PV modules - sizes and outputs

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The sizes and outputs of different PV modules vary greatly depending on the application. From handy modules that are only intended to maintain the operation of a camera to large modules for PV fields, but also maritime or other demanding applications.

Space issue

In addition to applications for PV systems or balcony power plants, where space is not particularly important, motorhome owners are often in a dilemma in this respect, as space is in short supply, as is electricity. In most cases, shore power is not available and the noisy generator is not something you want to put yourself or others through. So the only option is to install a PV module on the roof.

However, various roof hatches, fans, vents, satellite antenna, etc. already reside there, all of which compete for space with a PV module. One rectangular area is still free - 175 x 120 cm.

That must be enough!

Well, as always, everything is relative: in summer, in southern, sun-drenched climes, with a cloudless sky and bright sunshine - it may indeed be enough. However, in the latitudes of Germany or further north, the sun is increasingly low and not almost vertical, as in southern regions. The PV module lies flat on the roof and therefore only has its highest efficiency when the light is vertical.

Determination of requirements

To find out whether "must be enough" actually covers the demand, it makes sense to calculate which consumers demand which power and for how long.

Even a large battery is of little use if the energy consumed cannot be replaced by solar radiation. At some point, the battery will run out. Without sunlight, you will then have to wait a long time until it is halfway ready for use again.

All the more so if it is not a LiFePo4 battery but an AGM battery. While a LiFePo4 battery offers around 95 % of its capacity for use, an AGM battery is content with 50 % of its nominal capacity and is then already considered to be in need of charging.

The battery should only be considered as a buffer between incoming and outgoing power if you want to prevent a battery blackout.

Size brings performance

PV modules generate electricity with individual cells, Wafer electricity from solar radiation. The more cells a module has, the more electricity it generates, but also the larger it is.
Therefore, the possible size of a PV module is ultimately the limiting factor in terms of the energy generated.

• 380 .. 400 W - 1767 x 1041 mm - 60 wafers - 1.84 m²
• 520 .. 540 W - 2094 x 1134 mm - 84 wafers - 2.37 m²
• 540 .. 560 W - 2278 x 1134 mm - 91 wafers - 2.58 m²
• 695 .. 720 W - 2384x 1303 mm - 112 wafers - 3.11 m²

Modules that generate electricity on both sides offer a way out. bifacial PV modules. The energy gain on the rear side depends on the mounting height and the substrate: a white painted surface or even a mirror guarantee a significant energy increase of up to 30 % additional yield.

What to do?

If you are not in the fortunate position of having plenty of sunshine, the battle to keep the battery charged can quickly turn into a race against the sun. Some people have installed mounts on the roof that allow them to tilt and rotate the module individually. Others place "portable" modules next to their motorhome and lean them against the bodywork at a suitable angle. Apart from the fact that such modules are not "mass-produced" goods, but are produced in small numbers and are therefore more expensive, not everyone wants to go to the trouble of stowing them away while driving, and also wants to do without solar charging during the journey.

Alternative solution

It is therefore important to look for installation alternatives that ensure maximum surface area and thus maximum yield. In times of heavy hail, neither insurers nor owners were happy when hailstones smashed the relatively thin plastic domes of the roof windows and created an involuntary footbath inside.

So why not install one or two large PV modules across the entire roof surface from front to back, over the - open - roof hatches?

The usually 14 ... 16 cm distance from the roof also ensures perfect rear ventilation of the modules. With bifacial modules, the distance is sufficient to utilize the light reflected from the roof and increase the yield. And in the event of hail, the modules protect the sensitive roof hatches from damage - a win-win situation for everyone involved in every respect. And the loss of light is very limited, especially as side windows generally account for the lion's share of light gain. Another advantage: the hatches can remain open even in the rain!

Knowing how ...

Only the mounting of the modules, the elevation, requires some constructive effort, which is a welcome change for a skilled locksmith.
As an idea: take two 3 mm thick aluminum angle plates as an L-profile. Stabilizing triangular plates are welded in at suitable intervals. The short side of the L-profile is used for fastening to the vehicle roof, the long side for holding the PV module frame, which is screwed flush with the top of the aluminum sheet using V2A screws or, more simply, riveted.
If you also pay attention to aerodynamics, give the profile a spoiler at an angle of about 30 ... 40°, which diverts the wind upwards but allows it to pass between the two profiles below the modules (for cooling purposes).
If you want a chic look, you can have the profiles powder-coated in the color of the car.

Use wind power?

Many people who have been confronted with the problem of not being able to charge their battery reliably and sufficiently with sunlight have already thought about this.

Finally, there are vertical wind generators that can supply 240 W or even energy in the kW range.

Several aspects speak against this: first of all, the systems, even in the small design, are still a good one meter high and therefore less predestined for installation on motorhomes, even if you stay below a permissible height of 4 meters.

The next shortcoming is that the data used for advertising purposes usually represent maximum values that are achieved at wind speeds of 72 km/h (20 m/s), but can only be achieved at sea or when driving on the highway. At the usual wind speed of 5 m/s in central Germany, the yield is therefore close to 0.0 W. Apart from the fact that the measured wind speeds are recorded at a height of 10 meters. That's where it gets tight on the car roof ...

And last but not least, the system's vibrations must be absorbed to prevent damage to the bodywork, not to mention the corresponding noise development.

Yes, the wind at high altitudes is certainly capable of supplying large amounts of energy, but we rarely travel at these altitudes in a motorhome 😉 - So, a nice idea, but unfortunately not practical.