Solar Charge Controllers: PWM vs MPPT
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⚡ PWM vs MPPT Solar Charge Controllers: What’s the Difference?
Both PWM and MPPT regulate solar charging — but the way they approach it is very different, and the efficiency you get can vary signficantly. Let's have a look at the differences between the two technlogies, and how that can affect your production numbers.
Skim this if you’re new — or skip if you already speak solar.
- Watts (W)
- Unit of power — how fast energy is delivered. W = V × A.
- Voltage (V)
- Electrical “pressure.” Higher voltage can push the same power with less current (useful for long cable runs).
- Current (A)
- Flow of charge (amps). More current means more heat and voltage drop in wires.
- Voc — Open‑Circuit Voltage
- Panel voltage with no load attached (no current). Used for safe design and not exceeding controller limits.
- Vmp — Voltage at Maximum Power
- Output voltage wWhere the panel will naturally makes the most watts.
- Imp — Current at Maximum Power
- The current (amps) at Vmp. (Vmp × Imp = panel’s rated watts).
- Isc — Short‑Circuit Current
- Max current (amps) the panel can produce when the panel leads are shorted (in full sun). Important for wire/fuse sizing and controller input specs.
- Vdrop — Voltage Drop (Cable Losses, I²R)
- Voltage lost in cables due to resistance. As current flows through a cable, the resistance of the cable requires voltage to be sacrificed to keep the electrons moving (Vdrop = I × R). Higher currents and longer runs increase these losses.
PWM vs MPPT at a Glance
PWM (Pulse Width Modulation) is like a fast on/off switch. It holds the battery at its charge voltage by rapidly connecting and disconnecting the solar array. If your battery sits near 13 V but the panel’s maximum‑power voltage (Vmp) is 18–20 V, the PWM controller pulls the panel's output voltage down to the battery's voltage. The panel is forced out of its most efficient operating point, so a chunk of potential power is simply never harvested.
MPPT (Maximum Power Point Tracking) is a smart DC‑DC converter. It continually tracks the voltage/current combination where the panel can produce the most watts (its MPP). It does it's best to hold the panel there, without forcibly lowering the output voltage. The MPPT controller then converts the power to the lower battery voltage (while increasing current so the watts are preserved).
In short: MPPT turns volts you can’t use into amps that you can.
Visual: How PWM vs MPPT Work
MPPT holds the panel at it's optimal voltage (Vmp) and converts excess voltage into higher current for faster charging.
Efficiency in Real Systems
Let's have a look at an actual example, and try to understand the math. Below we will compare the charging potential of a 200W panel using both technologies, and then 2 panels in series.
With a 200 W panel rated roughly Vmp ≈ 18 V and Imp ≈ 11 A:
- PWM typically delivers ~140 W into a 12 V battery (about 70–75% of panel rating).
- MPPT commonly delivers 195–200 W into the same battery (up to ~99% of panel rating).
Example A: Single 200 W Panel → 12 V Battery
Example B: 2×200 W Panels in Series → 12 V Battery
MPPT converts nearly all the available power into usable charging energy.
Why the gap changes with wiring:
In series, voltage rises and current drops for the same power (at the panels). Lower current means lower voltage drop (Vdrop) and lower I²R heating in cables. This translates to potential cost savings, as well as improved charging.
In parallel, array voltages sit closer to battery voltage so PWM wastes less headroom (vs PWM with Series) — but the current in the cableing rises, which increases Vdrop and losses (unless you use thicker, more expensive wire). MPPT with series strings usually wins on both harvest and wiring.
Example C: 2×200 W Panels in Parallel → 12 V Battery
Even though parallel wiring looks better than series in a 12 V system with PWM (because the panel voltage is already closer to battery voltage), it still wastes about a third of the available power compared to MPPT. The trade-off is higher current: ~22 A from just two panels means more voltage drop, more heat in the wires, and often the need for thicker, more expensive cabling. It also means the wiring is already heavily loaded, leaving little headroom to expand the system later without rewiring. Series wiring, by contrast, keeps current lower and wiring losses minimal — but requires MPPT to unlock the extra voltage. In practice, most modern systems favor MPPT with series strings, while parallel is reserved for shade-heavy installs where each panel needs to contribute independently.
Voltage Drop & Cable Loss: Series vs Parallel (Worked Example)
Assumptions: two 200 W panels (Vmp≈20 V, Imp≈10 A each), 10 AWG copper ≈ 0.001 Ω/ft, 15 ft one‑way run (30 ft round‑trip).
The Math (Using Easy Numbers)
Cable resistance: 10 AWG ≈ 0.001 Ω/ft × 30 ft round‑trip = 0.03 Ω
Parallel (20 V, 20 A): Vdrop = I×R = 20×0.03 = 0.60 V → 0.60/20 ≈ 3.0% • Ploss = I²R = 20²×0.03 = 12 W
Series (40 V, 10 A): Vdrop = 10×0.03 = 0.30 V → 0.30/40 ≈ 0.75% • Ploss = 10²×0.03 = 3 W
What these numbers show is that current is the enemy when it comes to wiring efficiency. In parallel, the array pushes 20 A through the cable, which multiplies losses (I²R) and creates a 3% voltage drop — 12 W of wasted solar power before it even reaches the controller. By wiring the same panels in series, the current is cut in half, so the losses fall to only 3 W and the drop is less than 1%. The math confirms what many installers see in practice: higher voltage, lower current wiring keeps more of your solar energy usable, especially as cable runs get longer.
When to Choose Which
- PWM — Small/simple setups where panel voltage ≈ battery voltage (e.g., one 100W ~18Vmp panel to a 12 V battery). Lowest cost, but headroom is wasted.
- MPPT — Higher power, Multi‑panel arrays, higher‑voltage strings, or long cable runs. More energy will be harvested, using lower cost and cleaner wiring.
Conclusion
For RVs, vans, boats, and off‑grid cabins, we typically recommend MPPT charge controllers with series strings. You’ll collect more energy, manage voltage drop better, and keep wiring simpler. In partial shading scenarios, parallel panels can produce better, but the MPPT will still harness more of the available power.
If you’re building a very small weekend system with a single panel, PWM can still make sense. It works well when you simply don't need the maximum harvest, but can instead rely on longer charging times.
If you're still unsure, send us a message with your plans. We’re happy to help you choose.