Will a 40 Watt Solar Panel Charge a 12V Battery?

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A portable 40 watt solar panel is ideal for camping and is light enough to carry. But can it charge a 12V battery? And if so how long will it take? Knowing the solar panel’s output is necessary before you go camping with it, and that is what this guide is for.

A 40 watt solar panel can charge a 12V 50ah battery in 3 days. A partially discharged battery can be recharged in even less time with 5 hours of sun available.

How Long Does It Take a 40W Solar Panel to Charge a 12V Battery?

To get the most accurate estimate, you have to account for the battery size and how many hours of sunlight are available. Here are some examples.

A 40 watt solar panel can supply up to 40 watts an hour. This is the maximum output you can expect, but it could be lower than that depending on the weather.

It will take a 40 watt solar panel 7 days to charge a 100ah 12 V battery. This is assuming the solar panel produces 200 watts / 3.3 amps 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.

Key to this is getting a quality solar panel. The Newpowa 50W Solar Panel is perfect for charging 12V batteries, and the extra 10 watts might come in handy.

If you camp out during summer and get 5 hours of sunlight, charging time will be close to this. If you get 6 to 7 hours of the sun and you didn’t fully discharge the battery, charging could be even faster. But unless the weather is perfectly sunny, topping the battery could take longer than expected.

40 Watt Solar Charge Time Calculation

So what then is a realistic charging time like? There are many possible scenarios and the following are some examples.

You have a 40 watt solar panel and a 12V 100ah battery. The battery is fully discharged (empty). There are 5 hours of sunlight available. At noon the solar panel output can reach 40 watts, but as the afternoon goes on the output will drop to the 30s.

Assuming an average of 33 watts with 5 sun hours, that is 165 watts or about 13.7 amps a day. Provided the weather holds, it will take 8 days to fully recharge the battery. If the battery is lead acid and you recharged it at 50%, it will take 4 days to fill.

If the sky is overcast the panel output could drop to 20 watts on average. The disparity between solar panel output and rating occurs during cloudy days, so make adjustments to the charge time.

If your solar panel output averages close to 40 watts an hour and there are 6 to 7 hours of sun, your battery will get 19.8 amps a day. You can charge a 35ah battery in a day and half and a 50ah in two. This is the ideal scenario and can happen during summer and in the right spots.

Of course no camper expects this kind of weather every day, so keep your expectations in check. Always look up the weather in your specific area so you can get an idea of how much sun you will get. While 5 hours is the average, the range could be 4 to 7 hours depending on the season.

What Battery Can I Charge with a 40 Watt Solar Panel?

A solar panel can charge any battery provided the panel voltage is equal or higher than the battery. You should also have a charge controller installed to protect the battery from overloading.

The largest battery size you should use with a 40 watt solar panel is 200ah. Technically there is no limit, but 200ah may already be too much as it is. Even if the panel produces 17 amps a day it would take 12 days to recharge a 200ah battery.

If the output is less than 17 amps the charge time will probably take two weeks or more. Can you do it? Yes it is possible, but for batteries more than a 100ah, you should use a larger solar panel. Most people will use a 40W solar panel to charge small batteries like the 35ah Mighty Max.

For 35ah, 50ah, 75ah and 100ah batteries, a 40W PV module will do provided there is enough sun. That is the thing to remember for any solar charging. Even the biggest solar panel will produce near zero if there are clouds covering the sky.

It comes down to portability and how much weight you are willing to carry. If you have a camper with plenty of space, you can bring several solar panels and a large battery bank. If you travel light and don’t need a lot of solar power, a 40W PV module and small battery will suffice.

Is a 40 Wat Solar Panel Enough?

With PV modules becoming larger, does it make sense to still use a 40 watt solar panel? Well it depends on what you want to do with it.

40 watts is enough to charge a battery up to 100ah. You can power a larger battery but it will take several days. The longer charging takes, the greater the possibility the weather becoming unfavorable.

Besides batteries, you can use the solar panel to charge mobile devices. 40 watts is not enough to run a laptop, fan or other appliances. But if you just need a power source for your battery or tablet it is enough.

If you have an RV you can run appliances and other devices off its generator, battery or the installed solar panels. You can then use your 40W PV module exclusively to charge batteries.

If that is all you need – a solar battery charger – this panel is enough. But to run appliances and other electronics, you must have a larger solar panel. Or you can connect several 40 watt panels to produce more power.

Rigid vs. Flexible 40 Watt Solar Panels

So you have decided to go with a single 40 watt PV module. The question is, which do you get, a rigid or flexible panel?

A flexible solar panel is easier to carry and can be used on irregular surfaces, but it is less efficient. A rigid solar panel is heavier but can generate more watts.

To start with the power output. Flexible solar panels or thin film modules have seen significant improvements, but they are still behind rigid modules when it comes to power. Both rigid and flexible solar panels can produce up to 40 watts, but rigid panels are faster because of its construction.

The drawback with flexible solar panels is there has to be a compromise between flexibility and the ability to convert solar energy into current. Exactly how much longer it will take for a flexible panel to charge a battery is hard to estimate. It depends on the weather, panel quality, efficiency and other factors.

You just have to ask yourself, which is more important for you, charging speed or portability? If it is portability, a flexible PV module is the way to go. If it is speed, buy a rigid solar panel.

Tips and Suggestions

  • Decide what battery size you will be using before you buy a solar panel. The last thing you want is to be stuck with an empty battery that your solar panel cannot recharge because it is too small.
  • If you already have a 40 watt PV module, it means you will use it mainly for battery charging and to power some mobile devices. In that case a small 12V battery will be enough.
  • RVers and campers cannot rely on small solar panels alone. You must have another power source whether it is a solar array on the RV, a generator or a battery bank. If you frequent RV camp sites they have power available so minimal solar power is required.
  • Never let your lead acid batteries be completely discharged. If you do this often it will take longer to recharge and cause irreparable damage. If you can afford to spare the power, topping off at 85% is ideal.


So while PV modules continue to increase in output, portable 40 watt solar panels remain appealing for people who do not need a lot of power. If you have reasonable expectations, this panel can charge batteries without problems.