It is well seen that here are solar energy boards everywhere. With the developing consciousness of saving energy and money, more and more people believe that the home solar system is a great choice. When sun raise, solar system will convert solar energy into electric energy to support home daily usage.

However,

What will happen if sun sets? Then we need a lithium battery to solve this problem. When extra electric energy appear which is generated by solar system, the rechargeable battery is in the charging process to save extra electricity. Once there are a series of windy or rainy days, the lithium battery work as energy resources.

It is generally necessary to configure battery to work in solar energy equipment because the input energy of solar photo-voltaic power generation system is extremely unstable and the amount of battery capacity is limited. In addition, climate is various.

The key point is how to get a proper solar battery size.

It will be wasteful for money to choose a very-large-capacity lithium battery because you do not need so many electricity and lithium battery will damaged in long term charging process.

You may meet this problem when you are immersed in all kinds of batteries. You just follow those steps：

—Step 1—You should know how much energy storage you need.

If you don’t have enough battery capacity, your equipment will run out of power so you need to add solar battery backup and fire up the backup generator. On the other hand, if you buy too many batteries, you add unnecessary expense to your system, with extra components, complexity and maintenance.

Your energy usage decides the amount of battery storage. Energy usage is measured in kilowatt hours over a period of time. For example: 1 kilowatt multiply by 10 hours per day is 10 kWh per day. Here is a more convenient way to get this information. It is quite easy to find out your daily ,monthly and annual electricity consumption — according to your bill. You just divide daily, monthly and annual kilowatt-hour by days your bill included. It’s faster, isn’t it?

                                                                                 energy usage

By the way, you’d better notice special seasons like summer and winter. Both of them have extreme temperature. People use more energy in the winter and summer. The sun produces more power in the summer than in the winter. Following charts show that energy usage datas of every month are different because of temperature. You need to consider about them although they are not influenced battery size.

Tips: You can take 80% of the average amount of electricity used in extreme weather as a more accurate figure

—Step 2—You should estimate how many days your solar devices will be without sun.

Load electricity during continuous cloudy and rainy days must also be obtained from the battery. Maybe you don’t know the exactly information, you can look up the annual average of cloudy and rainy days recent years in your block online. If you want to get solar energy all year around, this step is extremely crucial. Or you just DIY the number of days backup required.

—Step 3—Third, you need to estimate the lowest temperature your battery bank will experience

When battery is colder, the lithium ion battery is going to be able to hold less power than it’s warm, thus you’d better estimate the lowest temperature the lithium battery pack will experience. You can also get this data online like the average cloudy days above. Heat or cold has a big impact on battery performance and capacity.

—Step 4—Battery Type

At present, solar batteries are generally lead-acid batteries and lithium batteries. Both of them have their own unique performance characteristics and their sizes are different. You choose one according to your need.

Lithium Battery Design Design Ebook Download(2M, 20 pages, PDF)

Well, all data is prepared, we need to calculate the solar battery size.

Based on energy usage of 5 kWh per day and there are 2 days of the continuous cloudy and rainy days. The lowest temperature is 0℃. And the DOD (depth of discharge) for Lead-acid battery is 50% and the DOD for lithium battery is 80%

Lead Acid Sizing Lithium Sizing

5kWh*2(day)*1(TO)/0.5(DOD)=20kWh                                     5kWh*2(day)*1(TO)/0.8(DOD)=12.5kWh

10kWh*2(day)*1(TO)/0.5(DOD)=40kWh                                   10kWh*2(day)*1(TO)/0.8(DOD)=25kWh

15kWh*2(day)*1(TO)/0.5(DOD)=60kWh                                   15kWh*2(day)*1(TO)/0.8(DOD)=37.5kWh

Convert

Battery capacity is specified either in kilowatt hours, or amp hours. If you convert into amp hours,it is easier and more clear to choose battery type.

“E = V * Q”

This formula can help you to convert kilo-watt hours to amp hours.

For example, 

20 kWh = 1000amp hours at 20 volts → 1000 Ah x 20V = 20 kWh

= 2000amp hours at 10 volts → 2000 Ah x 10V = 20 kWh

40KWh = 2000amp hours at 20 volts → 2000 Ah x 20V = 40 kWh

= 4000amp hours at 10 volts → 4000 Ah x 10V = 40 kWh

= 8000amp hours at 5 volts → 8000 Ah x 5V = 40 kWh

Depending on the size of the system, battery banks are typically wired for either 12 volts, 24 volts or 48 volts. Here are example battery banks for both lead acid and Lithium, based on an off-grid home using 5 kWh per day:

For Lead Acid, For Lithium,

20kWh=1666.7 Ah×12V                 12.5kWh =1041.7 Ah×12V

20kWh=833.4Ah×24V                    12.5kWh =520.8Ah×24V

20kWh= 416.6 Ah×48V                  12.5kWh =260.4Ah×48V

There is another formula to calculate it. This is a method to calculate solar battery size, too. You just choose one to design your solar battery size.

“BB = P ×D×DI/100 / V”

“BB” is the battery bank size (amp hours) It is the final answer you get.

“P” is the power consumption per day (the unit is “Wh”). In other words, it means the power you want to use for your solar installation per day. You just design the parameter according to yourself. You’ll want to look to your electric bill for this information.

“D” is the number of days backup required. In other words, it means the time of discharging.

“DI”is the percentage of maximum depth of discharge (%).The percentage of battery capacity that has been discharged expressed as a percentage of maximum capacity. A discharge to at least 80 % DOD is referred to as a deep discharge. In this formula, you should input the maximum depth of discharge

“V” is the voltage (voltage, abbreviated as” V”, its unit is “V”). You just design it according to your

Installation.

Let’s have a try

The following are some example problems on how to calculate the battery bank size.

Example 1 Example 2

P=5000 watt-hours P=4000 watt-hours

D=20 days D=7 days

DI=90 DI=80

V=250V →BB=5000×20×90÷100÷250=360 aMh V=220 v→BB=4000×7×80÷100÷220=101.81818181…aMh

It’s just like a simple math calculation, isn’t it? You just input all parameters you need, you can get a result.

That’s all about how to calculate solar battery bank. Now, are you clear about how to choose the right size? Experienced engineers in DNK Power Limited will solve your problems about batteries. If you are interested in batteries, you can leave a message about your doubts. We will solve them as soon as possible. We will update a lot of popular science about batteries, you can store up this website now then you can find us quickly next time.

Lithium Battery Design Design Ebook Download(2M, 20 pages, PDF)

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