MPPT vs PWM charge controllers: which one do you need?
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Between your solar panels and your battery sits a charge controller. It stops the battery being overcharged and decides how much of the panels’ energy actually reaches it. Many hybrid inverters have one built in; otherwise it is a separate box. There are two types.
PWM: simple and cheap
A PWM (pulse width modulation) controller connects the panels almost directly to the battery and switches them on and off rapidly to control charging. The panels are forced to work at roughly the battery’s voltage.
That works well only when the panel voltage is a close match for the battery, such as small “12 V” panels charging a 12 V battery. With the large panels sold for homes today, whose working voltage is far above 12 V, a PWM controller wastes much of the power they could produce.
MPPT: harvests more energy
An MPPT (maximum power point tracking) controller constantly finds the voltage at which the panels produce the most power, then converts it down to what the battery needs. That means:
- more energy from the same panels, often noticeably more, especially in cool weather or when panel voltage is well above battery voltage;
- panels can be wired in series at higher voltage, which means lower current and thinner, cheaper cables over long runs;
- freedom to use large modern panels with any battery voltage.
The trade-off is cost, though the gap has shrunk, and most home hybrid inverters now include MPPT.
Which should you choose?
- MPPT for almost any home system, any system with large modern panels, and anywhere the panels are a long way from the battery.
- PWM only for very small, low-cost setups with panels matched to the battery voltage.
Sizing a charge controller
Two numbers must fit:
- Current: the controller’s charging current must handle the panels’ output. A quick estimate:
controller amps ≈ total panel watts ÷ battery voltage × 1.25 For 1,800 W of panels on a 24 V battery: 1,800 ÷ 24 × 1.25 ≈ 94 A, so a 100 A controller.
- Input voltage: the panels’ combined open-circuit voltage, with a margin for cold mornings when panel voltage rises, must stay below the controller’s maximum input voltage. Exceeding it can destroy the controller. Your installer should check this against the panel and controller data sheets.
Higher battery voltage means lower current, which is one reason bigger systems use 24 V or 48 V. See 12 V, 24 V or 48 V.
The calculator suggests a controller current for your panel array.