RV Electrical and Solar

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Why Many Solar Systems Do Not Work Well

Many people complain that their systems do not provide them the time off-grid that they expected. I've been designing and installing systems since 2000, and I routinely hear these complaints. Almost always when you evaluate these systems it is an installation issue. Very few systems installed by RV manufacturers are done in an optimal fashion. Even dedicated "solar installers" often do not match components correctly or configure the system optimally. That is one reason I encourage people to implement their own systems, where they have the desire and the minimal necessary skills. Even if you do not do the installation yourself, designing the system will teach you enough to ensure a good installation by others.

The common problems/issues I encounter are:

• The system is under-wired. The wire run from the solar panels to the controller, and then on to the battery bank, is sized too small. It should never be less than #6 cable, and I use #4 routinely on 12-volt nominal systems. Manufacturers commonly use #10. That is way too small for all but the smallest system. The only exceptions to this are with higher-voltage systems (more on that later). USE the wiring tables or online calculators to determine the correct size wire, and then go a little heavier. The wire size is not an "opinion" - it is simple physics. Use the calculators. High voltage systems can use smaller wire than 12-volt nominal systems - determined by the wire calculators.
• The solar panels are shaded at certain times of the day. Why an installer would place panels where they KNOW they will get shaded is a mystery. But it is not that uncommon. Even the shadow of the shaft of a TV antenna can kill the output of a panel. You want NO SHADOWS. More on this later....
• The solar controller is too far from the battery bank. Put it as close as practical - but not in the same compartment. Do not use a controller that has an in-built display and place it in the RV so you can read it, instead use a controller with a remote display capability. Separately calculate the wire size needed from the controller at max output to the battery bank - this will likely be heavier than what is required from the panels to the controller.
• The solar and charger settings are not optimal. On flooded cell batteries the absorption setpoint (the bulk charge rate) should be 14.8 volts UNLESS your battery manufacturer says otherwise. (Only pay attention to the battery manufacturer. Installers and even controller manufacturers will routinely provide you with bad information.) The default settings for wet-cell batteries in almost all controllers/chargers is 14.4-14.6 volts. That is not adequate to get a good charge on the bank. The other common issue is that the controller does not allow enough time during the absorption phase of the charge. Thus, the bank never approaches a "proper" charge.
• Battery temperature sensors are not employed. To get a proper charge, both the inverter/charger and the solar controller should have a battery temperature sensor placed on the battery bank. The charge voltage varies depending on battery bank temperatures. It is difficult to get a good charge without the temperature sensors. If the inverter or solar controller offers a voltage sense line then that should be used as well.
• Batteries are not checked and equalized when they should be. You need to check the battery bank water levels at least monthly until you learn your system. You need to check the batteries with a hydrometer at least two times a year and equalize if required.
• Battery terminals are dirty and/or loose. You would not think this would be that common, but it is.
• There is no instrumentation that records cumulative amphours drawn from the battery bank. Without this information it is difficult to evaluate the current battery condition. As a result, many battery banks are drawn down too far and their life is unnecessarily shortened.

The Golden Rules of RV Solar and Electric

This is a summary. Details are covered in the following sections. These are my opinions based on experience and education - you certainly do not have to follow these guidelines. But if you do, you will have a successful system if properly implemented.

Panels

• Use high voltage (over 28 volts) on any but the smallest systems (small: under 400 watts)
• Optimal input voltages for most MPPT controllers outputting to a 12-volt battery bank is in the 30-50 volt range.
• Price panels on a per-watt basis. There is not much difference in panels unless you have special needs.
• Use serial/parallel connection to get higher voltage, when required. Panels must be matched.

Solar Controller

• Use an MPPT controller; high voltage; boost in the 10%+ range is realistic; price differential over PWM is not that great these days and for a larger system it allows many benefits ("larger system" = around 500-600+ watts)
• Controller must allow adjustable voltage and charge times
• Position close to the battery bank
• Make SURE the wire size to the batteries is correct. It will be bigger than what comes from the roof in most cases.
• Temperature compensation is NOT an option - use it. If a voltage sense line is available, use that too.
• Fuses/breakers on input/output sides.

Batteries

• Balance the system; have enough batteries for the amount of watts of panels you have (you can have more, but having less is wasteful)
• Rule of thumb: 1 amp of storage for each watt of solar panel.
• Flooded cell batteries charge at 14.8 volts NOT at 14.4/14.6 volts that you commonly see
• Wire correctly: large enough wires, +/- connections on diagonal corners, equal length wire runs.
• AGM batteries have advantages, but cost much more
• Solar alone often will NOT bring a bank up to "full" state of charge because the system is continually in use. But if properly designed it can.
• Use a battery monitor with a remote display (like a Trimetric, Link, or Magnum BMK)
• With flooded cell batteries check specific gravity at least every 6 months. Equalize if required.
• A desulfator "may" be helpful. Reports vary in RV use.

Inverter

• Wiring is critical. Never less than 2/0 and usually 4/0. READ the manual - there is no excuse to use a lighter wire than the manufacturer requires.
• Short distance to the batteries. NEVER more than 10' max.
• Catastrophe fuse
• Remote display/control is important
• Do not use too large an inverter for your needs. It is inefficient.
• Charge section is critical if using AGM batteries. You want a LARGE charger with AGMs.
• On flooded cells properly set the charge amperage. C/20.
• Wire through a subpanel. Wired in-line is OK for a 30-amp RV, but a subpanel is preferred. Do not wire 50-amp in-line unless the inverter has a 50-amp rated transfer switch (which is no longer available).
• Temperature compensation is NOT an option - use it.
• Build in provisions for removing inverter for service or upgrading your RV - AC wire length and junction box. If you have a converter leave it in place but disconnected from shore power. This can be used if the inverter/charger fails.

Wiring

• Wire size is CRITICAL. It is the single-most common issue with installations. Use voltage/distance calculators. Then go heavier
• Manufacturers almost never provide adequate wiring
• Wire for 2% loss or less. I wire for 1% from the controller to the bank.
• Use quality closed-end, coated lugs, and properly attach them; use dielectric grease and adhesive heat shrink
• Fuse before/after controller; catastrophe fuse at battery bank
• Use combiner on roof; I prefer a breaker box on larger systems. With high voltage systems the combiner can sometimes be in the main compartment and not on the roof, but calculate the loss on the #10 wires from the panels to see if this works.
• Use distribution buss bar(s) near battery to tie loads together (if required)
• Do not attach loads between shunt and battery.

Equipment Recommendations

I get asked often what I recommend, and that changes over time. The industry is constantly developing new products. What follows are my recommendations at the time I wrote this. Make sure you check these against your own needs, and against current technology. Although I try to keep this up to date, there is no guarantee. If you see something new that you think is better, feel free to write me about it, and why....

• Magnum inverters. Also look at the BMK (battery monitor kit). Many people prefer the Trimetricfor battery monitoring - as do I.
• Morningstar solar controllers. Personally, I like the MPPT 60 and its ability to directly network to your router. For larger installations, MidNite Solar has the Classic 150 which allows more panels to be used (otherwise you have to "stack" 60 amp controllers).
• Solar panels:  Sun Electronics solar panels (lots of choice and reasonable prices, look at some of the blemmed products). AM Solar always carries narrow-width panels that are pretty good. These narrow panels are great on RV roofs where space is sometimes tight - in general four fit across an RV roof, and a single panel fits next to an airconditioner. But they generally are not cheap. Wholesalesolar.com has very good prices on a variety of panels. They "can" be cheaper than Sun, it just depends. Look at them both. I like the SolarWorld panels available at Wholesalesolar. USA made, excellent warranty, good efficiency and priced reasonably.
• MidNite Solar breaker boxes, and combiner boxes. I like the ones with breakers in them, but there are other methods of protection that do not use breakers. AM Solar has a new combiner box that allows for larger wires.
• Bogart Engineering Trimetric battery monitor RV2025 or RV 2030 is still my favorite. I had a Magnum BMK in my 2012 coach, and wished I had a Trimetric. My 2015 coach has a Trimetric TM-2030.
• Look at the Magnum mini-panels (MPP) or the MidNite Solar E-Panel if you are doing a higher cost installation. They run about $600 but solve most of your wiring issues in one UL approved box. On a higher-end implementation you likely will be 75% of that with your own wiring. And there are extra advantages to these boxes. The MidNite E-Panel is probably best suited to most RV installations because of the dimensions (it mounts the inverter on the front of the panel), but in many cases neither of these will fit. This is for high-end systems only...otherwise the cost is not justified. And they will likely only fit in a 5th wheel installation - not in most motor coaches (except bus conversions).

Residential Refrigerators

In the past few years I often get asked to design solar/electric systems for RV's with residential refrigerators. Since the late 2000's the energy efficiency of these refrigerators has improved enough that it is possible to both boondock and have your residential refrigerator. For avid boondockers having the electric refrigerator is likely too much of a compromise unless you have a very large solar/electrical system. But for people that do not boondock for months on end, it is now a viable alternative with a large solar system, or a combination of a medium system and an hour or so of generator time a day. As always, your usage habits and the compromises you are willing to make will factor into the decision to go with a residential refrigerator.

A Norcold 1200 RV refrigerator (12 cubic feet) uses about 40 AH of DC per day to run the control circuits, so they are not "free" of power use. Residential refrigerators, in general add about 100 amp hours (DC) to your electrical burden (per 24 hrs). So the difference is about 60-80 AH. You need to replace both that, and whatever else you use for power. There are things you can do to minimize this usage some, but in general plan for around 100-110 Ah.

I have a spreadsheet you can download with examples of generator runtime, various refrigerators, and some of the other planning factors involved with designing a system around a residential refrigerator. Take a look at it and see if this direction meets your needs. If you have suggestions or see errors in the spreadsheet let me know. Most of it has been validated with actual in-use systems.