Determining How Much Solar Fits on an Enclosed Trailer Roof

Restrictions:

• no overhang
• no panels in V-nose area
• can't block vents or rooftop air conditioners
• no complex folding or layering of panels

Not hard to answer for one trailer, just go out there with a sturdy stepladder and a tape measure and jot down the length, then width of the roof. Say it's 16ft long and 7ft wide (192in x 84in).

But I’m not designing for my trailer — I’m designing for your trailer in your backyard. I can’t go measure it for you, so I need a way to solve this problem across the common trailer width/length categories.

Also quick note: trailer sizes are usually referring to the outside of the box (not the inside), and the length is the box length (not tongue-to-back, and not counting the V-nose). The V-nose area is basically a no-go zone for panel placement anyway — especially on slanted V-nose designs.

Example 7x16ft Trailer:

Say it's 16ft long and 7ft wide (192in x 84in). You want to use a 400W panel with the dimensions of 67.9in x 44.6in. 

If you take the trailer roof width of 84in and divide by the panel width of 44.6in, you get:

• 84/44.6 = 1.88 Panels

Hmm, can't exactly cut down one panel to 0.88 of it's original width, and if you were to try and squeeze two of those side by side, you'd overhang the trailer edges by 5.2in (2.6in each side) and that's without any room for brackets or mounts. You might not think 2.6in is much, but if you've towed enclosed trailers before, you know it's likely you might scrape some low-hanging tree branches going through tight neighborhoods, not to mention it doesn't look, professional.

If we turn the panels the other way (long edge of panel transverse on trailer roof) then we know the 67.9in would fit one full panel, with 16in of extra room (8.05 each side). Now we can figure out how many would fit down the length of the trailer by:

• 192/44.6 = 4.30 Panels
  

Keeping with our no overhang restriction: 4 panels on our 7x16ft trailer, which would be 

• 4x400W = 1600W
  

What if we have an air conditioner?

Say goodbye to 400W! That's if you're lucky, if the AC happens to be just perfectly placed you might not be able to fit two panels in front or behind the AC, so you're out 800W.

And vents can cause the same kind of problem depending on how the panels are mounted. Sometimes you can mount over a vent (great). Sometimes you can’t. Deleting a vent is possible (I did it on my old lawn care trailer), but it’s a hassle and takes time. An AC unit is worse: you’re not mounting over it unless you’re trying to build a skyscraper on your roof.

You could try to solve these weird gaps by sprinkling smaller panels around the empty spaces, but now you’ve got a lot more connections, more mounting solutions, and you can’t really mix and match panels without creating other issues (like needing separate solar charge controllers). It gets inconvenient fast.

Why does any of this matter?

When I built my original Solar Powered Lawn Care Trailer, I wanted the maximum amount of Watts so that I could power as much of the day to day all-electric lawn care that I was doing, (including the actual mowing!) so I needed every watt. While it was just a technological experiment, it taught me a lot and made me realize just how valuable every Watt is.

If I'm offering a solar powered kit, I want the owner of that kit to have the greatest potential for whatever their use case (SOLARPUNKclippy just wants to help), whether it be lawn care, a mobile makerspace, homeless support trailer, or even a cigar shop! I've thought of lots of ideas for a mobile solar powered trailer, but my viewers keep coming up with new ones! I love it.

The option to have less than the max solar will be available too, or to even use your own panels. But first I have to find the best way to maximize the amount of solar, while also keeping in mind sourcing, shipping, warranty, support, and installation of whatever panels I choose to offer with my kit.

Theoretical max vs real-world max

One thing that helps me reason about this is separating:

• Theoretical max: if brackets and spacing didn’t exist
• Real-world max: what you can actually install cleanly

A typical ~400W panel is around 45in × 68in. That’s about 21.25 square feet of panel area, which works out to roughly 18–19 Watts per square foot. I’m going to use 18 Watts per square foot because it keeps the numbers simple.

A 7×16 trailer roof is 112 square feet, so the theoretical max looks like this:

112ft² × 18W/ft² = 2016W

That’s the ceiling. It’s hard to actually hit, but it’s a great number to compare against because it tells you how close your real-world layout is to “perfect.”

Why big panels are a pain (even when they fit)

Even if you can fit the big residential-style panels, there are real issues:

• Weight: big panels can be 45–70+ lbs and it’s awkward to carry them up/down a ladder solo
• System flexibility: if your trailer only fits 1–2 panels, your voltage/amperage options get limited fast

• Shipping / Minimum Order Quantity (MOQ): a lot of these panels effectively only ship in bulk (often a pallet/MOQ of 10) which is a terrible experience if someone only needs one panel — and it’s also annoying if a panel gets damaged later and you need a replacement

That whole pallet delivery experience is not what I want people dealing with.

“Goldilocks” panels (200–300W) and a scorecard

Right now, what looks promising is a “Goldilocks” panel size in the 200–300W range:

• they can ship individually (wrapped in foam + cardboard)
• they’re much easier to handle (around mid-20 lbs)
• they typically have 12V / 24V options

• it’s easier to order exactly what a kit needs and to replace one later if it gets damaged

Using this approach, I can still fit about 1800W on a 7×16 trailer.

And if we compare that to the theoretical max:

• 1800 / 2016 = 89.3% space efficiency

I’ll take it. Here's what that looks like in the real world:

Scaling this beyond a single trailer

I have to solve for nearly every trailer combination out there if I want to sell a kit. Sure I could start with one or a few trailer sizes and see who wants one, but that's not my style.

After spending a significant amount of time wandering trailer dealer lots, hunting online for every listing I could find, and quite literally stopping every time I see a trailer on the side of the road to measure, I think I have a pretty accurate idea of the most common trailer width, and lengths. No thanks to AI.. Trust me, don't ask it, it will only tell you the incorrect info about trailer widths because it's never gone out to a trailer lot and measured them. Maybe now after this log it will know a little better... The irony.

Trailers are mostly broken into about 8 width categories, and then two foot length increments, starting at 4ft wide, and 5ft long. However, for a number of reasons, including some design constraints I'll get to in another log, I've decided to start my kit at 5ft wide and up to the max allowed on roads without a wide load sign and permit: 8.5ft* wide.

*8.5ft wide is actually 102in to the outside of the fender wells, which are small little 1 or 1.5in wide aluminum strips around the wheels. So the actual box width when measuring from the outside is anywhere between ~98-100in wide. This will be important later.

A typical 5ft wide trailer starts at around 6ft long and goes up to 14ft, and the 8.5ft wide trailers start around 10ft long and go all the way up to 50ft (rarely past 36ft). This means for my kit design, excluding custom trailers, I'll have to design solar arrays for approximately 58 trailer length/width combinations.

I could chase the theoretical max on every trailer combination for optimization, but the goal is consistency and repeatability. I don't just need a solar array, I need a system.

It's worth pausing for a moment to think about why I even want to get close to the theoretical max wattage. When designing my very first solar lawn care trailer, I wasn't entirely sure how much solar would be enough, and I figured if I have the roof space I might as well fill it up. This turned out to be a good idea, because the power required to mow a client's property (or 10+ in one day), is significant. However, this isn't just in regards to lawn care. I want someone to be able to power their idea, whatever that is (there are a lot of ideas out there!), with as much daylight as possible, without being incredibly complicated, just in case they need every last drop. If they don't need all that, I want the kit to be flexible in the sense you could install a smaller array initially, and upgrade later if needed. If I start with the practical max and work backwards, it's a heck of a lot easier than going the other way.

Using CAD to Scale the Design

I'm often using Cardboard for Aiding my Designs (and storytelling: see video), but this time I needed the computer version. In my case I've been off and on using Fusion for years (which offers a free "hobby use" license last I checked). I'm still an amateur, but I've got just enough practice to get me in trouble, and I bought a startup license, so you know I'm official now..

I started with a very basic 5x6ft enclosed trailer:

Modeling each length increment for every width category, making sure to try different panels and placements, without overhanging and maintaining some realistic gaps between panels. Eventually worked my way up to a 8.5x36ft:

(Solar panels look weird due to something with my computer rendering them poorly, but you get the idea)

Now I have a dimensionally accurate model of each combination, and I can tweak them individually if needed for best placement/layout. This felt daunting when I was getting started, but the more I pushed through each layout, the more that system presented itself.

As you saw above in a teaser image from my workshop, I've already graduated to a real world prototype of my kit design. However I'm still not done, and there's a lot more to validate and iterate on, like mounting strategy, installation, and aerodynamic/external forces.

More SOON™