MPPT vs PWM: Which Charge Controller Should You Use?

Intro

Off-grid power lives or dies by charging efficiency. Here’s a field-tested breakdown of MPPT vs PWM so you pick the right controller for your array, climate, and budget—without guesswork.

What a Charge Controller Actually Does

• Prevents overcharging.
• Sets proper charge stages (bulk/absorb/float).
• Protects batteries from reverse current at night.

PWM in Plain English

• How it works: Rapidly connects/disconnects panels to hold battery at target voltage.
• Best when:
  • 12V panels into 12V batteries (voltage matched)
  • Small arrays (≤400W)
  • Warm climates with short wire runs
• Pros: Simple, cheap, reliable.
• Cons: Wastes panel voltage headroom; lower harvest in cold/shoulder sun.

MPPT in Plain English

• How it works: DC-DC converter “tracks” the array’s max power point and steps voltage down while boosting current.
• Best when:
  • Higher-voltage strings into 12/24/48V banks
  • Cold climates (panel voltage rises in cold)
  • Long wire runs (use higher array voltage to cut amp losses)
• Pros: 10–30% more harvest in many real conditions; flexible stringing.
• Cons: Higher cost; needs correct sizing.

How Much Extra Power Does MPPT Get?

• Cold mornings / winter: biggest gains (15–30% common).
• Hot summer noon: small gains (0–10%).
• Cloud edge / variable irradiance: MPPT reacts faster, often +10–20%.

Sizing: Don’t Cook the Controller

• Controller input limits are for STC; cold can push Voc higher.
• Use array Voc @ min temp from the panel datasheet and keep below controller max Voc with headroom (~10%).
• Output current = array watts ÷ battery volts ÷ efficiency (~0.98).
• Pick next size up if near limits or if you want expansion.

Quick Picks (rule of thumb)

• ≤200–400W, 12V battery, panels close by, warm climate → PWM is fine.
• >400W, cold winters, long runs, 24/48V bank, mixed panel Vmp → MPPT.

Wiring & Safety Notes

• Use proper DC breakers/fuses, combiner if multiple strings.
• Keep controller near the batteries (short, fat battery cables).
• Match charge profile to chemistry (LiFePO4 vs AGM).

FAQ

Q: Can I upgrade to MPPT later?
A: Yes—wire the array for the future voltage now if possible.

Q: Two smaller MPPTs or one big one?
A: Two can be more flexible/efficient with differently oriented sub-arrays.

Q: Will MPPT fix shade issues?
A: No. Shade kills current. Use parallel strings/microinverters if shade is frequent.