This is a slightly terrible question.
It is because it is extremely easy to be misled by ‘professionals’ who seem to know exactly what you need and immediately dish it out to you without much solicitation.
In reality however, when it comes to estimating the things that make a system work for you, there is no standard set up. None whatsoever. Everybody has different needs and are each in situations/conditions peculiar to themselves alone; none of which can be reproduced in the exact manner. These situations/conditions are exactly what to consider when sizing up solar panels for a good working system; thus, a better question to ask is – how much power do I use in my mobile Home?
You will find in our article – the best solar panels for Caravans/motor homes that our first suggestion for this determination is to run a practical class of your own by taking the Motorhome away for a short break while monitoring the electricity expended while it was in use – if this is a rather impractical means of reaching an estimation, you would have to estimate it using the power ratings given on every single piece of equipment you intend to power from the battery.
You’ll find a thorough explanation given on the fitrv.com piece on How much solar you need on an RV
This is best estimated thus;
Power (Watts) = Current (Amps) × Voltage (Volts)
Depending on the value or the number of volts provided on the battery, you can estimate the drain on the battery from each appliance by dividing the power rating of each appliance by the volts provided by the battery.
In order to determine how many amp hours you may need thus, you would need to arrive at the drain value for each appliance and then add them up.
Each appliance usually has the wattage (or its estimated value) it uses written on the label, which may either be on the back or the bottom of the appliance; the list below gives the wattage use of some commonly used appliances on caravans or mobile homes.
|Mobile Phone Charger||2|
|Water Pressure Pump||120|
As we earlier established;
Power = Current x Voltage Thus;
P = I x V
I = P divided by V
V = P divided by I
P = power in watts (W) P = power in milliwatts (mW)
V = voltage in volts (V) V = voltage in volts (V)
I = current in amps (A) I = current in milliamps (mA)
A milliwatt is a 1000th of 1 watt
A milliamp is a 1000th of 1 amp
For Example: The 100W Photonic Universe Mono-Crystalline Solar Panel in peak light conditions will give approximately the following current using one of the above equations. We have the 2 variables, watts and volts so the following equation can be used – If the working voltage of the panel is 18 volts (to enable charging of the battery).
I = P divided by V I = 80 divided by 18 = 4.44 amps So as the formula states that a 100 watt solar panel in peak light conditions will supply a current of 4.44 amps an hour.
If we say that a 100 watt solar panel will supply 4.44 amps for a period of 5 hours (in the summer) it will replenish the battery with 22.2 amps in a day. If you have equipment which is connected to the battery that uses more than 22.2 amps per day then the solar panel you have may not be sufficient for your needs, as it will eventually drain your battery.
That is, also putting into consideration that conditions such as indirect sunlight, cloudy, hazy conditions and partial shade will decrease the output. Also the length of daylight i.e.: summer vs. winter.
In order to also find out how many amp hours you need from your battery every day, you may find this handy;
|Appliance||Power rating in watts (W)||Current drawn at 12V in amps (A)||Number of hours appliance is on||Amp hours (Ah) required||Energy top-up per day in watt hours (Wh)|
|(Power/12)||(Current x hours)||(Power x hours)|
|Living Area Light (each)||16||1.3||3||3.9||48|
|Halogen downlighter (each)||10||0.8||2||1.6||20|
|Kitchen Ventilation Fan||20||1.7||0.5||0.9||10|
|14in 12V colour television||45||3.8||2||7.5||90|
Table culled from Camping and Caravanning Club
The number as well as the size of the solar panels you need depends on the amp hours you eventually arrive at – the solar panel you then choose should supply approximately 20% more than your needs to compensate for variables such as cloudy days, shading etc.
While a single, large capacity solar panel might suit you just fine; having a smaller one is not bad either, as they can be connected to ideal capacity – our article on how to install solar panels on caravans explains how this can be achieved during installation.
How Long Will It Take To Charge My Battery?
Besides the multitude of variables that could affect the performance of a solar panel – some of which may include the weather, shading, and its position relative to sunlight, the time that it takes for a solar panel to charge your battery depends on the current of the panel you choose. As battery capacity is measured in A/h (amp-hours), you need to know the solar panel current to estimate how quickly it will charge your battery –
If a 100W Caravan solar panel produces approximately 4-5A per hour in bright sunshine.
If you have a 200A/h battery, this will take approximately;
200/5 = 40-45 hours of sunshine to charge from 0%-100%.
Ultimately, with choosing a solar panel the question is hardly what the panel can provide, but what you need to power your RV – but while it is a good stride to research thoroughly and draw conclusions based on your findings, it is also important to remember that the overall performance of the Solar system is dependent on environmental conditions they are subjected to, as well as the maintenance techniques you employ after their purchase.