Battery And Solar Panel Capacity

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Assuming 60/40 distribution i.e. 60 percent of the power is consumed in morning and 40% of the power is consumed at night.

At 620 units for 2 months, it would be 10 units per day. 6 units in the day and 4 units in the night.

1 unit of power is equal to 1000 watt-hours (or) 1 Kw-Hr . It means consuming 1000 watts in 1 hr (or) consuming 500 watts per hour for 2 hrs and so forth.

6000 watt-hours in 12 hrs and 4000 watt-hours at night (12 hrs)/

6000/12 = 500 watts per hour would be the needed minimum generation.

To be able to power devices at night...also assuming 9hrs of sunlight per day.

10000/9 = 1100 watts per hour need to be generated.

To generate 1100 watts of single phase AC power , 880 (1100*0.80) watts of DC power per hour need to be generated. With 0.80 being the efficiency of the inverter accounting for some losses among the lines.

To go 20% more than capacity approximately solar panels need to be of 1KVA capacity. This would mean 4 mono crystalline panels of 250 Watts each (or) 8 poly crystalline panels of 125 watts each. Keep in mind poly crystalline panels are less efficient , so they can produce power only till 4:30 p.m or so.

Lets do some reverse calculations now,

Say the battery pack is 1 KVA,

Under no load, After 1 hr the battery will be charged to 1 KVA (100%). During the day the power gets consumed at 500 watts per hour. The power gets produced at 1000 watts per hour. So the battery gets charged at 500 watts per hour.At this rate the battery gets full in just 2 hrs...not ideal.

Taking 3 KVA,
The battery would take 6  hrs to fully charge it self...ideal. There will be at least 6 hours of sunlight on any given day. This would mean that the battery needs to be charged before connecting it to the solar system for the first time only.

At night the battery discharges at the rate of 500 watts per hour. In 10 hours it would discharge 5000 watts. In order to get the maximum life of the deep cycle battery its stored capacity is not to fall below 50% . If it does, it is counted as 1 battery life cycle used.

In Summer, the sun (in Tamil Nadu) lasts at least till 6:30 p.m. So the battery will supply from 6:30 p.m to 9:30 p.m in summer. Then battery will switch to grid from solar after 9:30 p.m.

This is where some life style changes like turning on the fans in all bedrooms at 5:00 p.m to cool of, not using water heaters after 5:00 p.m etc..are necessary to reduce battery usage. If not, then simply switch to grid power...Life is fun after all.

To have a maintenance free system that lasts 2-4 years, it requires 3 KVA battery. But this is very expensive and would take up 70% of the total cost of the solar system.

A good set of deep cycle batteries, connected in parallel (daisy chained) would cost as much as Rs 50,000. This is the biggest drawback of the solar system.

This site shows a good energy calculation.

http://www.vmaxchargetank.com/

Note here that if the battery is used to full 100% every day it will last only 300 cycles i.e. less than 1 year. If it is discharged to 50% then it will last 2.5 years.