Solar system on the motorhome - an installation guide

If you live and work in your mobile home like we do, you use it not only as a home but also as an office. As a result, we have a few more consumers that we need every day to work and that need their dose of electricity several times a day. Since we can’t and don’t want to stand on a pitch every day, we need a power source that is independent. A power generator, i.e. a chugging aggregate, is out of the question. It stinks and makes noise.

If we want to keep it affordable, the best option is a solar system on the roof of the motorhome. Here I present the basics, considerations, planning, purchasing and implementation online.

So you too can build your own solar system on your motorhome.

A list of our consumers gives an indication of how much electricity we need every day. I will go into the calculation further below:

• Laptops
• Tablet
• Smartphones
• Mifi Router
• Raspberry Pi Mini Computer
• E-Book Reader
• Loudspeaker and Radio
• Water pump
• Heater

For budget reasons, we decided to plan and install the solar system ourselves. Even though I’m not the super handyman, this shouldn’t be a problem for me. Everything I’ve learned I’m making available to you for free in this article. If you read this article, you will save yourself a lot of research 🙂

12 volts or 220 volts?

An important question when it comes to electricity and motorhomes. Many technical devices that we have used in our everyday lives, such as a hair dryer, work with 220 volts. However, these often have a power supply unit that transforms the electrical voltage down from 220 to 24, 12 or even less volts. To operate 220-volt appliances in your motorhome, you need an inverter. If you now connect a 220 volt device with a power supply unit, the voltage is transformed from 12 volts to 220 volts by the inverter and back to 12 volts by the power supply unit. You realise that this is nonsense and that energy is lost and converted into heat.

Inverters also put a strain on the batteries because of the high current consumption. The same goes for hair dryers, mini ovens and coffee machines. That’s why I advise you to do without 220-volt appliances altogether and to use 12-volt appliances. There are 12-volt versions of almost every appliance.

Solar basics

I’m not going to start writing about the sun, sand and photons here. If you are looking for such basics, then read up here: http://www.solarstromerzeugung.de/grundlagen/

I’m more concerned with the basics you need to buy a solar system for your motorhome.

The electricity in your motorhome comes from two or more batteries. One is the starter battery. This is usually located in the engine compartment. It is used to start the engine and power the lights, indicators, etc. The other battery (can be 2 or more) is used to power the motorhome. The other battery (can be 2 or more) is the consumer battery. This supplies the lights, pumps, refrigerator, etc. Both batteries are charged by the alternator while driving. When your motorhome is stationary, the batteries are not charged, of course. The solar system is intended to change this state and charge the batteries even when the motorhome is stationary. You can find a detailed article on batteries in motorhomes here.

The solar system consists of solar modules (solar cells) and a charge controller. The charge controller checks whether the battery is full and if not, it is charged by the current coming from the solar cells. If the battery is full, the regulator cuts the current. If a consumer draws current, it is charged again until the battery is full.

Solar on the roof of the motorhome

The solar modules are glued to the roof, where the sun shines the most ;-). There are special adhesives and brackets for this. It is also important that the modules are ventilated. If you stick them directly on the roof, they get too hot and do not perform as well. There should be at least 2 centimetres of space between them.

I wanted to glue the modules directly onto the alcove. The idea behind this was that the solar modules absorb the sun. This means that not so much heat gets through and the temperature of the alcove is more pleasant in the sun. However, since it is slanted towards the front and therefore does not have a straight surface, this did not work.

Solar modules have the best efficiency, i.e. the best electricity yield, when the sun shines on them at an angle of 90 degrees. Since the sun moves throughout the day and also has different angles depending on the season, the solar modules would always have to follow the sun. They would have to be mounted so that they can be rotated and tilted. Ideally, they should also automatically track the sun. Such a system can generate twice as much electricity, but at the same time it is expensive and often not worthwhile for such a small system on the roof.

The most common solution is to use mounting brackets that allow you to manually raise the modules on the roof so that they are better aligned with the sun. The disadvantage of this is that you have to climb onto the roof every time. And the brackets are expensive.

For this reason, I decided to put one more module on the roof. It’s cheaper and I don’t have to climb onto the roof of our motorhome several times a day to open and close the modules. Since I will be installing several modules, it is important to connect them in parallel. I connect the positive pole to the positive pole and the negative pole to the negative pole, because I want to keep the voltage of one module and gain more current.

Polychrystalline or monocrystalline

If you have already dealt with the topic, you will certainly have heard these two terms. I will make my explanation brief:

Monochrystalline modules often have a better efficiency than polychrystalline ones. However, they are also more expensive to produce because more silicon is used. Another advantage of mono-cells is that they are less sensitive to partial shading. So if a branch casts a little shadow on part of the solar cell, a monochrystalline one will produce a little more electricity.

However, for the small areas on a motorhome, the differences should be rather small. Therefore, I would say it doesn’t matter which modules you buy.

How should I dimension the solar system?

At the beginning, I listed which consumers we needed to supply with electricity. In order to realise this with a solar system, three things have to be sensibly dimensioned:

• the solar modules
• the solar controller
• the battery(ies)

 To find out how to size your system, the first thing you should think about, like we do, is what loads you have in your motorhome. Then how much voltage (volts) they need and how many watts they consume and how long each one runs per day. This gives you the required power, energy and electrical charge. If you add up the individual charges of the devices, you get the amount of charge you need per day. From this you can determine which batteries you need and how large your solar panels should be.

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Calculation of the battery size

I first created a table with all the consumers: [su_table responsive=”yes”]

[/su_table] We have a consumption of around 55 Ah every day. This value is calculated very comfortably. For example, I charged the two laptops twice a day in my calculation. For lighting, we also save electricity by using LEDs instead of normal halogen lamps. You can find an article on the use of LEDs here.

To finally test the required storage capacity of the battery, I still need to know how many days I want to be autonomous. Or in other words, how many days of bad weather (when the battery is not charged) do I want to calculate? Since we are flexible on the road, 3 days are enough for me. By then we have either gone to better weather or, if necessary, to a place with a power supply.

So I need 3 x 55 Ah = 165 Ah. We have 2 85 Ah starter batteries in our motorhome. These are anything but optimal for the purpose. These batteries are designed to provide very high currents for a short time and then almost no power at all. With consumer batteries, however, we tend to need little power in the long term. Also, such batteries should not be discharged more than 20%, otherwise they will break down quite quickly. So we are left with 26 Ah if we don’t want to damage them. That’s not even enough for one day. So we have to buy new batteries together with the solar system. It is important here to look at the depth of discharge. You can take 50-70% out of most cycle-proof batteries. More on this in the article about batteries in motorhomes.

Update: In the meantime, we have installed two cycle-resistant wet batteries with 120 Wh each as body batteries. This means we are now comfortably free for many days.

The solar modules

Solar modules have various parameters that you should know in order to make a decision:

1. First, there is the maximum power Pmax in watts. You will find this number on every solar panel. I bought 100 watt panels.
2. Next, the voltage in volts. On my panels this is 18 volts.
3. Then there is the maximum current in amperes, which is 5.9 for me.

You need these three figures for the purchase and the comparison. The panels should supply enough current in one day to fully recharge your batteries. I need about 55 Ah a day and want to get by for 3 days. So my system has to supply 165 Ah a day. Less is also enough, as there is always some energy coming from the roof, even if it is cloudy.

I have therefore bought 2 pieces of 100WP solar modules to start with and can build a third one on the roof if necessary. If you don’t have that much space, then buy one or two with more power. I recommend different modules below.

The charge controller

The third part of a solar system in a motorhome, which is also called an island system, is the charge controller. This takes the current from the solar modules and directs it to the battery. It is therefore there to charge your batteries properly. So you have to make sure that your charge controller matches the battery or at least offers a mode for charging. Most current controllers can handle wet batteries as well as gel and AGM batteries.

What is important for the regulator is the maximum power and especially the maximum current. I wrote above that my panels deliver 5.9 A maximum. Since I have two and want to have a reserve for a third, I need a regulator that can deliver at least 18 A and 300 Pmax.

The cables

The correct dimensioning of the cables is important. If your cables are too thin, you will quickly have high voltage drops and your solar system will produce much less yield than it could. The cables between the solar panels and the charge controller as well as between the charge controller and the battery must be correctly dimensioned. The cable between the charge controller and the battery should be as short as possible. I have described in another article how thick the cables need to be and also built corresponding cable thickness calculators. Just read on there (opens in a new window): Cable thickness for solar system.

Purchase recommendation solar system motorhome

You can order the products from a retailer of your choice: [su_table responsive=”yes”]

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The delivery

I’ll explain to you why I chose exactly these components:

• For the solar modules, I chose a supplier that was well rated.
• I bought 100 watts because I had calculated it.
• For the charge controller, I opted for a brand-name product rather than a cheap product from eBay. On the one hand, I wanted to have the guarantee and, on the other, a display to see how much electricity I was producing. I also wanted the charge controller to have a reserve for connecting another solar module.
• I bought the two spoiler sets because simple aluminium angles were just too unsafe for me. I don’t want the panels to simply be swept off my roof at 100 km/h and possibly injure following drivers.
• I deliberately bought the connection cables with a diameter of 6mm. If the cables are too thin, more power is lost in the form of heat. On the one hand I want to avoid the losses and on the other hand a potential cable fire. (More in the article on cable cross-section in the motorhome).

Mounting the solar system on the motorhome

You should take a weekend for the assembly. I needed two days for it. I worked from noon until about 7 pm. First I built the spoilers on the panels, then I mounted the solar panels on the roof and then I connected everything. I explain it in detail here:

1. Pre-clean the roof

First, I carried a solar panel and two spoilers individually onto the roof and tried out how they lay best. It is important that no shadow from the roof structure falls on them. I then drew the outlines with a pencil so that I knew where I had to clean and sand.

The first cleaning step is simply to remove the coarse dirt. So I just did it with soapy water and a sponge and then wiped the roof dry with a cloth.

2. Prepare the solar spoiler

In the next step I prepared the spoilers. These are already pre-drilled on one side. The first thing I had to do was drill the holes. I used a 3.5 mm drill bit for this.

Then I put the first spoiler on a shelf and inserted the solar module. Because next I had to drill the holes in the frame of the module. Pre-drilling is important so that the screws go in well. I drilled briefly through the spoiler into the frame with the same drill to get a mark. Then I removed the spoiler and drilled through the frame.

Then I screwed the spoilers to the solar modules with the screws provided. The spacers have to go inside the spoiler.

The next step was the roof. In the marked area for the spoilers, I roughened the surface with the supplied sanding fleece. This is not to remove the paint completely, but to roughen the surface so that the glue holds better. Quite a job on the roof at 30 degrees and in the sunshine…

After roughening, I masked the edges flush with masking tape. This makes it easier to remove the excess glue after gluing.

3. Gluing the solar modules onto the roof of the motorhome

Next, I pre-cleaned the adhesive surfaces with methylated spirit and then treated them with the activator/cleaner supplied. I used a soft and non-fluffy cloth for this. After that, the whole thing has to flash off for at least 10 minutes. After this time, the primer is applied, also with a soft, lint-free cloth. Simply apply a thin layer to the gluing area. At the same time, I also applied the activator and the primer to the solar spoiler. The primer must flash off for at least 30 minutes.

After the waiting time, I glued the first module. I turned it over on the roof and applied the adhesive, Sikaflex 252, in beads.

Then turn the module over and stick it carefully and accurately onto the area marked with masking tape.

After gluing, you must press the solar module lightly into place. The adhesive should be between 2 and 3 mm thick between the roof and the spoiler. Once you have glued the first module, continue with the second module.

4. Gluing on a cable duct

As mentioned above, I connect both modules in parallel. This adds up the current and the voltage remains at 12 V. In the cellar I had a little cable duct that was 5 cm wide. I mounted the C4 Y-adapters in it and connected both modules.

5. Mount the roof duct on the motorhome

Somehow the cables from the solar panels on the roof have to get into the motorhome to the batteries. The next thing I did was to install the roof duct. I chose the place for it near the modules and the former satellite antenna. This allows the cables to come out in our large cupboard and I can wire them in a great way.

First, I laid out the lead-through, marked it and roughened the spot. Next, I drilled a hole in the roof with a 23 mm drill bit.

Then I glued a small piece of empty pipe into the hole with Dekasil. The pipe protrudes a little at the top. If a little water somehow gets into the roof duct, it won’t run directly inwards.

Then I masked off the edge with masking tape, cleaned and primed the surface and the roof duct (as already described above for the solar module).

Now I pulled the cables through and pushed them to the right length. After the necessary waiting time, I applied the Sikaflex and glued on the lead-through.

Then I sealed the two openings of the roof duct from the outside with Dekasil sealant, because the two cables were thinner than the rubber seals. I didn’t use Sikaflex adhesive because I might have to get to the cable again.

6. Assembly and connection of all components of the solar system

Before I wired everything up in the next step, I covered the solar modules with cardboard. As soon as light falls on them, they produce electricity. Since I didn’t want to get anyone wiped and also didn’t want to sizzle anything, covering them makes a lot of sense from a safety point of view!

Now it’s time to find a place for the solar controller. I bought one with a display so that I can always see how much electricity we are producing. Gimmicky, but nice for us and maybe also a little important 😉

So the solar controller had to be mounted outside the cabinet and as close to the battery as possible (because the cable between the charge controller and the battery should be as short as possible). I found a place near the seat and attached the regulator with four screws.

There should also be a fuse between the battery and the charge controller. This way, if something goes wrong, the whole motorhome won’t burn down. I used a normal car fuse (unfortunately you can’t see it so well in the photo), which I can also connect to my thick battery cables. It costs 3 euros on eBay.

After installing the fuse, I connected the regulator to the battery. First I connected both cables to the regulator, then the red one to the positive pole of the battery and then the black one to the negative pole. This sequence is important because in cars and motorhomes the negative is normally also connected to the frame and you can avoid short circuits when connecting the cables.

Next, I connected the positive cable of the solar modules to the controller and then the negative cable. To make it look nicer, I packed the cables into cable ducts that I still had there.

As soon as I took the cardboard off the solar panels, electricity was immediately produced and the batteries charged. A really great feeling. And this is how the roof of our motorhome looks now:

I needed about 14 hours for the complete installation. With this text, you can also completely assemble your own solar system yourself. If you have any questions, feel free to ask me here in a comment.

photos: (c) Sebastian / camperstyle.de