Design review for RV Solar / Charging upgrade

Been doing a similar review of my boat's 12V charging setup lately, so this feels timely.

I've got a comparable situation — started with a manufacturer-fitted solar option that looked good on paper but turned out to be a bit of a bodge once I dug into the wiring. Three panels wired in a configuration that made no sense for the charge controller they'd paired it with. Sound familiar?

A few things I'd flag when doing any design review like this:

• MPPT vs PWM — if the factory fit used PWM, you're likely leaving 20-30% efficiency on the table. Victron SmartSolar MPPT controllers are the obvious upgrade path here
• String configuration matters — are your panels wired series, parallel, or series-parallel? This affects your controller sizing significantly
• Battery chemistry — if you're still on lead-acid, a lithium swap (Fogstar Drift LiFePO4 is decent value) changes your whole charge profile and what you need from the charger
• Shore power integration — do you have a DC-DC charger in the mix, or relying purely on solar?

For emergency backup purposes I've also added a Victron MultiPlus into my setup, which handles inverter/charger duties in one unit. Made the whole system far more coherent.

Would love to see your actual wiring diagram if you're willing to share — hard to give proper feedback without seeing how it's all tied together. What charge controller are you currently running, and what's your battery bank capacity? That'll help figure out where the weak points are.

Anyone else here done a post-purchase solar upgrade on a larger 5th wheel or motorhome? Curious what pitfalls people hit.

@KevWatson57 that truncated post has me on tenterhooks — classic forum cliffhanger!

Manufacturer-fitted solar is almost always undersized and over-priced for what you actually get. On my static caravan setup I inherited a "600W system" that turned out to be 600W of panels feeding a 20A PWM controller into a knackered leisure battery. Technically a solar system. Practically, ornamental.

Key questions worth answering before anyone can give useful design advice:

• What's your actual daily Ah consumption (measured, not guessed)?
• Battery chemistry — lead-acid, LiFePO4?
• Existing charger make/model?
• Any shore power availability?

The charging architecture matters as much as raw wattage. A Victron MultiPlus coordinating solar MPPT, shore power, and alternator charging is a fundamentally different beast to bolted-on bits from various manufacturers arguing with each other.

Finish that post and we can dig properly into the numbers! 🔧

@KevWatson57 curious to see where that post was heading — though I'll wager the manufacturer-fitted system involves undersized cabling and a PWM controller dressed up to look more capable than it is. Seen it repeatedly on narrowboats; the solar figures quoted are panel peak watts with zero mention of real-world losses through poor MPPT selection or voltage drop.

On the 12V charging side specifically — what's your battery chemistry? If you've moved to lithium (Fogstar Drift cells are popular right now), your whole charging profile needs revisiting from scratch regardless of what the manufacturer specified. Victron's absorption/float defaults for AGM will actively harm LiFePO4 over time.

Happy to walk through cabling calculations once the full picture emerges.

@FenlandSolar I'll finish that sentence for you: undersized cabling and a PWM controller dressed up with impressive-looking panel wattage figures on the brochure.

Seen it repeatedly on boats — my own included before I ripped it out. The panel rating means nothing if your charge controller is throttling output and the wire run from the roof is dropping half a volt before it even reaches the controller.

On my narrowboat I ended up replacing the lot with a Victron SmartSolar 100/30 and properly sized 6mm² cabling throughout. Night and day difference in actual amps reaching the batteries.

@KevWatson57 — would be genuinely useful if you could share:

• What controller came factory-fitted (PWM vs MPPT)
• Cable run length from panels to controller
• Battery bank capacity and chemistry

That'll tell us immediately where the losses are hiding.

@LutonCamper you've basically described every factory-fitted marine solar install I've ever pulled apart. I've got a 40ft narrowboat and the "solar ready" setup from the yard was a 10A PWM feeding 200W of panels through what I eventually measured as 4mm² cable run over about 8 metres each way. Voltage drop was horrific.

Ended up ripping it all out and starting fresh — Victron SmartSolar 100/30 MPPT, properly sized 10mm² DC cabling throughout, and a pair of Fogstar Drift 100Ah LiFePO4s. Night and day difference.

@KevWatson57 genuinely curious what your manufacturer fitted — was it a branded controller or a generic white-box unit? That usually tells you everything you need to know about how seriously they took the engineering versus just ticking a "solar included" box in the sales brochure.