From 110 Wp to 780 Wp: how I upgraded the solar system without drilling any new holes in the roof

When I took delivery of my Phoenix 8200 BML in 2016, it came with a single factory-fitted 110 W Solara panel. Completely normal at the time — enough to maintain lead-acid batteries during summer camping. But the way I travel today is a different story. Starlink, laptops, air conditioning, fast chargers for the e-bikes. One 110 W panel wouldn't last me through a single night.

This article isn't a catalogue reprint. It's the real evolution of one specific motorhome over ten years — including a mistake I made that nobody warned me about in advance.

Original state: Solara 110 W and a fuse that kept blowing

The original Solara S440M34 panel was protected by a 10 A fuse on the 12 V side. That fuse kept blowing — and the result was that the entire solar system stopped charging without any warning. You'd only notice when the batteries were nearly flat. No alarm, no indicator, no notification.

After eight years of use the panel had also lost a share of its output — monocrystalline panels degrade at roughly 0.5 % per year, so about 4 % over eight years. In practice that meant around 90 W under ideal conditions. The contribution to charging was minimal.

First upgrade: Phoenix Service Center Schlüsselfeld (2024)

In 2024 I decided to go straight to the source — the Phoenix factory service centre in Schlüsselfeld, Germany. I wanted significantly more power, but I didn't want to drill new cable penetrations or dismantle the interior. The service centre proposed replacing the old panel with two SUNSET AS 220-6 units.

The old Solara came off. Two new panels went on, along with a new MPP 440 CI controller, new mounting brackets and new wiring. Total output went from 110 W to 440 W — four times the original. The difference was immediately noticeable in practice.

Second stage: full electrical rebuild

A year later came a complete overhaul of the entire electrical installation — a switch to LiFePO₄ batteries, a new Victron MultiPlus-II 3000 VA inverter-charger, a new Victron MPPT 150/60 controller and, as part of the project, two additional Büttner Dometic MT Black Line 170 W solar panels (from Nomádem.cz — a specialist caravan e-shop). The rebuild was carried out by Jiří Hnízdil.

Total installed capacity: 780 Wp. 2 × 220 W + 2 × 170 W. Seven times the original.

The problem I didn't notice at first

The panels were wired in a single series string — all four in a row. Many installers use this approach because it's fast, simple and perfectly correct when the panels are identical.

But my array wasn't made of identical panels. I had 2 × 220 W and 2 × 170 W — four panels, two different specifications. In a series string the current is always set by the weakest panel, just as the weakest link determines the strength of a chain. The result: the more powerful 220 W panels were working essentially as 170 W panels, and a significant share of their potential was wasted.

Series strings are also very sensitive to shading. A shadow from the air conditioner, the satellite dish, the Starlink terminal or a tree branch causes the output of the entire string to drop significantly — not just the shaded panel. When you're travelling with equipment on the roof, this is a real, everyday problem.

Why I didn't want to change the wiring or drill a new hole

Every new roof penetration is a potential leak. I didn't want any — and it turned out I didn't need one. But I do need to explain why I couldn't simply wire all four panels in parallel and call it done.

The original cabling was sized for much lower power. Connecting all four panels in parallel would nearly double the current in the existing conductors. Higher current means more losses, more heat, and a need for heavier cable.

The solution: two branches, MC4 Y connectors, no new holes

The answer turned out to be surprisingly simple — change the wiring on the roof, without touching the interior or adding any new penetrations. Two separate series strings:

Branch A: 220 W + 220 W in series — matched panels, each working at its optimal point.
Branch B: 170 W + 170 W in series — same principle.
Both branches are then joined in parallel using MC4 Y connectors, and the original roof penetration still carries just one positive and one negative cable.

Each pair now works at its own optimal operating point. The 220 W panels are no longer held back by the 170 W panels. If one panel is shaded, only its string loses output — the other string carries on normally.

When travelling with an air conditioner, satellite dish and Starlink on the roof, the difference is noticeable every day.

Results: then vs now

Panel weight — output isn't the only number

Most people focus on watts when choosing solar panels. For a motorhome with a maximum permissible mass of 5.99 t (or 3.5 t), weight matters just as much. Every extra kilogram on the roof is one less kilogram for water, e-bikes or other gear. And roof weight is more costly than floor weight — it raises the centre of gravity of the vehicle.

What to watch out for during installation

Always verify your MPPT controller's maximum input voltage before changing the wiring. The Victron 150/60 handles up to 150 V — two strings of 2 × 220 W (Voc ≈ 45 V each) sit safely below that. Calculate the current in the existing cabling before you change anything. Don't add roof penetrations if you can avoid it — every new hole is a future leak risk. And think about shading from your AC unit, satellite dish or awning when planning panel orientation.

What Victron VRM data showed — panel wattage isn't everything

During the stress test in Croatia, I had Victron VRM open the whole time, watching production in real time. And I saw things that surprised me at first.

The system woke up early — production started well before the installed peak power would suggest. Even under an overcast sky, the panels were putting out a surprisingly decent number. And the production curve wasn't a sharp midday spike — it was a fairly flat, wide arc with solid daily totals.

That's when I first realised that for a motorhome, peak output at noon in July isn't necessarily the most important thing. What matters much more is:

— how much you produce from 7 to 10 in the morning, before the heat kicks in and the awnings go out,

— how much you produce from 17 to 20 in the evening, when the air conditioning is no longer needed,

— how the panels behave under partial cloud cover,

— how they perform under partial shading from the AC unit or roof windows.

So I've become more cautious about the claim that "240 W is automatically better." On paper, yes. In real motorhome use — with early morning starts, summer cloud cover, and daily shading from the AC unit — it's not nearly that clear cut. The combination of 2 × 220 W + 2 × 170 W performed very well in Croatia. And the VRM data confirmed it.

What I would choose today if I were starting from scratch

If I were designing the whole system today, I probably wouldn't choose a mix of different panel types. I'd go with four identical panels — that eliminates the whole headache of different ratings in a series string and gives a properly designed system from day one.

I see two realistic paths today:

Option 1: Büttner Elektronik MT SM 210 Power-Line (premium choice)

Büttner Elektronik MT SM 210 Power-Line — a German manufacturer that specialises in motorhomes. Panels engineered for road vibration, quality aluminium frame, dimensions that fit Phoenix roofs precisely. Four of those: 840 Wp, one type, no compromises.

Option 2: Solarfam 240Wp monocrystalline (performance choice)

If I wanted maximum output at an accessible price point, I'd go with Solarfam 240Wp from ecoprodukt.cz. Four panels = 960 Wp at a combined weight of 4 × 12.8 kg = 51.2 kg. Nearly 1 kWp on a Phoenix roof is a real leap. Aluminium-framed glass panel, 1,520 × 760 mm — slightly larger than the Büttner, but still easy to work with.

Conclusion

Over ten years the Phoenix went from one ageing 110 W panel to a 780 Wp solar plant with Victron control and LiFePO₄ batteries. Seven times the original output.

But the biggest gain wasn't the jump in installed capacity. The biggest gain was getting the wiring right — two series strings, each with matched panels, connected in parallel via MC4 Y connectors. No new roof holes, no new cabling, the original penetration left exactly as it was.

That's the kind of solution I always look for: technically sound, minimal intervention in the vehicle's structure, and results that hold up in real use — even in the shadow of a Starlink dish and an air conditioner.