In off grid systems the main issue experienced is regarding the battery back up recommended by supplier. This issue as well the life of the battery guaranteed by the supplier makes a lot of difference in costing of the solar power off grid systems.

Issue 1: Life of the battery

If the warranty is 1 year the cost is XXX , but if the warranty is 2 years the cost is XXX*1.5 and if the warranty is 4 years the cost is XXX*2.The customer needs to consider this factor seriously and be vigilant about the same while making a buy decision.

Issue 2: Capacity of the battery

The capacity of the battery depends upon the no of hours of back up required by you.

Example:

You are buying a 5 kw solar power system and you tell the supplier that you require a 4 hrs back up time. The supplier will do the following calculations.

1. Power factor : Assumption 0.8 pf then the capacity of inverter 6.25 KVA * 0.8=5KW, so the inverter to be used for a 5 KW system should be 6.25 KVA.
2. Current : 5KW will draw a current of 5/230=approx 22 amp, safer is to say 25 amps ,AC current.
3. Battery voltage : 24 V
4. Current from Battery : 25*230/24=240 amps , say 250 amp DC current
5. Back up required : 4 hrs
6. Battery capacity : 250 * 4 ( Current required X back up hrs required)= 1000 Ah provided full discharge is taken from the battery but since it is safer to discharge the battery up-to 50%( This is called d-o-d , depth of discharge factor) only so the battery capacity should be 1000/0.5( Ah required divided by D-O-D factor)=2000ah at 24 V
7. If you are buying 12 V 150 Ah batteries then buy 4000ah 12 v batteries.27 Batteries of 150 ah 12V.
8. Cost will be about 11500 each battery i.e 300000 or 60/w. This is with 4 Yrs warranty. For 2 yrs warranty the same will be about Rs. 37/w and for 1 yr warranty the same shall be about Rs 30/W.