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# Thread: Calculation of battery back up for Solar PV Power Plant

1. ## Calculation of battery back up for Solar PV Power Plant

Hi,
Can anyone pls help me to calculate the battery sizing. I have gone through various websites and have found different formulae and not able to decide which formula is effective. pls suggest a effective formula for battery sizing and also how to decide the system voltage.

The following are the formulae i have come across and i would appreciate it if anyone can verify them

Total KWhr load = Total No. of Load*Wattage of each unit*total No. of working hours

Solar PV Module Capacity = (Total KWhr Load)/4 Considering insolation to be 4 hours

Inverter Capacity = Solar PV Module Capacity

Ampere hours per day = (Effective Load)/(System Voltage)

Total Battery Bank Capacity = ((Total Module Capacity)*(No. of backup hours))/((System Voltage)*(Inverter Efficiency)*(Battery deep discharge))

Thank you,
Regards

2. It is real easy. First decide on how many days of reserve you want. Minimum is 5 days which gives you 2.5 real days before you hit the 50% DOD mark. So let's say you use 1 Kwh/day, that means you need 5 days x 1 Kwh = 5 Kwh total capacity.

All you do now to find the amp hours is factor out the battery voltage gives you Wh / volts = AH. So lets say you want a 12 volt battery system. 5Kwh / 12 volts = 417 AH.

3. Originally Posted by Sunking
It is real easy. First decide on how many days of reserve you want. Minimum is 5 days which gives you 2.5 real days before you hit the 50% DOD mark. So let's say you use 1 Kwh/day, that means you need 5 days x 1 Kwh = 5 Kwh total capacity.

All you do now to find the amp hours is factor out the battery voltage gives you Wh / volts = AH. So lets say you want a 12 volt battery system. 5Kwh / 12 volts = 417 AH.
Thank you Sunking, could you please tell me what are the factors that are to be taken into consideration to choose the system voltage i.e the input voltage to the inverter from the battery bank. Also i would appreciate it if you could tell me if the formulae i used for the design is correct or not.

Regards

4. Total KWhr load - a useful number can only be obtained by measuring each individual device over a couple of days.

Solar PV Module Capacity = (Total KWhr Load)/4 - a meaningless number really - the 4 hours is a number grabbed out of the air and is possibly right for some location somewhere

Inverter Capacity = Solar PV Module Capacity - roughly though the module capacity can often be slightly higher than the inverter rating - the efficiency of the panels etc is in the 77% range
Effective Load = (Total KWhr Load)/(Inverter Efficiency) - a made up number by someone - the inverter efficiency (MPPT) is in the 93% to 97% range - for quality units - no name junk from China or elsewhere is most likely far lower

Ampere hours per day = (Effective Load)/(System Voltage) - there is the effective load word again?

Total Battery Bank Capacity = ((Total Module Capacity)*(No. of backup hours))/((System Voltage)*(Inverter Efficiency)*(Battery deep discharge)) - salesman's smoke?

5. Originally Posted by russ
Total KWhr load - a useful number can only be obtained by measuring each individual device over a couple of days.

Solar PV Module Capacity = (Total KWhr Load)/4 - a meaningless number really - the 4 hours is a number grabbed out of the air and is possibly right for some location somewhere

Inverter Capacity = Solar PV Module Capacity - roughly though the module capacity can often be slightly higher than the inverter rating - the efficiency of the panels etc is in the 77% range
Effective Load = (Total KWhr Load)/(Inverter Efficiency) - a made up number by someone - the inverter efficiency (MPPT) is in the 93% to 97% range - for quality units - no name junk from China or elsewhere is most likely far lower

Ampere hours per day = (Effective Load)/(System Voltage) - there is the effective load word again?

Total Battery Bank Capacity = ((Total Module Capacity)*(No. of backup hours))/((System Voltage)*(Inverter Efficiency)*(Battery deep discharge)) - salesman's smoke?
Thank you Russ for your valuable reply, I understand that most of the calculations seem to be traditional assumptions in sizing of a solar power plant, I would appreciate it if you could guide me through the proper standard calculations that are required. I have been having a tough time since every person or every source I find to learn these calculations have a different approach.

Regards

6. what are the factors that are to be taken into consideration to choose the system voltage

1) for every 1KW of load, add 12V to the battery system:
800W could work on 12V
1200w should have 24V
2300w should have 36V - but no gear exists so use 48V
2300w & greater = 48V

2) increase voltage to meet your wattage demand with a single string of batteries. Adding batteries in parallel is possible, but can lead to more troubles than a proper single string.

7. Originally Posted by Mike90250

1) for every 1KW of load, add 12V to the battery system:
800W could work on 12V
1200w should have 24V
2300w should have 36V - but no gear exists so use 48V
2300w & greater = 48V

2) increase voltage to meet your wattage demand with a single string of batteries. Adding batteries in parallel is possible, but can lead to more troubles than a proper single string.
Thank You Mike your post was of great help.

8. ## calrification

I have Solar modules 3kwp no. of bakup hours is 10hrs and system voltage 120(battery voltage),inverter eff .85,DOD is .8

so please teel me the AH capacity?i am little bit confused?

Originally Posted by seshu
Hi,
Can anyone pls help me to calculate the battery sizing. I have gone through various websites and have found different formulae and not able to decide which formula is effective. pls suggest a effective formula for battery sizing and also how to decide the system voltage.

The following are the formulae i have come across and i would appreciate it if anyone can verify them

Total KWhr load = Total No. of Load*Wattage of each unit*total No. of working hours

Solar PV Module Capacity = (Total KWhr Load)/4 Considering insolation to be 4 hours

Inverter Capacity = Solar PV Module Capacity

Ampere hours per day = (Effective Load)/(System Voltage)

Total Battery Bank Capacity = ((Total Module Capacity)*(No. of backup hours))/((System Voltage)*(Inverter Efficiency)*(Battery deep discharge))

Thank you,
Regards