backchannel: buying many (very) wrong panels

TL;DR put away the credit cards until you understand how stuff works and what you actually need.

chain of errors

As with airline disasters, solar disasters are made up of a chain of smaller errors. I share this tale so others in the planning processes might benefit. I am not affected by and have no stake in choices others make regarding their own rigs.

the Big Picture

OP on a reddit thread bought six expensive, odd-spec flex panels in “a good deal because they were on sale”.¹ Now they are trying to figure out how to use them.

error in choosing type of panel

Panels were intended for mounting on top of a van. Flex panels are typically ill-suited and needlessly expensive for this application.

Read on.

error in choosing panel voltage

He picked 12v 175w panels, which are difficult to work with in multiples. He now finds that any more than 3 in parallel will be more than what his combiners can handle.

Note: they are niche, but there are valid reasons for using higher-wattage 12v panels. Consider a 12v bank with a PWM controller. One or two of these panels would work fine with that setup. Flex panels are harder to justify but let’s pretend he had a teardrop, canned ham, boat, or other gently curved surface to work with.

failure to measure space

It turns out that only 5 can fit in the available space. Oops. Both 4P and 5S are more than his setup can handle. So either we run 3 on one controller and 2 on another, or drop one panel and go 2P2S.

discounting effects of shade

OP is wary of series because of shade.

Seriously, people, if you are planning for shade then solar is probably not a great fit. A small inverter generator will make more power, more reliably, and for less money.

a different approach

OP spent $1,974 on 1,050w of panel, only 875w of which can fit on the van in any case. This is ~$2.56/w at best.

Used high-voltage panels cost about 1/5th as much and would fill the roofspace in parallel safely under his current limit. Putting it altogether,

1. decide what you want to do. If what you want to do is “camp in the shade” then rule out mounted solar as a main charging source.
2. measure the space to see what will fit
3. understand panel, wiring, and controller specs
4. find sanely-priced panels that fit (physically and electrically)

my original response

[I bailed on this response and posted nothing. This is what I started to write. Since he already owns the panels my intent was to make best use of the present equipment.]

Since there is only room for 5 up top I would

1. place 4 on top in a series-parallel configuration. This would result in
   • Vmp 39.0v
   • Voc 47.8v
   • Imp 17.78A
2. If you will be in excellent solar conditions I’d put that on a 50A controller. Under poor-to-medium conditions a 40A would likely suffice; I rarely break 40A with 750w in my normal use case.
3. since they are light, use the other two as portables on a separate controller, perhaps with a quick-disconnect to the side of the van. These can be in series as they will be in direct sun; it will help with voltage losses along the relatively long wires.

I saw somewhere that if I park in the shade with panels in series that if one panel stops it’s output than the whole system is kaput.

Solar is intended to work in direct (or at least even) sunlight; if one is planning to have panels in shade then solar is a poor fit for primary charging. Building a large array and parking in shade is like buying a Bugatti Veyron for driving in mud. Wrong tool for the job, and a waste of money.

Having said that, the effects of shade on panels are highly variable depending on exactly where the shade hits them, panel construction, quality of light, the controller’s mppt algorithm (if any). It is exceptionally difficult to predict How Bad It Will Be in advance.

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1. Narrator: “it was not a good deal”. ↩