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Charge controller: sizing MPPT vs PWM
The charge controller protects the battery from overcharging and regulates energy transfer from the PV array to the battery bank. The choice between PWM and MPPT directly affects efficiency and how the array should be sized.
| Criteria | PWM | MPPT |
|---|---|---|
| Principle | Near-direct panel/battery connection | Maximum power point tracking, DC/DC conversion |
| Voltage constraint | Panel Vmp must be close to battery voltage | Panel Vmp can be much higher than battery voltage |
| Efficiency | ~75–80% | ~93–98% |
| Cost | Low | Moderate to high |
| Use case | Small systems, tight budget, 1–2 panels | Systems > 500W, long strings, cold/cloudy climates |
I_controller ≥ (P_PV_kWp × 1000 ÷ V_battery) × 1.2525% margin for high-irradiance conditions (cloud edge, reflection)
On a PWM controller, wiring a 36-cell panel (Vmp ≈ 18V) to a 24V battery doesn't work properly — panel voltage must exceed battery voltage by enough to charge it. This kind of mismatch is common with salvaged equipment.
For solar pumping or strings of more than 2 panels in series, SolarDim Pro always recommends MPPT — the efficiency gain (15 to 20 points) quickly offsets the extra cost, especially in overcast conditions or early/late in the day.
Common mistakes
- Using a PWM controller with a string whose Vmp is far above battery voltage (massive efficiency loss)
- Undersizing controller current based on Impp instead of adding the 25% margin
- Choosing PWM to save money on a pumping system where efficiency drives the water flow rate