Choosing a Solar Panel For Your Motorhome
You will have heard the saying 'You get what you pay for'.
This is very applicable to solar panels, so buy the best that your wallet will allow. Do not be fooled by cheap offers, they rarely exist in the solar panel world.
If your location has a general lack of sunshine you might think that there is no point in buying a solar cell of any description, but you are wrong. Cells produce electricity from light, not heat, but sunshine will increase the output. Many people buy a panel and find that it does not do the job they bought it for because they have not considered the required wattage or allowed for lack of sunshine.
This article will help you to choose the solar cells to meet the requirements you demand of them.
There are three types of solar cells, Mono-crystalline and Polycrystalline which are both a silicone composition and the third type is Amorphous Solar Cell (Thin film.)
Mono and Polycrystalline Cells.
This type of cell is wired in series, with each cell producing approximately .55 of a volt, so 36 cells are required to produce approx. 20 volts which are sufficient to charge a 12 volt battery under most conditions. For solar panels on caravans and motor homes, it is recommended to have glass-fronted mono-crystalline type that supply maximum power generation and lowest cost.
Although the theoretical efficiency of mono-crystalline cells is slightly higher than that of polycrystalline cells, there is little practical difference in performance. Crystalline cells generally have a longer lifetime than the third type of cells, the :-
Amorphous cells. (Thin Film).
An example of these is found on garden lights and calculators.
This type of cell is flexible and is less expensive to manufacture but is less efficient than the two types of crystalline cells and thin films have to be nearly double the size for the same output as crystalline. Their power output reduces over the first few months of use after which they become stable.
Solar power panels are rated by how many watts per hour they supply.
To find the Amperage output, divide the panels wattage by 17.
There are numerous power requirements, for example, caravan alarms discharging the main leisure battery in storage to requiring power on rallies. Solar Panels are great technology but for us caravanners they can be confusing.
Now, let us suppose you are requiring a solar panel to keep the leisure battery topped up and your alarm active when the caravan is in storage.
First, you have to find the amp-hour rating of the battery to be charged and you must plan for a wet-cell battery loosing about 0.3 percent of its amperage per day, depending on the ambient temperature.
So, for a 75 Ah hour battery, the solar panel must be capable of supplying 0.22 amps per day to maintain the charge. Add to this, say, 2 Amps per day the alarm is demanding then a solar panel should supply 2.22 Amps per day at least.
Allow for a crystalline panel facing south and un-shaded to supply for a maximum of 2 hours of optimum output in the winter, and 4 to 5 hours in the summer. If the panel is going to be placed in the caravan, note that any window could reduce the amount of light reaching the solar panel.
So a 20 watt panel should produce:-
20 watts divided by 17 = 1.18amps. per hour in optimum conditions.
In the winter approximately 2.36 amps will be supplied, enough to keep a 75 Ah battery charged and to power the alarm.
A 20 watt panel is 300mm x 640mm x 26mm and weighs about 2.3 Kgs.
Sometimes there will be too much power output, and sometimes there won't be enough so the battery will be damaged if it is allowed to be overcharged or over discharged, so a controller is needed to protect it. Panels rated at 15 watts and greater will have to have some type of power control.
A solar charge regulator / controller, is a device that regulates the voltage and current sent from a solar panel to a battery.
The output of solar panels can vary. If a solar panel is listed as 8 volt, the output may be within a range of around 4 to 12 volts. If the battery requires less than this, it could be damaged from overcharging.
The charge controller prevents this by keeping the charging voltage at a safe level. Only if a very small solar panel such as a battery saver is used to charge a large battery is it possible to do without a controller.
Solar charge controllers are specified by the system voltage they are designed to operate on and the maximum current they can handle.
CALCULATING WATTAGE REQUIREMENT
For those of you who have not nodded off yet, and who want to calculate your own power requirements, read on.
Draw four columns on paper.
List all items that will use AC and DC electricity in column 1.
List all AC watts that appliances draw in column 2.
If you power AC appliances with a DC battery through an inverter, you will have to calculate an approximation of what the inverter may take. Find the power (wattage) rating of the appliance you are using with the inverter and divide by 10. This will give you the amps per hour that will be taken from the battery and this includes the running current of the Inverter.
So if you have a small TV taking 100w (watts at 240vac) you will be using approximately 10 amps an hour from your battery.
List how many hours in a 24 hour period you will use each item in column 3.
Make sure you list every item separately.
Multiply column 2 by column 3 to get the watt hours for each appliance and put the totals in column 4. Add together all the numbers in column 4 for the total watt hours.
Remember solar power panels are rated by how many watts per hour they provide. Let us assume your panels produce 100 watts an hour. If you average four hours of sunlight a day, each panel will produce 400 watts a day at the Solar Panel working voltage, (around 17 Volts) which will equate to approximately 23 amps that could possibly put back into the battery (if the battery requires it).
I hope the above removes some of the confusion surrounding solar panels.