Solar charge controllers: PWM, MPPT and how to size one
A charge controller is the box between the panels and the battery. It lets the battery charge fully and stops it there. In a system built from separate parts, it is the part that decides whether the battery lasts years or months.
Below: what it does, the difference between the two kinds, and a sizing example worked from a panel sold here and a named controller’s datasheet. It is the same check we run when we design an off-grid solar system. For the battery on the other side of it, the battery page reads one lithium battery’s datasheet, and what is solar energy sets out the three kinds of system.
What it does, and where it sits
Panels deliver whatever the sun gives them. A battery needs charging at set voltages, and once it is full, it needs the charging to ease off. The controller does that, and in the evening it lets the battery run the loads while stopping current from leaking back into the panels.
An inverter for household appliances connects to the battery, not to the controller’s load terminals. Morningstar, a controller maker, gives the reason: an inverter can draw a surge at start-up larger than the controller is rated for, and the battery can supply it.
When it goes in, the order matters. Morningstar’s instruction is battery to the controller first, then the panels, then the loads, and the reverse when taking it out, because the controller takes its own power from the battery.
PWM and MPPT
- PWM, pulse width modulation
- Connects the panel to the battery and switches it on and off quickly to hold the right voltage. The panel ends up working at the battery’s voltage, whatever it was built for. Simple and cheap, and fine with a small panel made to charge a 12 V battery.
- MPPT, maximum power point tracking
- Lets the panel run at the voltage where it makes the most power and converts that down to what the battery needs, so the extra voltage becomes extra current. Victron’s datasheet claims up to 30 percent more harvest than PWM under changing cloud; that is the maker’s figure, not ours.
The difference is largest with the panels sold for houses today. The JinkoSolar JKM600N-72HL4-V makes its 600 W at 44.06 V. Held at a 12 V battery’s 14.4 V charging voltage by a PWM controller, its current cannot go above its short circuit figure of 14.25 A, so it delivers at most about 205 W. That is roughly 34 percent of what it is rated for. An MPPT controller lets it work near its rating.
Sizing one: voltage first, then current
A controller name like 100/30 carries its two limits: 100 V is the most the panels may ever present to it, and 30 A is the most it will push into the battery. Size against both, voltage first, because too much voltage damages a controller and too much current only gets limited.
1. Panels in series, against the voltage limit
A panel’s open circuit voltage rises as it gets colder, and it is highest at dawn, before the sun has warmed it but once there is light. This example uses 10°C, deliberately cold for a lowland roof; in the mountains, use your own lower figure.
- Open circuit voltage on the datasheet, at 25°C
- 52.95 V
- At 10°C, with the panel’s -0.25 percent per degree
- 54.94 V
- Two in series
- 109.87 V
Two in series is over the 100 V limit, so on either controller in this example the most in one string is 1 panel. More panels go in parallel instead, or the job wants a controller with a higher voltage rating. This is the check most often skipped, and the one that ends with a dead controller on the first cold morning.
2. Panels in parallel, against the current and power limits
Charge current is the array’s watts divided by the battery voltage. The datasheet also gives a nominal PV power for each battery voltage, above which the controller holds the input down, and a short circuit current the array must not exceed.
| Controller, on a 24 V battery | Nominal PV power | 600 W panels | Charge current |
|---|---|---|---|
| Victron SmartSolar MPPT 100/30Rated 30 A, short circuit limit 35 A | 880 W | 1 | 25.0 A |
| Victron SmartSolar MPPT 100/50Rated 50 A, short circuit limit 60 A | 1400 W | 2 | 50.0 A |
On a 12 V battery the same controllers are rated for 440 W and 700 W, so one 600 W panel on the MPPT 100/30 would be held back. That is the reason house-size off-grid systems run at 24 or 48 V: the same controller takes more panel.
When you do not need a separate controller
A hybrid inverter has MPPT inputs built in, so on a typical home system with a battery the panels connect to the inverter and there is no separate controller to buy. A standalone controller belongs in small DC systems, in off-grid setups built from separate parts, and in adding panels to a battery that already has an inverter.
Grid-tied, hybrid and off grid compared
What people ask about charge controllers
What does a solar charge controller do?
It sits between the panels and the battery and decides how much charge goes in. Without one, the panels would keep pushing current into a full battery and damage it. Most also protect against current flowing back to the panels at night, short circuits, and a battery connected the wrong way round.
What size solar charge controller do I need?
Two checks. First voltage: the panels in one string, at their open circuit voltage on a cold morning, must stay under the controller’s maximum PV voltage. Second current: the array’s watts divided by the battery voltage gives the charge current, which should be at or under the controller’s rated amps.
For one 600 W panel on a 24 V battery that is about 25.0 A, which a 30 A controller handles.
PWM or MPPT?
MPPT for any modern house-size panel. A PWM controller pulls the panel down to the battery’s voltage, and a 600 W panel built to work at 44.06 V delivers at most about 205 W into a 12 V battery that way. PWM suits small panels made for 12 V batteries.
Do I need a charge controller with a hybrid inverter?
Usually not a separate one. A hybrid inverter has its own MPPT inputs that do the same job, so the panels connect to the inverter. A standalone controller belongs in DC systems and in off-grid setups built from separate parts.
Tell us what you want to run, and where
Send the appliances, the hours and the site. You will get a written answer with a panel count, the battery voltage and the controller or inverter it needs.
Tell us the property and what you are trying to cut, and you will get a written answer with a size and a next step. No visit is booked and nothing is charged for asking.
Sources: Victron Energy, SmartSolar MPPT 100/30 and 100/50 datasheet, read 5 October 2026; Morningstar, How Does a Solar Charge Controller Work?, read 5 October 2026; JinkoSolar JKM580-605N-72HL4-(V) F8 datasheet, read 1 October 2026, the 600 W row. Named products are examples used for the arithmetic, not a quotation or a recommendation.