MPPT vs PWM Solar Controllers for UK Weather: Which is Best for Your Campervan?
The solar charge controller is the middleman between your solar panels and your battery. It converts the panel voltage to the correct charging voltage. The type you choose — MPPT or PWM — determines how much of your panel's potential reaches your battery, especially in UK weather.
The £52 upgrade paid for itself within weeks. This guide explains why.
How They Work
PWM (Pulse Width Modulation)
A PWM controller connects the solar panel directly to the battery. The panel voltage is pulled down to match the battery voltage. The excess voltage is lost as heat.
Example: A 200W panel produces 36V at 5.5A (open circuit). Connected to a 12.5V battery via PWM:
- 36V × 5.5A = 200W (panel capability)
- 12.5V × 5.5A = 69W (what reaches the battery)
- 131W (65%) is lost
The PWM controller essentially wastes the voltage headroom of the panel. It only uses the current.
MPPT (Maximum Power Point Tracking)
An MPPT controller is a DC-DC converter. It takes the panel's high voltage, low current and converts it to low voltage, high current while maintaining the same power (minus efficiency losses).
Example: Same 200W panel at 36V, 5.5A, connected to a 12.5V battery via MPPT:
- Input: 36V × 5.5A = 200W
- Output: 14.4V × 13.2A = 190W (after 95% efficiency loss)
- The MPPT extracted 190W vs the PWM's 69W — 175% more power
Why This Matters in the UK
The UK rarely gets full sun. On overcast days, solar panels operate at lower voltage and current. An MPPT controller is better at extracting power from these low-light conditions because it continuously tracks the panel's maximum power point.
On a bright overcast day (the most common UK weather):
- PWM: A 200W panel delivers 40–60W
- MPPT: Same panel delivers 70–110W
That extra 30–50W is the difference between charging your battery and barely treading water.
Efficiency Comparison
| Condition | PWM Efficiency | MPPT Efficiency | MPPT Advantage |
|---|---|---|---|
| Full sun, hot panel (60°C) | 65–70% | 93–96% | +35% |
| Full sun, cool panel (25°C) | 60–65% (panel Vmp drops) | 95–97% | +50% |
| Bright overcast | 50–60% | 90–95% | +60% |
| Heavy overcast | 40–50% | 85–90% | +80% |
| Winter (low sun angle) | 35–45% | 80–90% | +100% |
The MPPT advantage increases as conditions worsen. In UK winter (low sun, frequent cloud), an MPPT controller often delivers twice the power of a PWM controller from the same panels.
When PWM Is Acceptable
| Scenario | PWM OK? | Reason |
|---|---|---|
| Single 100W panel on a weekend van | Yes | Low power, low cost, the absolute loss is small |
| Panel voltage is close to battery voltage | Yes | 12V panels (Vmp ~18V) have less excess voltage to waste |
| Budget is critical (under £50 total for solar) | Yes | A PWM controller costs £10–15 |
| You are adding solar to a lead-acid battery | Possibly | Lead-acid is less sensitive to charging profile |
| Anything else | No | MPPT pays for itself in 1–2 seasons |
Cost Analysis
| System Size | PWM Controller | MPPT Controller | Extra Cost | Annual Gain (UK) | Payback |
|---|---|---|---|---|---|
| 100W | £12 | £40–60 | £30–50 | 20–30kWh | 1–2 years |
| 200W | £15 | £60–90 | £45–75 | 40–60kWh | 1 year |
| 400W | £20 | £90–150 | £70–130 | 80–120kWh | 6–9 months |
| 600W | £25 | £130–200 | £105–175 | 120–180kWh | 6 months |
Payback calculation: At 25p/kWh (typical UK electricity), 80kWh/year = £20 saved. The MPPT premium of £70 is paid back in 3.5 years at this rate.
But this misses the point: MPPT means you have more power when you need it. On a winter's day, the difference between 40W (PWM) and 80W (MPPT) can determine whether your battery reaches full charge before the sun sets.
Wiring Configurations
MPPT controllers also allow series panel wiring, which PWM controllers do not.
Series Wiring (MPPT Only)
Panels wired in series produce higher voltage at lower current. For example, two 200W panels (36V each) in series = 72V at 5.5A.
Benefits:
- Lower current = thinner cables (4mm² instead of 10mm²)
- Less voltage drop over long runs
- MPPT converts the high voltage efficiently to 14.4V charging
Parallel Wiring (PWM or MPPT)
Panels wired in parallel produce the same voltage, summed current. Two 200W panels in parallel = 36V at 11A.
Benefits:
- Partial shade affects only one panel
- Compatible with both PWM and MPPT controllers
Voltage Drop: The Hidden PWM Problem
When a PWM controller pulls panel voltage down to battery voltage, the cable resistance has a larger effect. At 12V, a 0.5V drop is 4% loss. At 36V (MPPT), the same 0.5V drop is 1.4% loss.
For a 5m cable run from roof panels to controller:
| Controller | Panel Voltage | Same Cable | Drop | Loss |
|---|---|---|---|---|
| PWM | 18V | 6mm², 5m | 0.4V | 2.2% |
| MPPT | 72V (series) | 6mm², 5m | 0.1V | 0.14% |
MPPT allows higher voltage panels/series wiring, which reduces cable losses by 10–20×.
Low-Light Performance
The most important differentiator for UK van life. On overcast days, solar panels produce less current but maintain near-full voltage.
| Light Level | Panel Current | Panel Voltage | PWM Output | MPPT Output |
|---|---|---|---|---|
| Full sun (1000W/m²) | 5.5A | 36V | 69W | 190W |
| Bright cloud (600W/m²) | 3.3A | 34V | 41W | 105W |
| Overcast (300W/m²) | 1.7A | 32V | 21W | 50W |
| Heavy overcast (100W/m²) | 0.6A | 28V | 8W | 16W |
PWM output collapses faster in low light because it cannot use the panel's voltage headroom. MPPT maintains better output because it finds the optimum voltage/current point for the reduced light.
Recommended Controllers
| Controller | Type | Max Panel (12V) | Price | Best For |
|---|---|---|---|---|
| Victron SmartSolar 75/15 | MPPT | 200W | £70 | Small systems, value king |
| Victron SmartSolar 100/30 | MPPT | 440W | £130 | Standard van system |
| Victron SmartSolar 100/50 | MPPT | 700W | £190 | Large systems, future-proof |
| Renogy Rover 40A | MPPT | 500W | £90 | Budget MPPT, wired display |
| EPEver Tracer 40A | MPPT | 500W | £70 | Budget, no Bluetooth |
| Generic PWM 30A | PWM | 400W | £15 | Emergency backup only |
Recommendation: Buy a Victron SmartSolar. The Bluetooth monitoring alone is worth the premium. You can see real-time panel voltage, charge current, battery voltage, and historical data on your phone.
FAQ
Q: Can I use a PWM controller if I already have one? A: Use it for now but replace it when you can. The upgrade from PWM to MPPT is the single highest-ROI improvement you can make to a solar system.
Q: Does MPPT make a difference in winter? A: Yes — more than in summer. MPPT doubles or triples winter output compared to PWM because it extracts power from low light and low sun angles more efficiently.
Q: Do I need Bluetooth on my MPPT controller? A: Not essential but highly recommended. Bluetooth monitoring shows you real-time performance, historical data, and helps diagnose problems. Victron's app is excellent.
Q: Can I connect an MPPT controller to a PWM panel? A: Yes. The controller does not care about the panel brand or type. It only cares about the panel's voltage and current output.
Q: What is the maximum panel voltage for an MPPT controller? A: Depends on the controller. The Victron 75/15 accepts up to 75V open-circuit. The 100/30 accepts up to 100V. Always check the controller's maximum PV voltage rating — exceeding it can destroy the controller.
