Solar Tutorial Part IV – Solar Panel Selection and Wiring

<- Solar Tutorial Part III – Full-timer Kits                                                         Solar Tutorial Part V – Battery & Inverter Selection ->

The two sample systems described in Part III of our Solar Power Tutorial series are essentially the same systems we have installed on our RV and sailboat.  The RV system is pretty standard for full-time RVers (four 120-140 watt 12-volt panels and 440 amp-hour battery bank).  The sailboat system is bigger than many cruisers carry (three 185 watt 24-volt panels and 710 amp-hour battery bank).

Cruisers often install less solar power and rely on additional charging methods via engine alternator, and wind/gas/diesel generators.  However, we have found our solar power alone is sufficient to live an ordinary house-like lifestyle on our boat (if we don’t use our standalone DC freezer).  We lived for 10 mid-winter weeks in southern Mexico on solar power alone, without using the alternator once (it was broken), and still used two laptops, the TV/DVD, stereo, microwave, chartplotter, autopilot, anchor windlass and vacuum as much as we wanted.

Our rationale for having enough solar power to live comfortably without alternative charging methods was:  we didn’t want to store a lot of gasoline to power a gas generator; we had found that boats with wind generators often suffered from the whirring noise and vibration; and we didn’t want the added cost, installation work and maintenance of an inboard diesel generator.  Solar power has been a great solution for us on both the boat and the fifth wheel.

Part III described these two basic full-timer systems with just a cursory comparison of the solar panel choices.  This page goes into more detail about the various options for sizing solar panels and suggests different ways to wire them.

Wiring in Series versus in Parallel and Wire Gauge Size

There are quite a few choices for solar panel configurations, each with its own pros and cons.  But before choosing a panel configuration it’s worthwhile to consider how to deal with the large current that will be flowing through the wires.  As much as 35 amps or more might be flowing from the panels to the charge controller and then from the charge controller into the batteries.  This requires heavier gauge wire which is more difficult to work with and is expensive.  However, there are several ways to reduce the amount of current in the wires.

If the panels are wired in parallel, the amps produced by each panel are additive while the volts remain constant (Ohm’s law).  Therefore, the cable leading from the connection point of all the panels to the charge controller and then on to the batteries will carry the full current load, or potentially as much as 35 or 40 amps at 12 volts DC.  Heavier gauge wire must be installed to handle this large current load throughout the system.

If there is too much current on a cable, then it will get warm (or hot), and lose some of its precious amperage to heat.  In other words, if the wire gauge is too small, not all the power produced by the panels will make it to the batteries.  It will dissipate as heat loss along the way.  And at the extreme, there’s the risk of melting the shielding off the cable (highly unlikely).

Larger wire is more expensive and is more difficult to handle because it is stiffer.  “Larger” generally means 6 or 8 gauge wire and “smaller” is generally 10 gauge.  The size is dependent on the current flowing through the wire and the length of the wiring run.  A detailed chart for selecting wire gauge is given here.  Note that some charge controllers can’t accommodate wire heavier than 4 or 6 gauge.

If the panels are wired in series, the amps produced by each panel stay constant while the voltage is additive (also Ohm’s law).  Therefore, the cable leading from the connection point of the panels to the charge controller will carry just the amperage produced by a single panel (7-9 amps) at 48 volts DC (if four 12 volt panels are installed in series), rather than the 25-35 amps at 12 volts DC that would flow when wired in parallel.  Because there is less current, thinner gauge wire can be used throughout the system.

In practical terms, most solar power systems on RVs and boats never reach their full potential current load.  During the morning hours, before the sun gets high and powerful in the sky, the batteries get quite a bit of charge.  Usually, by the time the sun is really cranking out maximum energy at noon — the time when the system could be producing max amps — the batteries have already gotten pretty well charged and are starting to ask for less and less current.  So the charge controller has already begun to throttle the panels back a bit and less current is flowing through the system.

Also, solar panels are rated for operating with the sun perpendicular to their surface, and anything other than a perpendicular orientation reduces their output significantly.  In all months except May-July, the sun doesn’t ride all that high in the sky.  We have rarely seen much more than 25-30 amps on either of our full-timer systems, although they are capable of 30 and 36 amps respectively.

Another important consideration is that when a small fraction of a solar panel is shaded — as little as a 4 square inch area on a 2′ x 5′ panel — the entire panel stops producing power.  That is because internally the panel is “wired” in series.  When there is resistance, caused by shade, in just one portion of it the panel’s internal circuitry, current can’t flow through any of it.

By extension, if the panels are all wired in series, when one panel shuts down due to a palm-sized bit of shade, then the entire array of panels shuts down.  A tree branch or part of a boat’s standing rigging or mast/boom can cause the entire array to shut down if it is wired in series.

marine solar panels on hunter 44ds sailboat

Shade from the mast and shrouds on our three 185 watt panels.

If the panels are all wired in parallel, a small amount of shade on one panel will only shut down that individual panel.  Current will still flow through the rest of the panels and then through the rest of the system.

We were persuaded by our solar panel vendor to wire our trailer’s panels in series so we could use small gauge wire throughout the system.  We have experimented with shading a small corner of one of the four panels and were stunned to see the entire array quit working!  However, almost everywhere we boondock we are in full sun.  So, in the end, it doesn’t matter for us. Wiring our RV’s solar panels in series has worked out just fine.   If, however, you anticipate camping under trees on a regular basis and you want to maximize the panels’ chances of getting access to the sun, wire the panels in parallel and use heavier gauge wire.

On a boat, this series versus parallel decision is much more critical than on the roof of an RV.  The mast, boom and shrouds often shade portions of the panels as the boat swings at anchor.  Under sail the shading can be even worse.  So the best wiring option on a boat is to wire the panels in parallel.  However, the cable runs in a boat can be much longer than in a comparably sized RV.  On our sailboat the wiring running from one end of the system to the other — panels-controller-batteries — is 45′.  Why so long?  The panels are high in the air on an arch at the back end of the boat, the batteries are at the bottom of the hull in the middle of the boat, and the all the wiring is routed so as not to be seen.

12 volt versus 24 volt panels

Another way to tackle this issue of having a lot of current flowing through the system is to use 24 volt solar panels instead of 12 volt panels.  When the voltage is doubled like this (24 versus 12 volts), the current is halved.  So the current produced by 24 volt panels is half that of equivalent wattage 12 volt panels (the watts don’t change whether the panels are 12 volts or 24 volts).  We chose to go this route on our sailboat, using three 185 watt 24 volt panels wired in parallel.

Since the batteries are 12 volt batteries, the input side of the charge controller coming from the solar panels is 24 volts while the output side going to the batteries is 12 volts.  Most large capacity charge controllers allow this kind of configuration.  This means that the current flowing between the panels and the charge controller is half that flowing between the charge controller and the batteries.  So, while the panels may be producing 14 amps at 24 volts, and those 14 amps may be flowing from the panels to the charge controller, the current will double to 28 amps at 12 volts when it flows from the charge controller to the battieries.

While the wiring run between the panels and the charge controller can be smaller gauge (less current flows in that portion of the system), the last wire run between the charge controller and the batteries needs to be as short as possible and wired with heavier gauge wire to accommodate the larger amount of current.

We made the mistake of placing the charge controller 25′ from the batteries at first and using 10 gauge wire (I suspect we didn’t explain our situation to the salesmen at the solar panel store well enough when we asked him for guidelines).  When the panels were running at full power we lost about 10-15% of the power they were producing.  Once we moved the charge controller to within 10′ of the batteries and replaced the 10 gauge wire with 8 gauge wire, we lost just 1.5% of the power between the charge controller and the batteries, which is considered acceptable.

Tilting Brackets

Tilting brackets make a lot of sense on an RV because an RV is parked in a stationary position.  In wintertime it is possible to tilt the panels towards the sun (tilt them about 45 degrees).  Most folks align the panels with the length of the RV and tilt them on their sides.  This means that either the driver side or passenger side of the RV will be situated to face due south and the panels will be tilted in that direction.  In most boondocking locations we find we can orient the rig any way we want to because there is so much space around us.

In experiments one December with RVing neighbors who had tilting brackets, we found that their solar power system produced about 40% more amp-hours throughout the day.  Their system was fully charged and their batteries were floating in the afternoon, while ours never reached the Float stage.  Some of that may have had to do with their batteries being better charged to begin with in the morning (we have no idea if they were or weren’t), but it is a pretty dramatic difference nonetheless.

RV solar panels on fifth wheel trailer

Four 120-130 watt panels on our fifth wheel’s roof

However, to get the advantage of tilting brackets, you have to get on the roof to tilt each panel every time you set up camp, and then remember to return them to their flat position before breaking camp and driving off. 

An alternative is to keep the panels flat in all but the most dire circumstances (a week of cloudy winter days), but have one more panel in your system than necessary.  Or don’t even bother installing tilting brackets at all. The trade-off is a few hundred dollars for an extra panel versus climbing up and down your RV ladder and fussing with the panels, as well as the risk that you might drive off with them raised up (we’ve seen plenty of people do that).

Tilting brackets don’t make much sense on a boat because boats move around so much at anchor.  Ours swings back and forth in a 90 degree arc.  Also, the tilting mechanism for a lot of boats introduces shade across the panels at certain angles.  On a boat, it is best to mount the panels as far from the mast and boom as possible and to focus on keeping the shade off the panels as much as possible by forcing the boom off to one side or the other while at anchor.  A fixed, flat mounting position works best.

 For more information about how to select the best solar panels for your installations, see this article:

Which Solar Panels To Buy – Flexible or Rigid? 12 or 24 volt? Monocrystalline or Polycrystalline?

 

Most of the components for an RV or marine solar power installation can be purchased at Amazon.

Shown here is a complete full-timer's kit (far left), a big charge controller (middle) and a big inverter (right). More comprehensive listings of each component type can be found at the following links:

Purchases at any of our Amazon links help cover our out-of-pocket costs for operating this site -- thanks!

 

This is the end of our solar power tutorial series.
We have lots of other info about solar power on this website. See the pages listed below to learn more.

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FURTHER READING and RELATED ARTICLES

SOLAR POWER OVERVIEW and TUTORIAL

BATTERIES and BATTERY CHARGING SYSTEMS

LIVING ON 12 VOLTS

 

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