How Many Solar Panels Does It Take to Charge a 400ah Battery?

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An off grid solar system, mobile home or RV will benefit from a 400ah battery. It takes up less space than several 100ah batteries and cost is going down too. But how many solar panels will you need to recharge a battery this size?

A 400ah 12V battery discharged at 50% requires two 300W solar panels to charge in five hours. The same battery can also be recharged by eight to nine 300W solar panels and it will take an hour under clear skies.

What Solar Panel Size to Charge a 400ah Battery?

There are several factors that decide what solar panel size and number are needed. The most critical are the depth of discharge, the panels you use, sun hours and how long you want to charge the battery.

A 12V 400ah battery requires a solar array that produces at least 4800 watts to do a full recharge. If you need to recharge the battery in one day ( with about 5 hours of sunlight), you can use any of the following.

Solar panel arrays to charge a 400ah battery:

  • 4 x 300W
  • 5 x 200W
  • 3 x 350W
  • 3 x 400W

These are the minimum solar panel requirements. If the weather is not favorable the array may not be able to produce the required power, so consider adding another PV module or two.

It does not matter which of the combinations above you choose as long as the output is at least 4800 watts (or 2400 watts, if the panel is 50% discharged). The 250W and 300W solar panels are the most popular, though 350 PV modules are becoming widespread.

There are a lo of options, but we like the Renogy 300W Solar Panel RV Kit because it comes with all the required connectors and a charge controller. You can also expand the panels if you need more power.

The conversion formula is Ah x V = W (400ah x 12V = 4800W)

Other 400ah batteries are 24V or 48V. The higher the voltage the more watts the battery has. However the conversion is still the same, just replace 12V with your battery voltage.

When people talk about solar panel sizes it usually refers to the power output in watts instead of the physical dimensions. The physical size of solar panels have little bearing to its output.

The physical size still matters of course. If you have to install solar panels on a roof, they need space so knowing the length, width and height is necessary. But for charging a battery, it is the watt output or rating output that matters.

How Long Does it Take to Charge a 400ah Battery?

It is not possible to give an exact time because so many factors come into play. Aside from the DOD, the weather, how long you want to charge, the solar panel efficiency and sunlight availability affect the charge time.

How Fast Do You Want to Charge the Battery?

Four 300W solar panels can recharge an empty 400ah battery with 5 hours of sunlight. If you need to recharge the battery in an hour, you have to use more solar panels

In an ideal climate, 16 x 300 solar panels can charge a 12V 400ah battery in one hour. It is possible for this array size to produce 4800 watts an hour, but the weather must be just right.

This runtime also assumes the battery is completely discharged. If it is lead acid you should recharge it at 50%. In that case you need 2400 watts. Instead of 16 you only require 8 x 300W panels to fill it up.

The more sun hours available, the better for your battery. With enough sun even a small solar array can fill up the battery, but the reverse is also true. Even a large solar panel cannot do much if there is very little sunlight.

Some AGM and gel batteries have a 70% DOD so you can use it until the capacity is at 30%. With lithium you can let it drop to 90% or even 100%. This is great if you have appliances or power tools like a welder that consume a lot of power.

But the more you discharge a battery, the longer it will take to recharge. The battery age is also a factor here. Older batteries take longer to recharge and lose power faster too. A high quality, durable battery such as the Weize 12V 100ah will last longer even with heavy use.

The DOD is just one factor though, because the current must still be provided by the solar panels. Whatever the battery type is, the rate which solar power flows into it will depend on the PV modules.

If you want to charge batteries as fast as possible, get a large solar array and charge during the warmest part of the day. You can also use smaller solar panels but it might take several hours to get the job done. Most campers base their calculations on 5 hours of sunlight, but it really is up to you and how fast you need that battery topped.

Solar Panel Efficiency

The efficiency rating indicates how much sun energy the cells are able to turn into DC power. The best solar panels have an efficiency rating between 21% to 25%.

When sunlight strikes the solar cells, a chemical reaction takes place that converts the energy into direct current. This current is transmitted into the battery.

Only 15% to 25% of this energy is turned into DC power. The higher the efficiency rating, the faster a battery can be recharged.

Take three 400ah batteries and connect each one to form a 6000W solar array. One set has 18% efficient panels and the other 23%. Even under the same conditions, the more efficient solar array will charge the battery faster.

The efficiency rating of solar panels continue to improve. Just a few years ago the standard was 15%, but now that is considered the low end, and high quality PV modules are rated at 20% or higher.

Solar Panel Output

The output refers to the maximum watts the solar panel can produce in an hour. However you should not expect the panel to deliver maximum performance all the time.

If you have 2 x 300W solar panels and need to recharge a 400ah battery bank with 50% capacity, how long will that take? During the summer it is possible for the two solar panels to produce 600 watts combined in an hour.

If the panels can sustain 300 watts an hour, that battery will be fully charged in 4 to 5 hours. 600 watts x 5 = 3000 watts, more than what the battery needs. But again this is supposing the weather is perfect for solar charging.

The presence of clouds will slow down the process and those two 300W panels may require 6 hours to recharge the battery. If there is not enough sunlight available it might take a day or two.

You can also use a larger solar array to convert more power, but it still depends on how much sun is present. Charging any battery is easy during the summer, but you will have to wait a bit longer during the fall and especially winter.

Sunlight Availability

In the end, all solar power systems depend on the sun and how many hours are available during the charge. Because sun hours per day varies by season, geography and other factors, we can only use estimates in our calculations.

Before you decide what solar panels to buy, consider how many sun hours are available. During the summer you can expect at least 5 hours, maybe 6 or 7 if you live in the sunnier parts of the country. If you live in New York or it is near winter, sunlight hours will be limited.

You have to make these adjustments each time you recharge a battery, no matter the size. However once you have been using solar power for a while, you get an idea of how many sun hours are available in a season.

Where you install the solar panels also matter. If it is a ground mount installation, the panels have to be the right distance from your house to work, including battery charging. You also have to make sure the cables are of good quality to reduce energy loss.


So it comes as no surprise that a 400ah battery, lithium or lead acid, requires a lot of solar power. If you are going to use one – or more – in your system, recharging should be done during ideal weather conditions so power transmission is fast and consistent.