How to size the battery for your solar power system?

1/8/2020

You can store solar energy in the batteries in the form of chemical energy which can be used later when required.

So, the batteries are the energy storage bank and are a very important part of the solar power system.

Their importance in the solar power system increases further when you want either more energy at night or the grid supply is erratic in your area or both.

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The sizing of the battery depends on the following factors:

Your energy requirements
Depth of Discharge (DOD) of the battery
Efficiency of the Battery
Efficiency of the Inverter

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Your Energy needs

In a normal day we have around 12 hours of sunshine and another 12 hours we see night.

When most of your solar energy needs are met in daytime then we have less scope left to attach batteries with the solar power system.

But when your energy need curve is tilted more towards night-time that is when you consume more energy in the night then need for battery becomes indispensable and its sizing becomes important than before.

In addition to this, if you live in an area where grid supply is not smooth or you often see lots clouds around sun then the battery is required even in the daytime.

In short, your need for battery depends on your energy needs specifically in the night, weather conditions and grid supply (smooth or erratic).

More energy needs mean more battery size

Let us understand all this with the help of an example:

The following electrical appliances along with their power rating are shown in the table below:

Total load is 150 watts + 80 watts + 150 watts
= 380 watts

I want to run this load on battery for 4 hours as I feel that I am without sunlight and without grid supply for a total of around 4 hours in day.

You can choose battery backup according to your needs.

The output energy required from the batteries = 380 watts x 4 hours = 1520 watts-hr or 1.52 units

The battery output will go to the inverter, it converts dc power into ac power which in turn will run the load (fans, CFLs and fridge).

Before going ahead, I feel it is important to understand some terms which are important in sizing the battery.

Battery basics

(i) Depth of discharge:
This term tells the amount of or extent of discharge of the battery.

If battery is fully or 100% charged, then depth of discharge is 0%.
If the battery has delivered 40% of its energy and 60% is reserved in it, then the depth of discharge is 40%.

Similarly, if only 20% charge is remaining in the battery, the depth of discharge in this case would be 80%.

The batteries come with rating for DOD, here I am taking a battery with DOD 80% i.e. 20% charge is reserved.

If the battery is discharged below its rated rate, it affects the performance and the life of the battery.

Therefore, never discharge the battery beyond its rated depth of discharge; it shortens the life of the battery.

(ii) Capacity of the battery:
It is the amount of the charge available in the battery, expressed in Ampere-hours (Ah). 1 Ah means that a current of 1 ampere is flowing for 1 hour.

(iii) Charge rate:
The charge rate is denoted as C. It is the maximum safe rate of discharge of the battery.

If the battery is rated as 12v/150 ah and C/10, it means that one can draw a maximum of 150ah/10= 15 amperes of current safely.

Similarly, a 12v/150 ah battery with C/20 rating will safely provide 7.5 amperes of the maximum current.

(iv) Cycle life of the battery:
It is the number of charge and discharge cycle a battery can support before its performance falls and reduces.

If a battery rated 1500 cycles, it means that it can with stand 1500 complete charging and discharging cycles before its performance reduces or falls.

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Calculating the capacity of the battery

Now, coming back to the example, the capacity of the battery will be calculated by considering the following points:

I am taking DOD as 80% that is the 20% charge remains in the battery.
Not all the chemical energy stored in the battery is converted to the electrical energy, assuming 90% efficiency of the battery.
There will be conversion losses when the dc power of the battery is converted into ac power through the inverter to run the load. We are assuming that 10% of the energy is lost in the conversion, i.e. the inverter is 90% efficient.

Look at the following energy conversion chart:

Now, the capacity of the battery which can back up the load for 4 hours = Output/(efficiency of the battery x DOD x efficiency of the inverter)
= 1520/(0.9 x 0.8 x 0.9)
= 1520/0.648
= 2345.7 watt-hours

As for the residential purposes, most of batteries have 12 volts, put this value in the equation, we get,

Capacity (x) = volts x Ah

2345.7 = 12 volts x Ah
Or
Ah = 2345.7/12 = 195.5 Ah

I want C/10 rated battery, so that I can withdraw a maximum current of 19.55 amperes for 10 hours.

Therefore, I will look for battery having specifications as:

12 volts/195.5 Ah and C/10 rating

The nearest specification in the market is 12v/200ah battery with C10 rating & look for solar tubular battery for your solar system.

You can customize your battery size based on your load requirements, hours of backup, the efficiency of your battery and inverter; the basic concept and the formulas will remain the same.

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