MPPT vs PWM charge controllers: which one do you need?

A charge controller is the small box between your solar panels and your battery. Shops sell two kinds, PWM and MPPT, and the price gap between them can be large, so it is fair to ask whether the expensive one is worth it. This guide explains both in plain words, when the cheaper one is fine, and the two numbers that matter most for safety: the voltage limit in cold weather and the current rating. It is for anyone building a small system for a shed, cabin, van, boat or backup battery, in the US or the UK.

The short version

• A charge controller stops your panels from overcharging the battery. Every system that charges a battery from panels needs one (an all-in-one "power station" has one built in).
• A PWM controller is basically a fast switch. It connects the panels straight to the battery, so the panels work at the battery's voltage, not at their best voltage. It is cheap and works well with one small panel that matches the battery.
• An MPPT controller is a converter. It runs the panels at their best voltage and turns the extra voltage into extra charging current. In a test example by one controller maker, it harvested 100 W where a PWM controller got 81 W from the same panel at 25 °C (77 °F).
• The gap depends on temperature. In hot panels, 45 to 75 °C (113 to 167 °F), PWM comes within about 10 % of MPPT. In cold, in weak winter light or with partial shade, MPPT wins clearly.
• The cold-voltage limit is the safety rule people miss. Panel voltage rises as panels get colder. A string that looks fine on a mild day can go over the controller's limit on a frosty morning, which can damage it.
• Size the controller by current: roughly panel watts divided by battery voltage, within the maker's limits. Our quiz checks all of this against your location's coldest recorded morning.

What a charge controller does

Think of your battery as a bucket and your panels as a tap the sun controls. The charge controller slows the tap to a trickle when the bucket is nearly full, so nothing overflows.

In electrical terms, it watches the battery's voltage (the electrical "pressure", in volts, V) and limits the current (the flow, in amps, A) going in, because overcharging shortens a battery's life or damages it. One maker's datasheet, for example, holds a 12 V battery at 14.4 V to finish charging, then 13.8 V to keep it full, both adjustable. New to these words? See our solar glossary.

PWM in plain words

PWM stands for pulse width modulation: once the battery is nearly full, it switches the panels on and off very quickly to hold the right voltage. Before that, it simply connects the panels to the battery. Victron Energy, a controller maker that sells both types, describes it in its white paper as "in essence a switch that connects a solar array to the battery", with the result that "the voltage of the array will be pulled down to near that of the battery."

Why does that matter? Every panel has one voltage at which it gives its most power, called the maximum power point. For the typical small "12 V" panel in Victron's example (100 W, 36 cells), that point is 18 V at 5.56 A, because 18 × 5.56 = 100 W. A battery being charged sits around 13 V. When a PWM controller ties the two together, the panel is forced down to about 13.5 V (battery plus about 0.5 V lost in wires and controller). The current barely rises, so the power falls. In the paper's example, the panel gives 13.5 V × 6 A = 81 W instead of 100 W: 19 % less.

MPPT in plain words

MPPT stands for maximum power point tracking. The controller keeps searching for the panel's best voltage, holds the panel there, and then converts the power down to the battery's voltage.

A good comparison is a bicycle gear. Your legs (the panels) are strongest at one pedalling speed. MPPT changes gear so your legs stay at that speed whatever the road (the battery) is doing. PWM is a bike with one fixed gear.

Because power is voltage times current, converting down in voltage pushes up the current. Using the same example panel: 100 W at the battery's 13 V gives 100 ÷ 13 = 7.7 A into the battery, where the panel itself only delivers 5.56 A. A small amount is lost in the conversion; one maker's datasheet gives a maximum efficiency of 98 %.

MPPT has a second benefit: you can wire panels in series (one after another, so their voltages add up) and send 36 V or more to a 12 V battery. Higher voltage means lower current, and Victron's paper notes that doubling the array voltage lets you cut the cable's cross-section by four for the same loss. Our guide on solar cable size and fuses shows this with numbers.

How big is the difference really?

• At 25 °C (77 °F) panel temperature, Victron's worked example shows MPPT at 100 W and PWM at 81 W.
• At 75 °C (167 °F), hot panels lose voltage. The same panel's best voltage drops to 13.8 V, very close to the battery's voltage, so PWM and MPPT both deliver about 77 W. The paper calls the advantage "nil".
• Between 45 and 75 °C (113 and 167 °F) panel temperature, and with battery voltages from 13 to 15 V, the paper finds PWM "performs nearly as well (within 10 %)" as MPPT.
• Outside that band, in cold panels, in weak light below about 200 W/m² (an overcast or winter sky; full sun is about 1,000 W/m²) or with partial shade, Victron's blog puts the MPPT advantage at "10 % to 40 %".
• Product pages often say "up to 30 %". That is a best-case marketing figure from a maker, not an average.

How hot do panels get? The paper says panels typically run about 25 °C above the air on a mild sunny day, and free-standing panels can reach 70 to 80 °C (158 to 176 °F) on a hot, still day.

So in a hot climate with panels that match the battery, PWM loses little. In a cold or cloudy climate, such as a UK winter or a northern US cabin, MPPT collects noticeably more, and winter usually decides how big your system must be. Our article on watts, watt-hours and sun hours explains why winter drives the size.

When PWM is fine

PWM is a sensible choice when all of these are true:

1. The panel's nominal voltage matches the battery: a "12 V" panel (36 cells, best voltage about 18 V) on a 12 V battery. PWM cannot convert voltage, so a higher-voltage panel wastes its extra volts.
2. The system is small. Victron suggests MPPT pays off above a few hundred watts on a 12 V battery, largely because low-voltage cable gets thick and expensive.
3. You do not rely on winter or cloudy-day output, and the cable run is short.

A single small panel keeping a boat or caravan battery topped up is the classic case. Choose MPPT for large panels (60 cells or more), panels in series, winter use, partial shade or long cable runs.

The cold-voltage limit: the rule people miss

Every controller has a maximum PV input voltage (PV means photovoltaic, the panels); in the datasheet we cite, the "100" in the model name is its 100 V limit. If the panels ever push more than that, the controller can be damaged. The trap is that panel voltage goes up in the cold.

The key number on the panel's datasheet is Voc, the open-circuit voltage: the voltage across the panel's two wires when nothing is connected, which is also what the controller sees at dawn before it starts drawing current. Voc is measured at a standard 25 °C (77 °F) cell temperature. The datasheet also gives a temperature coefficient of Voc, a small negative percentage per degree, for example −0.35 %/°C. Negative means voltage falls as the panel warms, so it rises as the panel cools.

In the US, the National Electrical Code (NEC) section 690.7 requires the maximum system voltage to be worked out at the lowest expected temperature, using the panel maker's coefficient where it is given, as described by the IAEI, the electrical inspectors' association. When there is no coefficient, the code offers a table of multipliers, from 1.02 near 24 °C (75 °F) up to 1.25 at −40 °C (−40 °F). UK installations follow BS 7671, where equipment on the panel side must also be rated for the highest voltage the panels can produce.

A worked example

Our calculation from the cited figures. Take the example panel from Victron's white paper: Voc 21.6 V, temperature coefficient of Voc −0.35 %/°C. You want to put four of them in series on a controller with a 100 V maximum input.

• On a mild day at 25 °C (77 °F): 4 × 21.6 V = 86.4 V. Looks safe.
• On a morning at −20 °C (−4 °F), the panel is 45 °C colder than the test temperature. Each panel's Voc rises by 45 × 0.35 % = 15.75 %, to 21.6 × 1.1575 = 25.0 V.
• Four in series: 4 × 25.0 V = 100.0 V (100.008 V before rounding). That is already above the 100 V limit.
• At −30 °C (−22 °F) it would be about 103 V. With the NEC table's −40 °C multiplier of 1.25, it would be 108 V.

Three panels in series would give 75 V at −20 °C, with a comfortable margin. That is the whole decision: the same panels, one fewer in each string.

Panels at dawn are at about air temperature, so the coldest morning gives the highest voltage. The formula: cold Voc = Voc × (1 + coefficient × (cold temperature − 25)) × panels in series, with the coefficient as a decimal (−0.0035 for −0.35 %/°C).

Sizing the controller's current

MPPT. The current out of the controller into the battery is roughly the panel watts divided by the battery voltage. Our calculation: 400 W of panels on a 12 V battery charging at about 13 V gives 400 ÷ 13 ≈ 31 A; on a 24 V battery (about 26 V), about 15 A. Makers publish this as a "nominal PV power" per battery voltage. One 30 A model's datasheet lists 440 W at 12 V and 880 W at 24 V, and says that if more panel power is connected, the controller limits its input. Extra panels are then partly wasted at midday, not dangerous.

Two more limits on the same datasheet matter:

• Maximum PV short-circuit current. Isc is the most current a panel can push (with its wires joined). The datasheet warns that an array with a higher short-circuit current "may damage the controller". US practice under NEC 690.8 multiplies Isc by 1.25 to allow for sunlight stronger than the test level; the IAEI notes this "represents a potential irradiance of 1250 W/m²".
• Start voltage. The same datasheet says the panel voltage "must exceed Vbat + 5 V for the controller to start", then stay above battery voltage + 1 V. Hot panels lose voltage, so a single "12 V" panel can be marginal on a 12 V battery in high summer.

PWM. A PWM controller passes the panel current straight through, so its rating must cover the panels' short-circuit current with margin, using the same 1.25 factor.

The battery has a say too. The charging current from the controller plus any other charger (such as a van's alternator charger) must stay within what the battery maker recommends. See how big a solar battery you need covers battery sizing.

What to check before you buy

1. Find your panel's Voc, Isc and temperature coefficient of Voc on its datasheet.
2. Work out the cold Voc of your string for your coldest morning and keep it below the controller's maximum PV voltage, with margin.
3. Match the controller's battery voltage (12, 24 or 48 V) to your battery.
4. Check the nominal PV power and the short-circuit current limit (against 1.25 × Isc × parallel strings).
5. MPPT: panel voltage on a hot day at least 5 V above the battery's charging voltage. PWM: panel voltage class equal to the battery's.
6. Keep the charge current within your battery's recommended limit.

What can go wrong

• Cold-morning overvoltage. The string that was fine in autumn exceeds the controller's limit in January and damages it.
• Panel voltage too low. Hot panels leave too little voltage for the controller to start: full sun, no charging.
• A big panel on PWM. A 60-cell panel on a 12 V battery through PWM wastes much of its power.
• Too much short-circuit current from parallel strings can damage the controller.
• Charging a cold lithium battery. A LiFePO4 battery must not take charge below about 0 °C (32 °F). See lithium batteries in cold weather.
• Mismatched system voltage between controller and battery.

What our quiz checks

When our quiz builds a system, the controller must pass each check below. If a value is missing from the maker's documents, the check cannot pass and the product is not recommended.

• Cold voltage: the coldest daily minimum air temperature in your location's multi-year weather record, with the panel's own coefficient, as in the worked example.
• Panel power and short-circuit current (1.25 × Isc × parallel strings) against the controller's or power station's limits.
• Start voltage: MPPT needs the hot-day panel voltage above the battery's charging voltage plus the controller's margin; PWM needs matching voltage classes.
• One system voltage (12, 24 or 48 V), charge current within the battery's recommended limit, and no charging of a lithium battery below its temperature limit.

Our methodology page lists the sources and assumptions behind each check.

Questions people ask

Can I use an MPPT controller with a single small panel? Yes. It works, but with a "12 V" panel on a 12 V battery in warm weather the gain over PWM can be small.

Do I need a controller for a power station? No, it is built in. You still check its solar input voltage and power limits the same way.

Do I need an electrician? For anything wired into a building, yes: US wiring follows the NEC and UK wiring BS 7671. See our cable and fuse guide.

Next step

Tell us where and how you will use solar, and our quiz gives you a ready, checked parts list sized for your location's real weather, with the controller's voltage and current already checked. Every number is explained.

Our results and guides are estimates and do not replace advice from a qualified electrician.