Battery is basically an instrument which is used to stored the electrical energy in the form of chemical energy. Battery is used to store the DC Supply. When the solar system is installed on any location whether it is Residential or Commercial, Batteries are required to store the DC which is going to be generated by the solar modules during sunlight.
The Size of the battery for a particular solar plant is calculated by following steps:
1. DETERMINE THE AVERAGE DAILY CONSUMPTION
The first step for calculating the battery size is, calculating the average daily consumption. The average daily consumption is the measure of total power consumed by the all the loads of particular location by period of one day. The consumption is measured in kWh or Wh.
EXAMPLE: Lets take an example of residential building. Suppose the average daily consumption of a house is 2kWh or 2000Wh.
For more details, the load is given following:
2. DIVIDE AVERAGE CONSUMPTION WITH THE INVERTER EFFICIENCY
The second step to calculate battery capacity or battery size is to divide the average daily consumption with inverter efficiency. Inverter is a device which convert DC into AC. In the household, most of the loads are works on AC supply. That is the reason why inverters are used. They take DC power from batteries and convert it into AC supply. There are different types of inverters which have different different efficiencies. The efficiency of an inverter is written in its specsheet and on nameplate also.
EXAMPLE: Suppose the efficiency of an inverter is 90%.
Average daily consumption/ Inverter efficiency
= 2000Wh/ 0.9
= 2222.22 Wh
3. ADD DAILY ENERGY CONSUMPTION FROM DC LOADS
The third step is to add the energy consumption from all the DC loads in Step 2. Most of the load in household is AC but sometimes there are loads in house which work on DC Supply. No doubt, the value of DC load in house is very less as compared to AC load, but we should consider all type of load for calculating the battery size.
EXAMPLE: Suppose the energy consumption from DC load is 120Wh
Total daily energy consumption = 2222.22 + 120 = 2342.22Wh = 2.342kWh
4. MULTIPLY ENERGY CONSUMPTION VALUE TO DAYS OF AUTONOMY
The fourth step is to multiply the total daily energy consumption value with the days of autonomy. The days of autonomy means the number of days the battery can provide power without recharging. The days of autonomy decided by the owner/ client according to situation of power outage in their area.
EXAMPLE: Suppose the days of autonomy are 3.
Total daily energy consumption * Days of autonomy
= 2.342 * 3 = 7.026kWh
5. DIVIDE CALCULATED VALUE WITH THE TEMPERATURE COMPENSATION VALUE
The fifth step is to divide the calculated value which comes in step 4 with temperature compensation value. The temperature compensation value is provided by the manufacturer of the battery. The capacity of battery is affected by the temperature. The battery capacity changes with change in temperature, thus the temperature compensation value is provided by the manufacturer.
EXAMPLE: Suppose the temperature compensation value = 0.9
= 7.026kWh/ 0.9
6. DIVIDE THE VALUE BY ALLOWABLE DEPTH OF DISCHARGE
The sixth step is to divide the value obtain in step 5 with depth of discharge. The depth of discharge, also known as DOD is basically an amount of energy drawn from the battery. The depth of discharge for deep cycle battery taken as 80%. Deep cycle batteries can be charged and discharged many times without any damage.
EXAMPLE: Suppose the depth of discharge is 80%.
= 7.806kWh/ 0.8
7. DIVIDE THE VALUE BY DESIRED NOMINAL VOLTAGE FOR BATTERY BANK
The seventh and the last step to calculate the battery capacity is, to divide the value obtain in step 6 with the desired nominal voltage for the battery bank. The battery banks comes in different range of nominal voltage, for example 12V, 24V, 48V etc.
EXAMPLE: Suppose the nominal voltage for battery bank is 48V
= 9.757kWh/ 48V
So The battery capacity needed for a solar system which have average daily consumption of 2kWh is 203.2Ah.