Battery selection for 3KW Solar power System

[dhawal]

Hi ,

i want to select a VRLA battery for 3KW solar power system. kindly guide me how do i calculate the best suitable battery (volts and amp) for my system. the details of Module is as given below

10 Solar panel of 300W each = 10 x 300 = 3 KW
Max Power Voltage of each panel = 36.8 V
Max Power Current of each panel = 8.20amp
module efficiency = 15.2%

with 2 days anatomy.

please help me with the calculations.

[Sunking]

Cannot be answered. You are putting the cart in front of the horse. You have not stated your daily watt hour specification and location.

At 3000 watts I have to assume you will use a MPPT charge controller as it would be very foolish not to. At 3000 watt panels means you have to use 48 volt battery. So based on 48 volts the minimum size FLA battery is 500 AH, and the maximum is 750 AH. Put another way minimum weight of 1400 pounds and maximum weight of 2200 pounds.

However I will warn you th eway you are going about this in all likely hood will fail. Systems are designed on how much power you use in a day. Not just buy what you hope might work. The cost of the batteries is going to get deep in your pockets of anywhere from $4000 up to $6500. Here is what should alarm you. YOu have no idea if that will do what you want. If you are like most you have grossly underestimated your usage and the system will be way undersized. If that happens you will destroy your batteries in short period of time. You will really regret it if you come to find out you need 6000 watt of panels and a 1500 AH 48 volt battery.

[Naptown]

battery selection will depend on a number of factors not only panel size.
What is the daily load on them assuming this is for an off grid application.
second if off grid what is the daily insolation where the system be installed for the worst or lowest month of the year.
What will the battery system voltage be
What type of charge controller are you planning to use
You will probably not like the answer as for that size array it will be big and expensive

[T1 Terry]

Hi dhawal,
As other have said, you location and ambient temps through the yr are important in your selection of a suitable battery pack in regards to both type and system voltage. As an example, panels with a Vmpp of 36.8v connected in parallel would suit a 24v system using an MPPT controller if you were in a cool climate because the Vmpp you quoted would be an STC figure, the NOCT figure is the all important one if you are in a warm climate.
If the panel temps are likely to climb into the high 40's or 50's C then that Vmpp figure of 36.8v will drop to a little over 30v, not enough to cover controller losses and cabling losses and still charge a 24v battery pack sufficiently, the end of boost for a VRLA 24v nom battery is around 28.8v.
Even series parallel to give an STC of 73.6Vmpp would drop to around an NOCT of 66Vmpp or less, cutting it close when the end of boost for a 48v nom VRLA is 57.6v but at 48v the cable losses would be less so you would get away with it at a scrape.
Now if you lived in the cold country where the chill factor was likely to keep the panels at 25C or lower even though they are in full sun then a 24v system would work fine.

Next step is calculating the required storage capacity to give you 2 days energy requirement in hand, you would need to know just how much you need per 24 hr. Remember, from your solar you need to supply all your needs for 24 hrs from the min number of peak sun hrs you will see in a year plus enough remaining to recharge the battery pack so that 2 days in hand can be recovered if and when you use it.
if you have the $$ to be considering setting up a system like you are suggesting I'd personally forget about lead acid anything and go for lithium ferrous. Build your own pack and you will be $$ ahead of a lead acid pack and have a far more flexible battery pack.

T1 Terry

Hi ,

i want to select a VRLA battery for 3KW solar power system. kindly guide me how do i calculate the best suitable battery (volts and amp) for my system. the details of Module is as given below

10 Solar panel of 300W each = 10 x 300 = 3 KW
Max Power Voltage of each panel = 36.8 V
Max Power Current of each panel = 8.20amp
module efficiency = 15.2%

with 2 days anatomy.

please help me with the calculations.

[dhawal]

Cannot be answered. You are putting the cart in front of the horse. You have not stated your daily watt hour specification and location.

At 3000 watts I have to assume you will use a MPPT charge controller as it would be very foolish not to. At 3000 watt panels means you have to use 48 volt battery. So based on 48 volts the minimum size FLA battery is 500 AH, and the maximum is 750 AH. Put another way minimum weight of 1400 pounds and maximum weight of 2200 pounds.

However I will warn you th eway you are going about this in all likely hood will fail. Systems are designed on how much power you use in a day. Not just buy what you hope might work. The cost of the batteries is going to get deep in your pockets of anywhere from $4000 up to $6500. Here is what should alarm you. YOu have no idea if that will do what you want. If you are like most you have grossly underestimated your usage and the system will be way undersized. If that happens you will destroy your batteries in short period of time. You will really regret it if you come to find out you need 6000 watt of panels and a 1500 AH 48 volt battery.

Thanks Sunking for the reply,

my daily watt hour is coming to 36000wh.

please help me on how you have arrived at "48 Volts" suitable for 3000w system.

regards

[dhawal]

battery selection will depend on a number of factors not only panel size.

What is the daily load on them assuming this is for an off grid application. max. 36000wh is my daily requirement
second if off grid what is the daily insolation where the system be installed for the worst or lowest month of the year. 2 days anatomy
What will the battery system voltage be: Thats what i want to find out. How to arrive at voltage for battery.
What type of charge controller are you planning to use. : MPPT of 3 KW
You will probably not like the answer as for that size array it will be big and expensive[/QUOTE]

hey , Thanks for your revert,

have replied your queries in Bold. i wana know the calculation on how to arrive on accurate battery voltage.

Thanks and regards

[Naptown]

You are not going to like this answer but here goes
Minimum battery voltage will be 48 Higher if possible
Minimum battery size will be 3750 AH of batteries @48V
Assuming you get 4 sun hours at the worst time of year (you have not given a location that was asked for)
this will take 13500W of PV panels. and 5 80A Mppt charge controllers.
You missed your system size by a factor of 4

[rreinha]

battery selection will depend on a number of factors not only panel size.

What is the daily load on them assuming this is for an off grid application. max. 36000wh is my daily requirement

With a daily use 36kWh?
If you lived in So Calif. your lowest production month would be Dec with only 9.5kWh Highest would be in Aug with 14kWh. you are already under sized without a battery.
This site will help with solar sizing:

http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/

there is a version 2 link on this site also

[russ]

For his location - annual average of 5.3 kW/m2/day

"Station Identification"
"City:","Bombay"
"State:","IND"
"Lat (deg N):", 19.12
"Long (deg W):", 72.85
"Elev (m): ", 14
"Weather Data:","IWEC"

"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 0.8 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 19.1"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 1.8 rupee/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (rupee)"
1, 5.43, 114, 2.10
2, 6.10, 116, 2.13
3, 6.62, 137, 2.52
4, 6.84, 136, 2.50
5, 6.20, 130, 2.39
6, 4.52, 92, 1.69
7, 3.57, 75, 1.38
8, 3.71, 79, 1.45
9, 4.20, 86, 1.58
10, 5.67, 114, 2.10
11, 5.64, 112, 2.06
12, 5.22, 110, 2.02
"Year", 5.30, 1300, 23.92

[Sunking]

36000wh is my daily requirement
2 days anatomy
Thats what i want to find out. How to arrive at voltage for battery.
MPPT of 3 KW

I warned you about putting the cart in front of the horse, now you just drove the cart over the cliff. For your location here is what you need. I hope you are a multimillionaire or billionaire:

Panel Wattage = 14,500 watts
48 Volt Battery Capacity = 3000 AH = 4500 Kg of lead.

No sense in going any further because you will never build the thing. If you already have the 3000 watt of solar panels all you can make is about 8 Kwh/day way short of your target of 36 Kwh/day. That is what happens when you put the cart in front of the horse and hope it will work. You would need a 60 amp MPPT charge controller, and a 48 volt battery with 680 AH capacity.

I think we are done here.

[Sunking]

please help me on how you have arrived at "48 Volts" suitable for 3000w system.

Because it is 3000 watts. The largest MPPT charge controller you can buy is 80 amps. The maximum power input is based on battery voltages as follows:

12 volts @ 1000 watts
24 volts @ 2000 watts
48 volts @ 4000 watts.
60 volts @ 5000 watts

That only leaves you with two choices of 48 and 60 volt battery, which is now a moot point because to generate 36 Kwh/day you would need a panel wattage of 14,500. That would mean you need a minimum of 3 80 amp MPPT controllers operating at 60 volt battery. Those 80 amp MPPT charge controllers will cost you about $900 USD each and you would need 3 of them. No need for me to post on this thread any more because this is never going to happen unless you happen to be the son of Shri Pranab Mukherjee.

[axis11]

At an electricity price of 0.034$/kwh , going off-grid is a no win idea.

[Sunking]

At an electricity price of 0.034$/kwh , going off-grid is a no win idea.

That is not what He wants to hear or know about. He wants to pay $3.00/Kwh so he feels better about himself.

[russ]

The 1.8 Rs cost is totally wrong - don't know where they got that figure.