How To Choose Solar Panel
How to Choose the Right 12V Solar Panel for Your Caravan, Motorhome or Boat 12V Leisure Battery
Our customers always ask us about the right size of solar panel to charge 12V leisure batteries in vehicles and boats. Unfortunately, based on our experience, there's no single right answer to this question. Solar panels for caravans, motorhomes and boats can all be very different.
As a rule of thumb, there are two factors which need to be considered when deciding on the solar panel size you require:
- 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
Please note, electric fridges typically have very high power consumption. Such a unit will often require a 100W-150W solar panel alone. This is in contrast to a gas fridge which requires much less energy to operate.
It is therefore important to estimate the power consumption of your fridge correctly and incorporate this into your calculations when choosing the right solar panel for your caravan.
Using Power Inverters to Convert 12V DC to 240V AC
If you use 12V DC to 240V AC power inverters to run your appliances this should also be taken into consideration when calculating your usage.
Inverters typically work at around 85-90% peak efficiency meaning that they draw 10-15% more energy from your battery than the devices actually require. This means the daily power consumption used to calculate the size of motorhome solar panel should be increased by approximately 10-15%. Large inverters used to power small loads will have even lower efficiency.
Capacity of Your Batteries
The capacity of your batteries is less important for the size of your solar panel, but it does need to be taken into consideration for two reasons:
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%.
A 100W motorhome solar panel normally produces 4-5A per hour in bright sunshine. If you have a 100A/h battery, this will take approximately 100/5 = 20-25 hours of sunshine to charge from 0% to 100%.
The current of 12 volt solar panels or charging kits is typically proportionate to their wattage. Therefore a smaller 50W solar panel would take twice as long to charge the same battery compared to a larger 100W solar panel.By Photonic Universe Ltd