What Size Solar Panel and Controller Is Needed For Lithium Ion Batteries?

What Size Solar Panel and Controller Is Needed For Lithium Ion Batteries -Featured Image

Are you thinking what size solar panel and controller is needed for lithium ion batteries? Lithium batteries are a great choice over lead-acid batteries, and they are safer, last longer, charge faster, are lighter in weight, and are maintenance-free.

But what application is the battery being used for? Is it for a recreational vehicle, boat, caravan, or trailer? Is it for a home installation to help reduce your fuel bills? Or maybe for an outbuilding such as a shed, garage, or workshop.

The correct amount of solar input and the right size charge controller is really important. Ensuring this is right will allow you to maintain the battery’s best operating performance. 

What Size Solar Panel And Controller Is Needed For Lithium Ion Batteries?

“As an example, 2 x 12V 100Ah lithium Ion batteries would require a minimum of 400 watts of solar input. That would be 2 x 200 watt or 4 x 100w solar panels assuming 6 hours continuous sunshine.  A solar charge controller rated at 40A would complete the system, and you would need to halve the amount of solar panel input and charge controller rating for 1 x 100Ah battery.”

However, if you wanted the fastest charging, most efficient system possible, take all environmental considerations into account, such as clouds, time of day, angle of the sun, and the season. Using 500w of solar panel input with a 50A solar charge controller charging 2 x 100Ah batteries would be better.

What Size Solar Panel Is Needed For Lithium Ion Batteries?

Renogy info video 100 Watt Solar Flexible Kit With Voyager Charge Controller

Currently, the best type of solar panels to use is the monocrystalline variety, and this is because they are the most efficient currently available. Characterized by their dark-colored solar cells, they are easy to spot, with some being black.

Other less efficient, less expensive solar panels are polycrystalline and thin-film. Characterized by their blue color, they will do a job. But if you want maximum efficiency, it is worth the extra cost of the monocrystalline solar panels.

Solar Panel Sizing

This will ultimately depend on how many Lithium Ion batteries you want to charge. As I explained earlier, the minimum for a 12V 100Ah battery would be either 1 x 200 watt or 2 x 100-watt solar panels.

The theoretical calculation for sizing as a minimum is;

Battery amp hours (Ah) multiplied by volts (V) equals Watt Hours (Wh). Then watt-hours (Wh) divided by sun hours (H) equals solar panel wattage;

100Ah x 12V = 1200Wh

1200Wh divided by 6H = 200 Watts 

In the real world, because of varying environmental considerations. The calculation would be better based on 5 hours of continuous sunlight, with 20% added to the total solar panel input giving maximum efficiency whatever the conditions. 

The calculation would then look like this;

100Ah x 12V = 1200Wh

1200Wh ÷ 5H = 240 Watts

240w x 20% = 288 Watts

This setup would require 2 x 150w  or 3 x 100w solar panels per 12V 100Ah Lithium Ion battery. Below is a table based on sun hours for charging 1 x 12V 100Ah lithium Ion battery. With 20% added to the total solar panel watts required for environmental considerations.

Sun Hours Solar Panel Watts Required Plus 20% Environmental
8H150 Watts180 Watts
7H171 Watts206 Watts
6H200 Watts240 Watts
5H240 Watts288 Watts
4H300 Watts360 Watts

Remember the above table is calculated per battery. Meaning 4 x 12V 100Ah Lithium Ion batteries with 6 hours of sunshine per day would require 960w of solar panel input for maximum efficiency. 

The lower sun hour calculations are most relevant to people living in the northern hemisphere. The further north you live, the lower the sun hours per day, so more solar panel watts are required. Those living in warmer climates can safely work around 6 hours of sunlight or more.

Related == >>>>>>>>>> Renogy Solar Panels 100w – Versatile 12v Charging Power

What size Solar Charge Controller Is Needed For Lithium Ion Batteries?

What Size Solar Panel and Controller Is Needed For Lithium Ion Batteries - Solar Charge Controller

Before buying a solar charge controller for Lithium Ion batteries, make sure it will charge them. This is because some controllers do not. So make sure of the manufacturer’s specifications. 

What Types Of Solar Charge Controllers Are Available?

Solar charge controllers are an essential part of any off-grid solar system. They are designed to protect your costly Lithium Ion batteries. Never use a solar panel to charge a battery without one. You will damage your batteries beyond repair, potentially costing you hundreds if not thousands of dollars. 

There are two types of solar controllers: Pulse wave modulation (PWM) and Maximum Power Point Tracking (MPPT). The best and most efficient are MPPT controllers. They not only provide the proper protection for your batteries. But are better suited to handling larger solar panel input arrays. 

Solar Charge Controller Sizing

Solar charge controller sizing is quite an easy thing to do. All you have to do is divide the total solar panel input by the number of volts. Then add 25% to take environmental considerations into account.

So using 400w of solar panel input and assuming a 12V system the calculation would be;

400 ÷ 12 = 33.33

33.33 x 25% = 8.33

33.33 + 8.33 = 41.66

You then round the 41.66 to the nearest whole 10, which is 40

This means that for 400 watts of solar panel input, a 40A solar charge controller is required. Below is a table showing the charge controller size needed for various solar panel inputs.

Solar Panel Input Wattage Solar Charge Controller Size Required
1000w (1Kw)100A

Of course, with solar panel arrays over 600 watts, you can use more than one controller. It is best to use the same type of controller, MPPT with MPPT and PWM with PWM. If you try to use different kinds together, you will burn them out. 

It would probably be better to use the same make/manufacturer type. This will keep your solar charge controller setup compatible with each other. After all, it’s about keeping those Lithium Ion batteries safe from damage.

Related == >>>>>>>>>> Solar Charge Controller Review (Renogy MPPT And PWM)

What Are The Advantages of Lithium Ion Batteries?

Renogy Smart 100Ah Lithium Iron Phosphate (LiFePO4) Battery

As I mentioned at the beginning of the post, Lithium Ion batteries are great because they have several advantages over lead-acid. Lead-acid batteries need quite a bit of looking after. Even if you follow all the rules, they can still quickly fail because they are easily damaged.

Lead-Acid Batteries Have Quite A Few Shortcomings 

One of the biggest reasons they fail quickly is because they are easily over-discharged. Many people don’t realize that they should only be discharged to 50% of their capacity. So if you have a 100Ah deep cycle battery, you really only have 50Ah of usable energy. 

Discharging over 50% even a few times can irrevocably damage the battery. They have limited life cycles, usually around 300, charge inefficiently, losing and wasting energy. Some need lots of maintenance, and they are large and heavy.

While some are pretty cheap to buy, and you might think so what? But constantly replacing them is both time-consuming and a waste of money. 

Yes Lithium Ion Batteries Are More Expensive

Lithium Ion batteries are more expensive, but there are several advantages to using them. I think their most significant advantage is that they have longer life cycles than lead-acid types, as long as you do not discharge below 80%. However, you should never discharge the battery to 0%; it will damage it !!!!

Below is a table showing the approximate number of cycles you can expect continually discharging a Lithium Ion battery to the same level. Discharging to ad-hoc levels, perhaps 50% one day and 80% the next and 30% the day after, makes it impossible to predict how many cycles you might achieve.

Depth of DischargeExpected Cycles
80% DOD500 cycles
50% DOD600 to 1000
30% DOD900 to 1500

They are also smaller and lighter in weight and can charge up to 5 times faster than a lead-acid. 

Being non-hazardous, much safer, and maintenance-free is also a big plus. A much longer shelf life means you won’t have a dead battery on your hands after lengthy non-use. 

They tolerate high temperatures, give constant power, and do not have a voltage sag. With a built-in battery management system, everything is kept running smoothly.

Having said all that, they do not perform particularly well at low temperatures. This means if you live in climates that are prone to temperatures below 32°F/0°C. It might be better to use a Lithium Ion battery with built-in self-healing. 

Related == >>>>>>>>>> Renogy Smart Lithium Iron Phosphate Battery Review 12V 100Ah

My Closing Thoughts

Choosing the right solar panel and solar charge controller for a Lithium Ion battery is essential. It all boils down to how many batteries need charging, and this will depend on the application you will be using it for.

For instance, fitting solar panels to the average recreational vehicle would probably require 400 watts of solar. This would need a 40A solar charge controller and 2 x 100Ah Lithium Ion batteries as a minimum. But taking environmental factors into consideration, it might be better to install 500w of solar and a 50A controller. 

Installing solar panels to a garage to power lights and tools in the northern hemisphere. You would need to factor the amount of available sunlight into the equation, especially further north, and this means you might calculate using 4 hours of available sunlight.

To charge a single Lithium Ion battery, you would need 360 watts of solar panels, rounding it up to 400w. It would probably be better to use 4 x 100-watt panels, and this is because it would create a larger surface area to collect the available sunlight. So a 40A solar charge controller would be required. 

And don’t forget you would also need an inverter to provide AC power. 

Related == >>>>>>>>>> DC To AC Pure Sine Wave Inverter Review (500w to 3000W)

I hope you enjoyed this post and have found it helpful. If you have any questions about Lithium Ion batteries and what size solar panel and controller you need for them. Or want to leave your own personal review, please feel free to leave a comment below.

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4 Comments on “What Size Solar Panel and Controller Is Needed For Lithium Ion Batteries?”

  1. Wow, this post regarding Lithium Ion batteries was fascinating. In my opinion, even if they are costly, they are well worth it because they have far longer life cycles than lead-acid varieties. Also, you mentioned that there are also smaller and lighter in weight and that they can charge up to 5 times faster than a lead-acid battery.
    Excellent article.

    1. Hi Lionel

      Thank you for your kind comment with regard to my article. Yes lithium batteries are a much better bet than lead-acid. There are many advantages for using LiFePO4 batteries one of the biggest being they are much more environmentally friendly. This is because they do not contain liquid acids. This also makes them much more versatile in their use.

      They are much more expensive but I can see them coming down in price over the next few years as they are being used in motor vehicles much more now.



  2. Thank you very much for helping me decide what lithium ion batteries I need. I was trying to make up my mind between lithium or an lead-acid equivalent. And inherently safe chemistry, having the option of finding standard industry sizes and the charging process being fast and safe has inclined the balance towards lithium. Now, before I buy, honestly, what are the problems with lithium ion batteries?

    1. Hi Paolo

      Yes the disadvantage of LiFePO4 batteries is that they do not like temperatures below 32°F/0°C. They also require a battery management system (BMS). Both these problems can be overcome by buying a battery with a built-in heating for use in cold weather. And making sure the battery has a built-in battery management system.

      The other obvious disadvantage is they are more expensive even compared to AGM lead-acid types. Having said that LiFePO4 lasts much longer. 



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