Solar Panels and Solar Controllers

When looking for panels search the web. The prices vary, but compare panels by the cost per watt. New panels and new models are always being introduced. Make sure the company manufacturing the panels is one you feel will be around for the duration of the warrantee. Panels do go bad from time to time. The minimum warrantee on any panel worth considering should be 20 years. I prefer the panels available from Sun Electronics, Wholesale Solar, or Northern Arizona Wind and Sun (see the Equipment Recommendations section for my current recommendations). 

Also, I never mount panels with the intention to tilt them - and I do not recommend that you tilt panels unless you have special circumstances. While there is a great benefit to doing so in the winter most people have no business routinely climbing on their RV roof, and there are issues with wind load that need to be carefully addressed if you tilt. I have seen MANY panels ripped from RVs in the desert SW when tilted. I would much prefer an extra panel or use of an MPPT controller.

And consider this: do you want to be going on your roof to lower your panels when a windstorm comes up?  Even assuming you are there? That is NOT the time to be on your roof!!

Techie Stuff You Need to Know

Solar panels are pretty simple in practice. Unlike lots of other areas, unless you have the interest you really can choose to ignore the base technology, how they work, and exploring the intricacies of panel design. But you do need to understand "some" of the techie stuff in order to pick panels, and to figure out if your energy needs are met.

Panels are all measured in watts, which is a measure of energy. DC or AC - it does not matter. A Watt is a Watt. You can see that if you look at the formula: watt = amps*volts. So whether the voltage is AC or DC  - 100 watts is 100 watts.

Panels specifications are derived under "standard test conditions" or STC. You will often see that in literature. STC is just a certain set of parameters used  during the measurements: light conditions, heat, etc. The thing to understand about STC is that it is NOT real world conditions, and you will infrequently, if ever, get the STC ratings from a panel. So do not expect to get 100 watts out of a solar panel rated at 100 watts very often.

Now for some  random terms.

Vmp is voltage at maximum power. It is an important thing to know about a solar panel. The Vmp can be used to determine if the panel is a "high voltage panel".  A typical 12 volt nominal panel will have a Vmp af between 1 and 18 volts (or so). This is used with a PWM controller. A high voltage panel will have a Vmp typically in the 28+ volt range. It is used only with an MPPT controller, which is designed to handle the higher voltage.

Voc is voltage at open circuit. This is exactly what it seems - the voltage measured when the panel is in the sun and not hooked up to anything. This is what you should see when you test your new panel with a voltmeter in good sun before you put it on the roof.  Near this, anyway, although it will not be exact. It is really not a figure used for much except determining charge controller size.

Isc is the current from the panel under short circuit conditions....eg. the max you could ever see from the panel. You use this when you figure out wire sizes using the interactive voltage drop calculators and when you figure out the breaker sizes required.

Imp is similar to  Vmp, except it is the current at maximum power when hooked up. It is less than Isc.

The above is a little more complicated than I've expressed it here - but I'm aiming for a practical use of the terms.  There are nuances in the interrelationships of power and voltage that panels produce. But for our purposes, the above should suffice.

Grid tie panels is a term used for panels intended to feed power back into the electricity grid, and not to charge batteries. There is no intermediate battery storage in a grid-tie application. These are also called "high voltage panels" since they typically have a Vmp of above 28 volts. In some cases significantly above that. Grid-tie panels are typically rated above 225-245 watts, and there are plenty in the 300+ watt area. In RV application grid-tie panels are used with MPPT controllers. These panels are physically larger since to get the 28+ volts they need more solar cells in them (remember, each cell provides around half a volt).

Micro-inverters relate to grid tie applications. A microinverter is a small inverter integrated with a solar panel (typically). It converts that panels DC power to AC right at the panel and then sends that power directly into the electric grid. Why would you want to do this?  Simplicity and efficiency, mainly.  Since each panel has its own inverter you do not have to have a larger central inverter, or group of inverters, and issues affecting a panel do not affect the entire string. Microinverters are not used in RV applications. I only bring it up because you will hear the term and should understand the basics. Remember, in grid-tie applications the array of panels is directly sending energy through an inverter, which converts the DC to appropriate AC, synchronizes the power to the grid, and sends it out on the grid.

Panel Warranties

Most panels carry a 20 or 25 year warranty. But read the fine print. Some have "stepped" performance warranties, eg, so much output after 5, 10, 15 years. A continuous output warranty is better (vs. stepped). But in the end, it boils down to price/watt vs. expected performance.  One thing that you should know - almost all grid-tie panels, and even many low voltage panels, have wording in their warranty documentation that procludes use in a "mobile" environment. Meaning RVs and boats.  Panels have been widely used in these applications for years, without issue. But it is important that you know what you are getting, and are willing to take the (minimal) risk. It will be hard to find high voltage panels that specifically allow use in a mobile environment. I have no qualms about using them in that application provided they are properly supported.

Panels and Shading

The conventional wisdom is that you never mount panels where they may be shaded. Shading any portion of the panel means that the output drops to virtually nothing. How much it drops depends on the panel technology used, but for discussion purposes consider a partially shaded panel to produce no output. And if wired in series to another panel the entire string of panels is affected in a big way. This is one disadvantage to series-wired panels.

So what is a "partially shaded" panel?  Any shading at all is what I mean. Such as: a cellular antenna casting a shadow. An air conditioner next the the panel casting a shadow. Or even the aluminum mounting angle casting an edge shadow on the panel when the sun is low. All are shading a portion of a cell(s). And all reduce the output to essentially nothing.

I'm now going to deviate from "conventional" solar panel advice. If you have filled your roof with panels that never get shade, are you finished?  I would contend no! Mounting some additional panels where they will be shaded "sometimes" may be a good strategy to increase solar harvest. A panel mounted next to an AC unit that receives shade only part of the day will still produce significant power depending on the RV orientation and the sun angle. Likewise, a panel that gets a shadow cast by a satellite TV antenna part of the day may still produce significant power during the other part of the day. With the price of panels  running around $1/watt you can take some liberties with mounting locations to maximize total energy harvest. But ONLY after mounting panels where they are never shaded. One caution: do not wire "shaded" panels in series with another panel. You will lose the output from both when any portion of the two is shaded. So careful panel design and layout is required.

Panels on Smaller RV's

Smaller RVs present special considerations because of lack of roof space. While you can find grid-tie panels in the $1watt range, in most cases these will not be suitable for smaller RVs. Grid-tie panels have dimensions of at least 32"x66", and are typically larger than this. Putting this size panel on a smaller RV is generally either difficult or impossible.

In the case of smaller RVs you want to look for a narrower panel. AM Solar sells some good narrow(er) panels that fit smaller RVs better than the huge grid-tie panels. Both the GS100 and GS135 are quite narrow. So look there - they are in the $2/watt range delivered to you. Remember, the grid-tie panels you see advertised for $1/watt do not include shipping. You really have to factor that in.

Brian Boone ("Solar Brian") also sells a low voltage 160 watt panel that is 59"x26". That works well in tight spaces. His panels are available on ebay, and at the time of this writing include shipping for $165. That is a very good price for a shipped panel.

Smaller RVs have special design considerations that need addressing. Mounting methods may vary because of roof curvature, MPPT controllers may be problematic because of panel size and resultant Vmp, and finding unshaded space is always an issue.

The Truth About Panels

Solar panels are not going to output the watts in the rating sheets. In a typical RV application there are a number of factors at work to prevent this.

• They are not tracking the sun and are typically flat on the roof.

• The insolation (solar energy) is not near what the STC assumes (standard test conditions used to rate the panel). Clouds and haze are typical culprits.

• They are not operating at the temperatures in the STC models. Often they are mounted without optimal airflow for cooling.

• They get dirty. Mounting them with a slight tilt is useful for self-flushing during rain. But they still get dirty.

• They get shadowed because of improper mounting.

You are far better off putting on more watts of panels than you think you need, even if you do not have optimal battery storage for it. When calculating solar needs I typically figure on 4-5 hours of sun and about 75-80% of the rated wattage for harvest.

I would go as far as to say that at current prices of around $1/watt (plus shipping) that most people should just design to maximize the number of panels on the roof. That is a generalization, and some larger coaches would not want to do that. But for most people that works out well.

The other thing to bear in mind is that some of the "huge" grid-tie panels are not really usable on RVs. They are simply too large. The limits in most cases are around the 300 watt panels that run in the 40"x65"+ area. Much larger than that and you start getting into issues with most RVs. And, as discussed above, smaller RVs really have difficulty using grid-tie panels.

Planning Your Layout

Planning on where the solar panels go is a critical step in your design process. Do not overlook this step or delving into the required details. "The devil is in the details" is an appropriate saying that applies to this step. Knowing how much space you have, and where it is can be the difference between a smooth install and a nightmare.

Where are your roof obstructions? Can you move things around on the roof to generate more panel space?

Each RV is different, but here are some hints:

• You can go over some objects on the roof. Things like vents can be under panels without an issue. Sometimes cutting a vent pipe down works, and sometimes you can replace the top with a lower profile vent. Look at this vent (360 Siphon RV Fume Extractor). When going over the top of items think through how you will access them. You will want panels mounted to tilt in most cases.

• You can move or eliminate batwing antennas. This may be more work than you want to do, but it can free up quite a bit of unshaded space.

• You "can" go over top of skylights. This allows light in, but keeps heat out for the most part, so offers some additional benefit. You do have to be cognizant of the height of the panel, though. Be aware that higher panels can catch tree limbs easier. There is no real problem with wind, though, if properly mounted.

Attaching Panels to the Roof

There are many ways to attach panels to the roof. The bracket on the far left is by UniRac (at the time of writing this, about $17 for 4 at Sun Electronics) and made to directly attach the panel to the roof. This type of bracket is generally referred to as a "Z" bracket. If you blow up the picture (click on it) you will see that on the panel side it has a slot that allows easy attachment to existing holes in the panel. If you mount it this way the only way to remove the panel for service is to get under the edge of the panel to the screw, or remove the portion on the roof. Both are difficult. It is much better to attach some aluminum angle to the side of the panel with the L extending out away from the panel. This provides an "extension" edge that you can then mount the bracket to. This makes it easy to remove the panel if required, without disturbing the roof portion of the bracket. Usually, I bond two panels together with a continuous run of aluminum angle, then fasten each side to the roof with two or three brackets (depending on the panel size).

Also very similar are the "smart feet" at RVSolarStore.com, and for self sticking feet from the same source look at the "sticky feet". The sticky feet come with VHB tape for mounting on fiberglass.

AM Solar has some very nice mounting systems. Take a look on their website here. I prefer their mounts.

How you secure the brackets (or whatever you use) to the roof depends on the RV roof construction. If you have a roof that is fiberglass, with good lamination to its substrate, then you can actually use adhesive to bond the legs to the roof. If you do this, you need to use 3M 5200 fast cure adhesive. This is a high performance polyurethane adhesive. It will hold any panel or set of panels to the roof IF the surface roof material is properly bonded to the substrate. I've used it on many installations with no issues. I recommend 3-4 brackets per side if you have two panels bonded together. This gives you better surface area coverage. Most people not familiar with high performing adhesives are nervous about this method of attachment. If it makes you feel better, put ONE screw per leg on just the windward (leading) edge of the panels.

The issue with using this attachment method on a rubber roof is that the rubber is often not well-bonded to the plywood below. I don't recommend just 5200 on a rubber roof. Generally, on a rubber roof I use dicor caulk under the leg, 2-3 screws (at least #12 for the bigger thread size) through the leg, and then Dicor caulk over the leg. Sometimes I cover the entire screw area of the leg with Eternabond after the Dicor dries.

Try to mount the panels with a little tilt to them. This makes them self cleaning. Rain water will help wash dust off them. Plus, you do not want water sitting on your panels in a puddle. Most RV roofs are sloped from the center to the edges so you will naturally get a small tilt to the panels. The large grid-tie panels are especially prone to a slight  "droop" in the middle. This collects water and dirt - something you do NOT want. With the larger panels make sure you mount them so they self-wash.

Also, the larger grid-tie panels that tend to droop in the middle may need some additional support. How you mount them is installation-specific, but avoid letting them droop in the middle. People are sometimes advised that "grid-tie panels cannot be used on RV's". That is not true - but they do have to be properly supported. ANY panel in the 60" length area needs support in the middle.