Roof-mounted solar panels are the primary way modern motorhomes achieve long-term energy independence without relying on campsites, generators, or shore power. Unlike residential solar installations, however, the roof of a motorhome presents strict limitations: the available space must be shared with air conditioners, roof windows, ventilation systems, antennas, and other equipment, while every additional kilogram counts against the vehicle's payload.
As a result, choosing the right solar panels — and wiring them correctly — has a much greater impact than in a typical residential installation. This article explains modern solar cell technologies, the physical limitations that are not immediately obvious from manufacturers' datasheets, and the practical aspects of wiring panels in series, parallel, or hybrid configurations.
Monocrystalline vs. Polycrystalline
To understand newer technologies such as TOPCon and HJT, it helps to first understand the basic differences in the silicon structure used to manufacture solar cells.
Polycrystalline Panels
Polycrystalline panels are manufactured by casting molten silicon into molds. As the material cools, numerous small crystals form with different orientations, giving the cells their characteristic blue, shimmering appearance. Typical efficiencies range between 15–17%. For motorhomes, where roof space is the most valuable resource, polycrystalline panels have become largely obsolete due to their relatively low power density.
Monocrystalline Panels
Monocrystalline panels are sliced from a single silicon crystal grown using the Czochralski process. Their uniform crystal structure gives them their characteristic deep black appearance. Modern monocrystalline panels typically achieve efficiencies of 20–22% while also delivering noticeably better performance under low-light and diffuse-light conditions. Today, monocrystalline technology is the standard choice for virtually all RV solar installations.
The New Generation: TOPCon and HJT
If you want to maximize energy production during the early morning, late afternoon, or under cloudy skies, conventional PERC monocrystalline panels are no longer the state of the art. Modern n-type silicon technologies provide significantly higher efficiency and lower long-term degradation.
TOPCon (Tunnel Oxide Passivated Contact)
TOPCon technology improves the rear contact of the solar cell by inserting an ultra-thin tunnel oxide layer and highly doped polycrystalline silicon between the silicon wafer and the metal contact. This dramatically reduces electron recombination losses and increases cell efficiency to typically 22.5–23% or higher. For motorhomes, TOPCon's greatest advantage is its improved spectral response, allowing it to generate more power under low sun angles and diffuse light conditions commonly encountered during cloudy weather.
HJT (Heterojunction Technology)
HJT combines crystalline silicon with thin layers of amorphous silicon. The resulting "sandwich" structure produces some of the lowest electrical losses of any commercially available solar technology, with efficiencies approaching 24–25%. For mobile installations, HJT offers further advantages:
- Excellent high-temperature performance — the lowest power loss coefficient of the three technologies.
- Virtually no susceptibility to microcracking — the amorphous layers are mechanically more forgiving than rigid crystalline contacts.
- Immunity to LID and LeTID degradation (explained below).
If you're looking for the highest-performing technology currently available for an RV roof, HJT represents today's benchmark — at a correspondingly higher price.
Physical Limitations in Real Life
Manufacturers measure solar panels under Standard Test Conditions (STC):
Conditions on the roof of a motorhome are rarely anything like this. This is where real-world energy production is determined.
Temperature Coefficient of Power
A solar panel is a semiconductor. As temperature rises, internal electrical resistance increases, voltage decreases, and therefore power output drops. During summer in Southern Europe, the roof of a motorhome commonly reaches 60–75 °C, which is 35–50 °C above STC.
Typical power loss at +40 °C above STC:
The difference between 14.8 % and 10.4 % may appear small, but during hot summer conditions it translates into significantly higher real-world energy production from an HJT panel compared with a conventional panel having the same rated wattage.
The Phoenix has 780 Wp of standard monocrystalline PERC panels on the roof: two 220 Wp and two 170 Wp panels. In a typical Croatian summer — roof temperatures easily touching 65–70 °C — I've observed output roughly 12–15 % below rated values, which matches the PERC temperature coefficient. If I were specifying the system today, I'd look seriously at TOPCon, particularly for summer-heavy off-grid use.
Material Degradation
Solar panels naturally lose performance over time due to UV exposure, moisture, and repeated thermal cycling. Traditional p-type PERC panels experience Light Induced Degradation (LID) during their first hours of sunlight exposure, typically losing 1–2 % of their original output almost immediately. Afterwards, annual degradation is usually around 0.55 % per year.
Modern n-type technologies (TOPCon and HJT) are essentially immune to LID. Manufacturers typically guarantee annual degradation below 0.4 %, while premium HJT panels often achieve around 0.25 % per year. Even after 25 years, these panels generally retain 85–90 % of their original rated output.
Designing the Solar Array: Series vs. Parallel Wiring
How the panels are connected has a direct impact on MPPT controller efficiency, cable size, resistance to partial shading, and overall system performance. Partial shading may come from trees, nearby buildings, roof windows, antennas, or satellite dishes.
Series Connection
- Voltage adds together.
- Current remains equal to that of one panel.
- More susceptible to localised shading.
The positive connector of one panel is connected to the negative connector of the next. Higher voltage offers several important advantages: the MPPT controller starts operating earlier in the morning because the solar voltage only needs to exceed the battery voltage by a few volts, and lower current means thinner cables (typically 4–6 mm²) with very low transmission losses.
The disadvantage is partial shading. If even a small section of one panel becomes shaded, its internal resistance increases dramatically. Since the same current must pass through every panel in the string, the output of the entire string is limited by the weakest panel. Although bypass diodes allow shaded cell groups to be bypassed, overall power loss can still be substantial.
Parallel Connection
- Current adds together.
- Voltage remains equal to one panel.
- More resistant to shading.
- Requires significantly larger cables.
All positive terminals are connected together (typically using MC4 Y-connectors), and all negative terminals are connected together in the same way. The primary advantage is excellent tolerance to partial shading — even if one panel is completely shaded, the remaining panels continue operating independently at full output.
The trade-off is very high current. Four 200 W panels producing approximately 11 A each will deliver about 44 A when connected in parallel. This requires heavy-gauge cables — often 10 mm² or larger — between the roof and the MPPT controller to avoid excessive voltage drop and overheating.
Another disadvantage is that during very hot weather, panel voltage decreases and may approach battery voltage, making it difficult — or even impossible — for the MPPT controller to start tracking efficiently.
With a purely parallel wiring and standard monocrystalline panels in summer heat, open-circuit voltage can drop close to 18–19 V. Some MPPT controllers require the solar voltage to be at least 5 V above battery voltage to begin tracking — if your battery is at 14 V during absorption charging, the margin disappears quickly. Always check your MPPT controller's minimum input voltage specification before going fully parallel.
Series-Parallel Combination
For larger solar systems (typically 800 W and above), the most effective solution is usually a hybrid configuration. Pairs of panels are first connected in series, doubling the operating voltage and allowing the MPPT controller to start producing power even under low-light conditions. These series strings are then connected in parallel, significantly reducing the impact of localised shading across the entire roof.
This arrangement combines the advantages of both wiring methods and is generally considered the optimal solution for larger RV solar installations.
The 780 Wp system on the Phoenix uses exactly this series-parallel approach. The two 220 Wp panels form one series string; the two 170 Wp panels form a second series string. Both strings are then connected in parallel to the Victron BlueSolar MPPT 150/60-Tr controller. This keeps operating voltage high enough for reliable early-morning start-up while limiting the shading impact of the roof ventilation unit, which occasionally casts a shadow across one panel in late afternoon.
Frequently Asked Questions
Is an HJT panel worth the higher purchase price?
For full-time or frequent off-grid users — especially in warm, sunny climates — the answer is usually yes. The lower temperature coefficient means measurably more energy output during summer heat, and the dramatically lower annual degradation (≈0.25 % vs. ≈0.55 % for PERC) translates to higher real-world output after 10–15 years. For occasional weekend campers who spend most nights on campsites, standard monocrystalline PERC panels offer a better cost-to-performance ratio and the difference may never be noticeable in practice.
I have four solar panels — should I wire them in series, parallel, or as a series-parallel combination?
With four panels of the same wattage, a series-parallel combination is almost always the best choice: connect panels 1 and 2 in series to form string A, connect panels 3 and 4 in series to form string B, then connect string A and string B in parallel to the MPPT controller. This doubles the voltage compared to pure parallel (better controller start-up, thinner cables), while maintaining reasonable shading tolerance because a shadow affecting one string leaves the other string unaffected.
Why does my solar system produce so little power during winter, even when the sun is shining?
Two compounding factors are at work. First, the sun sits low in the sky during winter, meaning sunlight hits your flat roof at a steep angle rather than perpendicularly — at 30° elevation, irradiance is roughly half of what it would be if the sun were directly overhead. Second, shorter days mean fewer usable hours. The only good news: cold temperatures actually improve solar panel efficiency slightly (voltage rises as temperature drops), which partially compensates — but not enough to offset the much lower irradiance and shorter day length.
Related articles
- MPPT Charge Controllers — how an MPPT controller converts panel output into usable battery energy, and what to look for when sizing one.
- Electrical Basics in a Motorhome — understanding 12 V, 24 V, and 230 V electrical systems, wiring, and protection.
- LiFePO₄ Batteries Explained — where all that solar energy actually goes, and how to size the battery bank correctly.