Portable Panel & Roof Solar: One Controller?

 

It is one of the most common questions we get: "Can I just run a portable solar panel through the same controller as my roof solar?" It sounds logical. The controller is already there, it is sized with plenty of headroom, so why not run both through it?

 

The short answer is no, you shouldn't - your portable panel needs a separate controller. Even when an existing controller is big enough, sharing one controller between your fixed roof panels and a portable panel will (at best) cut your charging significantly, and (at worst) drop your extra panel to zero output. That is money down the drain. Here is exactly why, what size solar controller you need instead, and how to wire it so your portable panel works efficiently. 

 

 

KEY TAKEAWAYS

  • The short answer is no: Never run a portable solar panel through the same controller as your fixed roof solar, even if the controller has spare capacity.
  • One controller drags both arrays down: Roof and portable panels see different sun, angle and shade, so the controller pulls the whole lot down to the weakest panel.
  • Yes, you need a separate controller: Give your portable panel its own dedicated MPPT controller so it can chase the sun and work independently of the roof.
  • Run it direct to the battery: The portable input should go straight to your caravan battery (or shunt if you have one), with cable and circuit protection sized for the appropriately.
  • Not sure how yours is wired? Contact Offroad Living and we will help you work out the right setup before you spend a cent.

 

WHY ONE CONTROLLER KILLS YOUR OUTPUT

A solar controller sees everything plugged into it as a single array and runs it at one operating point. The catch is that your roof panels and your portable panel almost never see the same conditions. The roof is fixed flat and pointing up, while the portable panel is angled at the sun, often in a completely different spot. One might be in full sun while the other sits in shade or at a different angle.

 


The angle of the sun in comparison to your solar panels can drastically effect power production.

 

When panels are on the same controller, two things go wrong. First, the panel voltages need to be within about 1 volt of each other, or the controller drags everything down to the lowest panel. Second, even if the voltages match, the controller limits the whole array to match the weakest performer. So a shaded roof panel will hold back your portable panel sitting in full sun. We have seen full-shade roof panels drag a portable panel in perfect sun all the way down to zero output when they share a controller.

The whole point of a portable panel is to chase the sun when your roof can't. Sharing a controller throws that advantage away.

 

"BUT MY VAN ALREADY HAS A SOLAR INPUT PLUG"

This is the trap. Many caravans and camper trailers come with an external Anderson plug or solar input already fitted, and that plug is often wired straight into the same controller (or a DCDC charger) that runs the roof solar. Plug a portable panel in there and you hit exactly the problem above.

You may even be told by the dealership that the existing controller "is all you need" because the roof panels won't work optimally anyway. That logic doesn't hold up. The existing controller is busy running your roof solar, and feeding a second, differently angled panel into it is what causes the losses in the first place. Before you plug anything in, find out whether that input runs directly to the battery or into an existing MPPT or DCDC charger. If you are not sure, check with the dealer your purchased it from, an autoelectrician, or drop us a line.

 

HOW TO SET IT UP: A SEPARATE CONTROLLER, DIRECT TO THE BATTERY

The fix is simple in principle: give your portable panel its own MPPT solar charge controller and run it directly to your caravan battery, bypassing the roof controller entirely. That way each array works independently and your portable panel can do its job.

What size solar controller do you need? Divide the panel wattage by your battery voltage and round to the nearest controller size. A 400W panel on a 12V system is 400 / 12 = 33.3, so a 30A controller is sufficient. For higher voltage panels, also check the controller can handle the panel's Open Circuit Voltage, and make sure it has a lithium battery charge profile if you are running LiFePO4. Then make sure the new cable run to the battery is sized for the current and protected with the right circuit breaker or fuse. One important note: if your system has a battery monitor and shunt, all charger and load negatives must connect to the load side of the shunt, not directly to the battery negative.

Connecting a new cable to the battery is DC electrical work. As a general rule, have it carried out or checked by a competent person, such as an auto-electrician, particularly on a lithium system where correct fusing and shunt wiring matter. 

If you want to run two portable panels, the principle is the same. Each panel should get its own controller, and you can then combine the two controller outputs with an Anderson double Y adapter into a single input running to the battery, again with cable and circuit protection sized for the total current. As a real example, two AllSpark 400W panels each on a Tracer 30A MPPT, combined into one input, want around 6B&S cable to the battery and a 70A circuit breaker.

 

A WORD ON FACTORY WIRING

If you are speccing a new van, our technical advice is to ask the manufacturer to run the external solar input directly to the battery (or shunt) rather than through the roof solar controller, and ideally to have a qualified auto-electrician make that connection. From there you simply add a separate controller for each portable panel and plug in. It is a small thing to get right at build time that saves a rewiring job later. If your manufacturer refuses to change anything (as some do) then you will need to organise for an auto-electrician to make the changes after you take possession. 

 

STILL NOT SURE? ASK US FIRST

Every van is wired a little differently, and the worst time to find out yours shares a controller is after you have spent a day in the sun getting nothing into the battery. If you are unsure how your solar input is wired, or which controller suits your panel, send us a message with your setup and we will point you in the right direction before you buy. Contact Offroad Living for expert help.

 

Want the full step-by-step on connecting a portable solar panel to your caravan battery, cabling, controllers and combining inputs? Read our complete guide, Adding a Portable Solar Panel to an Existing System.

 

FAQs

Q: Can I run a portable solar panel through the same controller as my roof solar?
A: No. Even with a controller that has spare capacity, sharing it between roof and portable panels drags both arrays down to the weakest panel and can cut your portable panel to zero output. Give the portable panel its own controller.

Q: Do I need a separate controller for my portable solar panel?
A: In almost all cases, yes. If your roof panels are already on a controller, your portable panel needs its own separate MPPT controller wired direct to the battery, so the two arrays work independently.

Q: My van has an external solar plug already. Can I use that?
A: Only if that plug runs directly to the battery (or shunt) and not into an existing, in-use MPPT or DCDC charger. Many factory plugs feed the roof controller, which is the setup you want to avoid. Check where yours goes, or ask us.

Q: What size solar controller do I need for a portable panel?
A: Divide the panel wattage by your battery voltage and round to the nearest size. A 400W panel on 12V needs a 30A controller. For higher voltage panels, also check the controller's voltage rating against the panel's Open Circuit Voltage.

Q: Can I run two portable panels together?
A: Yes. For optimal efficiency, give each panel its own controller, then combine the controller outputs with a double Y adapter into one input to the battery, with cable and circuit protection sized for the total current. Alternatively, if the controllers are installed internally, install two connectors, with each running to the battery (or shunt).