Choosing the right 12V Solar Panel for you
We are often asked us what the right size of solar panel to charge 12V leisure batteries would be. Unfortunately, based on our experience, there's no single right answer to this question. For examples, Solar panels for caravans, motorhomes and boats can all be very different.
- Your average daily power consumption (very important)
- Capacity of your batteries (less important)
Average Daily Power Consumption
In order to choose a motorhome solar panel, you need to know how much energy your appliances use each day in a 24 hour period. The average daily power consumption can be estimated individually for each device (type of load) and added together to create an overall requirement.
- 4 x 10W 12V energy saving LED lights, used 3 hours a day
- 1 x 40W 12V LCD TV, used 2 hours a day
- 1 x 20W 12V water pump, used not more than 30min a day
Based on the above, the average daily power consumption will be 4*10W*3 + 40W*2 + 20W/2 = 210W/h (watt-hours)
Once this estimate is available, it's very easy to decide on the size of 12 volt solar panel required for your caravan or motorhome.
Calculating the Solar Panel Size
The average daily power generated by your motorhome solar panel should be equal or more than the average daily power consumption of your appliances.
For example, let's assume that on a typical day we can expect 5 hours of sunshine. If we have a 50W solar panel on the roof of your motorhome, it will generate approximately 50W*5 = 250W/h (watt-hours) - this is almost in line with the average daily power consumption 210W/h.
Of course, in reality there will be some losses of energy generated by the solar panel, mainly in the solar charge controller and cables, but also due to battery self-discharge. These are typically kept within 20% of total output. It is therefore wise to choose a solar panel for your motorhome that offers a daily power rate of approximately 20% higher than the daily power consumption you require.
Sometimes it is difficult to estimate the daily power consumption – especially when the power rating of individual appliances is not available or have been lost. In this case we recommend the following high-level approach:
- If you simply need to trickle charge your battery to compensate for self-discharge in your caravan, a small 5W-10W solar panel or charging kit should be sufficient
- In the case of small loads such as energy saving lighting, a stereo, mobile phone charging and low usage of a computer (2-3 hours a day), we would recommend a 40W-50W solar panel or charging kit for your motorhome
- For larger loads or heavier use including non-energy saving light bulbs, large TVs, frequent use of water pumps or other equipment, we would recommend a larger 80W-100W charging kit or solar panel.
Calculating the Power Required for an Electric Fridge
Batteries often have a limit on the initial charging current - this is usually more important for more expensive batteries such as gel or AGM batteries. If you have a small battery (e.g. from a motorcycle), you can't really use a very big solar panel / charging system unless you are absolutely sure that the battery can accept this current.
Typically gel / AGM batteries have a limit on charging current equal to 20% of their capacity. Therefore a 50A/h battery cannot be charged by current higher than 20%*50A = 10A (this would be the equivalent of a 160W solar system current).
This will not normally be a concern unless the battery you use is really small. However it is wise to the specifications of your battery to see whether there are any restrictions on the charging current.
If the battery cannot store the charge, the energy is wasted. When you have invested in a solar panel for your boat or motorhome, it would be a shame to see the energy generated go to waste simply due to the limited capacity of your battery.
For example, if your 12v solar panel starts charging the battery in the morning and completes the job in 2-3 hours, the rest of the energy generated throughout the day is wasted. This is a complete waste, especially if you need the energy in the evening or at night for lighting etc. So think about whether your battery capacity is adequate to the size of the solar panel you use and how quickly it will charge the battery to 100%.
How to Calculate the Length of Time It Takes Your Battery to Charge from a 12v Solar Panel