To get the most accurate estimate, you must consider the size of the battery and how many hours of sunlight is available. Here are some examples.
A 40 watt solar panel can provide 40 watts of electricity per hour. This is the maximum output you can expect, but depending on the weather, it may fall below this value.
It will take a 40 watt solar panel 7 days to charge a 100ah 12V battery. This is assuming the solar panel produces 200 watts a day. If the battery is discharged at 50%, it will take 3 to 4 days to charge. But again this assumes the solar panel produces peak output consistently.
If you camp in summer and get 5 hours of sunshine, the charging time will be close to this time. If you have 6 to 7 hours of sunlight and you don’t fully discharge the battery, charging may be faster. But unless the weather is perfectly sunny, it may take longer than expected to charge the battery.
What Battery Can Charge With A 40 Watt Solar Panel?
A solar panel can charge any battery as long as the panel voltage is equal to or higher than the battery. You should also install a charging controller to prevent battery overload.
The maximum cell size you should use a 40 watt solar panel is 200ah. There are no technical restrictions, but 200ah may be too much. Even if the battery board generates 17 amps of current every day, it takes 12 days to charge the 200ah battery.
It depends on portability and the weight you are willing to carry. If you have a camper with enough space, you can bring several solar panels and a large battery pack. If you travel light, you don't need too much solar energy. A 40W photovoltaic module and small battery are enough.
Therefore, while the output of photovoltaic modules continues to increase, portable 40 Watt Solar panels are still attractive to those who do not need a lot of power. If you have reasonable expectations, this panel can charge the battery without problem.