
2
87.1V
94.1V
Fits, but only about 6V of headroom
3
130.6V
141.2V
Too high: needs a 150V or 250V controller
Manufacturers state their limits differently, so read the small print. EPEver's Tracer AN G3 datasheet lists 100V and 92V for its 10-series models, and the manual says the 92V figure applies "at 25℃ environment temperature". Victron's 100V is an absolute ceiling. Either way, two of these panels in series on a 100V-class controller is close to the line in a cold climate. Victron's manual says to use its MPPT sizing calculator, and a good installer will run the same check with the real panel and the real site.
2. Minimum voltage
An MPPT needs the panel voltage to sit comfortably above the battery. Victron's datasheet says "the PV voltage must exceed Vbat + 5V for the controller to start", and its manual says the nominal PV voltage should be at least 5V above battery voltage. For a 24V battery on a 100V controller, the manual gives a minimum of 72 cells in series and a maximum of 144 cells, which is four 12V-class panels.
That's also why a single 36-cell panel gains the least from MPPT: on a hot day its voltage isn't far above a 12V battery's.
3. Charge current and array watts
The controller's current rating limits how much power reaches the battery. Divide the array's watts by the battery's charging voltage to see roughly what current the array could push:
400W of panels on a 12V battery: 400 ÷ 13.5V ≈ 30A, so a 30A controller.
The same 400W on a 24V battery: about 15A.
Victron's datasheet gives the same picture as "nominal PV power": 440W on 12V and 880W on 24V for the 100/30, and 700W and 1,400W for the 100/50. Connecting more panel than that is allowed on these models, because "the controller will limit input power", but the extra is clipped on bright days. There is a hard limit on the input side too: the array's short-circuit current must stay under the controller's maximum, 35A for the 100/30, because "a PV array with a higher short circuit current may damage the controller."
EPEver states its ratings the same way. The Tracer 3210AN G3, for example, is a 30A controller rated at 390W on 12V and 780W on 24V.
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Lithium iron phosphate (LiFePO4) batteries charge differently from lead-acid. They don't need equalisation, they don't want temperature compensation on their voltage, and they must not be charged when they are too cold. A controller set up for lead-acid can overcharge a lithium battery or keep it at the wrong voltage.
What to look for:
A lithium profile. Victron's SmartSolar manual sets its LiFePO4 preset to 14.2V absorption for two hours and 13.5V float on a 12V system, with equalisation and temperature compensation disabled and a low-temperature cut-off of 5°C. Some PWM controllers have a lithium setting too: Victron's PWM-LCD&USB datasheet lists one for "LiFePO4 batteries (with internal BMS only)". The basic PWM-Light has none.
BMS coordination. Every lithium battery has a battery management system (BMS) that protects the cells and can cut charging. Better controllers can listen to it. EPEver's Tracer AN G3 manual describes a BMS-Link module that reads the battery's protection voltages and current limit and charges to "whichever is the smaller" of the BMS limit and the set limit. In a Victron system, a GX device coordinates charging with supported lithium batteries.
Recovery after a BMS cut-off. If the BMS disconnects a flat battery, the controller has to be able to wake it. Victron says its SmartSolar "will reconnect to a fully discharged Li-ion battery with integrated disconnect function", and EPEver lists a lithium self-activation function.
Always set the controller to the voltages on your battery's own datasheet, not a generic lithium preset, if they differ.
Built-in MPPT in hybrid inverters vs a standalone controller
If you are looking at a home system rather than a caravan, you probably won't buy a charge controller at all. A hybrid inverter contains its own MPPT trackers, works at much higher voltages and talks to the battery's BMS directly.
two MPPT trackers, one string each, at up to 13A per tracker;
an MPPT range of 150V to 425V, a 125V start-up voltage and a 500V maximum PV input;
up to 6,500W of DC input, a 48V battery and a stated MPPT efficiency above 99%; and
a lithium charging strategy described as "self-adaption to BMS", with BMS communication over RS485 or CAN.
Home panels of 555W to 630W run at about 40V each, so they are wired in strings of several panels to reach that voltage window. The cold-morning check is the same as above, just against 500V instead of 100V.
Standalone charge controller
MPPT inside a hybrid inverter
Typical system
12V or 24V caravan, cabin, gate, camera, small off-grid
48V home backup or full solar
PV voltage
Up to about 100V to 250V, depending on model
Hundreds of volts; 500V maximum on the Sunsynk 5kW
What it charges
The battery only (DC coupling)
The battery, and runs the house through the inverter
Expandability
Add another controller for another array
Fixed by the inverter's tracker count and input limits
Standalone controllers still appear in larger systems. Victron designs usually use separate SmartSolar MPPT controllers, one per array, feeding an inverter-charger, as our Victron guide explains. Fully off-grid homes and farms sometimes add an extra DC-coupled array the same way. Our off-grid system cost guide shows how those systems are sized and priced.
Quick checklist before you buy
Battery voltage: 12V, 24V or 48V, and does the controller support it?
Panel type: 36-cell or 72-cell for PWM; anything higher needs MPPT.
Cold Voc: series string Voc on your coldest morning, with headroom under the controller maximum.
Current: array watts ÷ charging voltage, against the controller's rated current and maximum short-circuit current.
Battery chemistry: a lithium profile, and BMS communication if your battery supports it.
Environment: check the operating temperature and derating. Victron's 100/30 gives full rated output up to 40°C.
If your needs have grown past a 12V setup into keeping the house running, compare home backup options with a free quote →. Alumo designs, installs and warrants the system, including strings that suit the inverter's MPPT inputs.
Frequently asked questions
Is MPPT always better than PWM?
It harvests more whenever the panel's voltage is well above the battery's, which covers most panels sold today. For one small 36-cell panel on a 12V battery the gain is smaller, and a PWM controller can be good enough. For larger panels, series strings or lithium batteries, MPPT is the better choice.
How much power do I lose with a PWM controller?
The panel runs at about battery voltage instead of its best voltage. A 100W panel with a Vmp of 18.0V and 5.56A delivers roughly 13.5V × 5.56A, about 75W, on a 12V battery at best. Manufacturers say MPPT can harvest 20% to 30% more, depending on conditions.
What size MPPT charge controller do I need?
Check two things. The panels' combined open-circuit voltage on your coldest morning must stay under the controller's maximum PV voltage, with some headroom. Then divide the array's watts by the battery's charging voltage to choose the current rating: about 30A for 400W on 12V, or 15A on 24V.
Do I need a charge controller with a hybrid inverter?
Usually not. A hybrid inverter has MPPT trackers built in and charges the battery itself. A separate controller is only added in some designs, such as a DC-coupled extra array in a Victron or off-grid system.
Is SolarGuide the installer?
No. SolarGuide is an independent referral partner. Alumo supplies, installs, finances and warrants the systems. We help you compare options and get a free quote. All pricing is indicative and subject to a site assessment and credit approval.
SolarGuide is an independent referral partner. We help you compare options and arrange a free quote — Alumo supplies, installs, finances and warrants the systems. Pricing is indicative, sourced from Alumo’s published catalogue, and subject to a site assessment and credit approval.
