How Many Solar Panels Do You Need on a Campervan?
Solar power has transformed campervan travel. A well-sized solar system allows you to wild camp for days without needing to drive or find a hook-up, run a fridge, charge devices, power lighting, and even run small appliances — all from the sun. But the most common mistake builders make is either over- or under-sizing their solar array, often because they have not done a proper energy audit first. This guide walks you through the calculation step by step, with a worked example and specific considerations for travelling in the UK, Germany, and the Netherlands.
Step 1: Calculate Your Daily Energy Consumption
Before choosing any solar panels, you need to know how many amp-hours (Ah) you consume per day. List every electrical load in the van and estimate how many hours per day it runs.
| Appliance | Typical Draw (A at 12V) | Hours/Day | Daily Ah |
|---|---|---|---|
| 12V compressor fridge (45 L) | ~3 A average | 24 | 30–40 |
| LED lighting (4 × 5W strips) | 1.7 A | 4 | 7 |
| Diesel/gas heater blower fan | 2 A average | 8 | 16 |
| Roof fan (Maxxair, medium speed) | 1.5 A | 6 | 9 |
| Phone charging (2 phones) | 1 A | 4 | 4 |
| Laptop (via 12V adapter) | 4 A | 3 | 12 |
| Water pump (intermittent use) | 7 A peak, ~2 A average | 0.5 | 3–5 |
| 12V TV or monitor | 3 A | 2 | 6 |
| Typical Total | 80–100 Ah |
A build used mainly for a couple on summer trips — fridge, lighting, phone charging, roof fan — will typically land in the 60–90 Ah/day range. Adding a laptop, a diesel heater (in winter), or regular inverter use for 230V devices pushes this to 100–140 Ah/day.
Step 2: Understand Peak Sun Hours in Your Region
Solar panels are rated in watts-peak (Wp), which is the output under Standard Test Conditions (STC): 1,000 W/m² irradiance at 25 °C cell temperature. In real-world conditions, a panel never consistently achieves its STC rating. The amount of usable solar energy available depends on latitude, season, and weather — expressed as "peak sun hours" (PSH) per day.
Average PSH figures for the UK, Germany, and the Netherlands:
| Location | Winter PSH (Dec–Feb) | Spring/Autumn PSH | Summer PSH (Jun–Aug) |
|---|---|---|---|
| United Kingdom (South) | 0.8–1.5 | 2.5–3.5 | 4.0–5.0 |
| Germany (South) | 1.0–1.5 | 3.0–4.0 | 4.5–5.5 |
| Netherlands | 0.7–1.2 | 2.5–3.5 | 4.0–4.8 |
These figures show why van life in northern Europe in winter is genuinely challenging on solar alone — you are working with less than 2 PSH on many days, which severely limits generation. This is why combining solar with a DC-DC charger (to charge from the alternator while driving) or a mains inverter/charger (for campsite hook-up) is essential for year-round travel.
Step 3: Apply the Panel Sizing Formula
The basic formula is:
Required panel Wp = (Daily Ah consumption × 12V) ÷ PSH ÷ system efficiency factor
The system efficiency factor accounts for real-world losses: cable resistance, charge controller efficiency, temperature derating, partial shading, and battery round-trip efficiency. A realistic combined efficiency factor for a well-designed system is 0.70–0.80.
Worked Example: A Two-Person Summer Build
Scenario: Two people, summer travel in the UK and northern France. Loads: 45 L fridge (35 Ah/day), LED lighting (5 Ah), phone charging (4 Ah), roof fan (7 Ah), laptop (12 Ah). Total: 63 Ah/day.
Available PSH: UK summer average, 4.0 PSH/day.
Calculation:
- Daily energy in watt-hours: 63 Ah × 12 V = 756 Wh
- Required panel output at STC: 756 Wh ÷ 4 PSH = 189 Wp at panel output
- Accounting for system efficiency (0.75): 189 ÷ 0.75 = 252 Wp minimum
Practical recommendation: install 300–400 Wp of panels. This provides a buffer for overcast days, partial shading, and the occasions when loads are higher than average. Two 175 Wp monocrystalline panels in parallel is a common and practical configuration for a standard wheelbase van.
MPPT vs PWM Solar Charge Controllers
The solar charge controller sits between your panels and your battery and regulates charging. The two main types are:
PWM (Pulse Width Modulation)
A PWM controller connects the panel directly to the battery when charging is needed, pulsing the connection to regulate voltage. It is simple, reliable, and inexpensive. However, it is only efficient when the panel's voltage is close to the battery voltage. If your panel's open-circuit voltage (Voc) is significantly higher than your battery voltage — as is common with 12V systems and modern panels — much of the panel's potential output is wasted.
MPPT (Maximum Power Point Tracking)
An MPPT controller uses electronics to operate the panel at its optimal voltage and current combination (the maximum power point), then converts that power to the correct charging voltage for the battery. MPPT controllers typically extract 15–30% more energy from the same panel in real-world conditions compared to PWM. They are more expensive — a quality MPPT controller from Victron, Renogy, or Epsolar costs £80–£250 depending on rating — but the efficiency gain pays back the extra cost relatively quickly.
For any system with more than 100 Wp of panels, MPPT is the correct choice. It becomes even more valuable in overcast UK and Dutch conditions where extracting maximum available energy from weak sunlight is critical.
Panel Types: Monocrystalline vs Flexible
Rigid Monocrystalline Panels
Rigid panels with aluminium frames and tempered glass are the standard choice for campervan roofs. They are robust, have a long service life (25+ years), carry meaningful warranties (typically 10–12 years product warranty, 25 years on output), and perform well in all conditions including cold weather. Monocrystalline cells are the most efficient type, with typical efficiencies of 19–22%. They are slightly more expensive than polycrystalline but have largely replaced poly in the campervan market.
Flexible (Thin-Film or Bendable Mono) Panels
Flexible panels are lightweight and can conform to curved roof profiles — useful on VW Transporter or Citroën Relay vans with pronounced roof curvature. Their disadvantages are significant: lower efficiency (often 16–18%), much shorter typical service life (5–10 years vs 25+ for rigid panels), poorer performance in heat (flexible panels mounted flush on a metal roof with no air gap overheat, reducing output significantly), and limited warranty coverage. They are also more expensive per watt.
Flexible panels are worth considering only where roof curvature makes rigid panel mounting impractical or where weight is a critical constraint. For most builds, two or three rigid monocrystalline panels properly mounted with a small air gap are the better long-term investment.
Partial Shading and Panel Layout
Partial shading is one of the least understood aspects of campervan solar. If a single cell in a panel string is shaded, it can reduce the output of the entire string disproportionately — depending on the system architecture. The practical implications:
- Park in shade during summer heat, but be aware this dramatically reduces solar generation.
- Roof-mounted obstacles (satellite dishes, aerials, roof racks, bike carriers) that shade even a corner of a panel can significantly reduce output.
- Panels with integrated bypass diodes mitigate shading effects within a panel but not across panels wired in series.
- For systems where partial shading is unavoidable, wiring panels in parallel (rather than series) generally results in less total output loss from partial shade.
- Some advanced MPPT controllers support individual panel tracking, further reducing shading losses.
Battery Storage and the Complete Picture
Solar panels generate energy only during daylight. Battery storage bridges the gap — overnight, during overcast stretches, and when parked in shade. Your battery bank should ideally hold two to three days of consumption without any solar input:
- For 80 Ah/day consumption: aim for 160–240 Ah of usable battery capacity.
- AGM batteries at 50% depth of discharge: 320–480 Ah nominal capacity needed.
- LiFePO4 batteries at 80% depth of discharge: 200–300 Ah nominal capacity needed.
Frequently Asked Questions
Can I add more solar panels later?
Yes, provided your charge controller has sufficient capacity for the additional panels. MPPT controllers are rated in amps and watts — check the maximum input before adding panels. If you plan to expand, buy a slightly oversized controller from the start.
Do solar panels work on overcast days?
Yes, but at significantly reduced output — typically 10–25% of their rated capacity on a fully overcast UK day. Diffuse light still generates some power. On a heavily overcast day with 0.5 PSH effective output, a 300 Wp system will produce around 38–75 Wh. This is useful but not sufficient to offset a full day's consumption without supplementary charging.
How do I mount solar panels to avoid leaks?
Use purpose-designed roof mounts with rubber gaskets and apply a quality UV-stable sealant (Sikaflex 252 or equivalent) around all roof penetrations. Aluminium mounting feet designed for van roofs are preferable to universal mounts. Cable entry glands should be marine-grade and properly sealed.
What cable size do I need for my solar panels?
Use 4 mm² solar cable (UV-resistant, double-insulated) for runs up to 5 m between panels and controller at up to 30 A. For longer runs or higher currents, increase to 6 mm². Undersized cable causes voltage drop that reduces energy harvested.
Will my van's metallic roof affect solar panel efficiency?
Metal roofs reflect heat but also conduct it. A rigid panel mounted with a small air gap (20–40 mm) stays cooler than one bonded directly to the roof, and panel efficiency is better at lower temperatures. For every 10 °C above the STC test temperature of 25 °C, panel output drops by approximately 3–5% for monocrystalline silicon.
