How Many 250W Solar Panels to Charge a 200ah Battery?

Solar panels have different power output ratings, but 250W is probably the most common for homes and in many RVs as well. It gives you sufficient power and works well with various batteries, including the 200ah. So it is no surprise that many people pair the two, but how quickly will this system take to charge?

Two 250W solar panels can charge a 12V, 200ah battery with six hours of sunlight. To charge a 12V 200ah battery in an hour requires 12-15 solar panels. The solar panel efficiency, batter depth of discharge, sunlight availability and battery voltage determine how long the charge will be.

How Many Watts Solar to Charge a 200ah Battery?

Because of the way solar energy works, several factors come into play and a definitive answer is not possible. To put it another way, the question “how many 250W solar panels to charge a 200ah battery” is incomplete. If we include the factors mentioned earlier we will get different answers.

Solar panel output is measured in watts, so we need to find out how many watts is in an amp. Multiply amps x volts to get the watts. In our example, 2400W = 12V 200ah. So our 250W panel has to produce 2400W to fully charge a 200ah battery.

If solar panels always generated maximum power then the calculation would be easy. However a 250W panel only reaches 250W when the sun is at its peak. Otherwise the output will probably be 200W at best. So 200W with 5 hours of sunlight gives us 1000W. Solar panels are getting better all the time though, so in a few years we will see solar panels with higher efficiency. For now though an 80%-85% efficiency is good enough.

Besides, you can always go for a higher rated panel. If 250 watts is not enough, you can go with the Huajin 300W Semi Flexible Solar Panel as it can produce more power than a standard 250W PV module.

Battery Depth of Discharge

Another factor that affects charging time is the battery depth of discharge. We have covered this in other posts such as this one so only a quick summary is required here. A 200ah battery like the ML4D from Mighty Max for example, does have a 200ah capacity. But for best results you should only use 100ah, or 50% of it. This is especially true with lead acid batteries.

There are a lot of reasons but the long and short of it is a 50% depth of discharge is good for battery health. Constant recharging from 0% to 100% puts s strain on the battery and could damage it. If you follow the 50% DOD rule then it has a huge effect on the solar panel charge time. Some batteries have a 75% maximum output (i.e. a 120ah battery maxes out to 90ah).

A 50% DOD means your solar panel only needs to recharge 100ah, not 200ah. 100ah is 1200 watts. Assuming an 85% output efficiency, a 250W solar panel can recharge a 100ah battery in six hours (200wh x 6 hr = 1500W). With peak sun and 20A of current, a 250W solar panel can charge the battery in 5 hours.

If you use lithium battery – and there are many reasons why you should- -the DOD can be 80% or 1920W. You can use more of the battery but it may take a bit longer to recharge. We say just a bit because lithium ion batteries charge faster than lead acid.

How Many Hours of Sunlight is Needed by a 250W Solar Panel?

Three 250W solar panel will generate 3000W with 5 hours of sunlight. If there are six hours then two panels will suffice as we mentioned earlier. Of course if sunlight availability is reduced it will take more solar panels / more hours to recharge the battery.

Sunlight directly correlates to solar panel production, more sunlight = more energy. In our example we use 5 to 6 hours of sunlight. But depending on where you are, the time of day and the season, you cold get just 4 hours, more than 6 hours or somewhere in between.

There is not much sunlight in the morning or late afternoon. The peak time is at noon and during summer you can get up to 5 hours. Even with five hours you should not expect the panel to always be at peak level, i.e. 250W. However you can still use the formula here and replace the sunlight hours with what is available in your area.

Obviously a lot of advanced planning is required here. How you charge solar panels and batteries will differ depending on the season. It is something you have to account for and should be done weeks or months in advance. But with the guide here you should have an easier time figuring out how long it could take regardless of the season.

Accounting for Energy Losses and Low Efficiency

Overcast days and less than efficient design slows down charging. Some charge controllers have up to 20% energy loss. Add low panel efficiency and unfavorable weather, and it could take a while to charge a 200ah battery even at 50% DOD. With a 12V system with 25% energy loss, a 350W output is needed.

another problem has to do with the question itself. Rather than ask “how many solar panels to charge X battery capacity”, the better question is “how many solar panels and/or batteries is required for my daily needs”.

That question is more specific and you will also get a more specific answer. Combine the total watts or amps of all devices you will use. Then you will know what solar panel size you will need. This will also tell you what battery capacity is required.

If your total watt usage is 1000W, it is easy to figure out how many 250W panels are needed using the methods given above. The solar panel draws the energy and the battery stores it. Again using the same formula, you can determine if 200ah is enough or not.

What is the Biggest Battery You Can Connect to a 250W Solar Panel?

In ideal condition a 250W solar panel produces up to 1000W or 83.3ah. You can connect a larger battery but it would be a waste. A charge controller keeps the current flowing normally and prevents overcharging. But yes, theoretically there is no limit to the battery capacity you can use. But you will be wasting money and time as the solar panel will not be able to fully charge it.

Conclusion

One of the first things you should learn about solar power is some math is involved. It is not that complicated as we have seen, but knowing the numbers is crucial. In this case, also knowing the factors that affect charge times is critical. But now that you have the facts at hand, you can use any solar panel and charge any battery with a good idea of how long it will take.