HOW TO DESIGN OFF GRID SOLAR SYSTEM WITH 4500WATTHOUR daily load

JAYMIN

hello dear friends

kindly help me the design of off grid solar system with 4500 watt hour daily load.
Regards

Jaymin Bavishiya

vtmaps

JAYMIN said:

kindly help me the design of off grid solar system with 4500 watt hour daily load.

Three questions... How much of that daily load is when the sun is not shining?  How many cloudy days do you want the system to operate before you run a generator?   What is the most number of watts that you will draw at one time?

--vtMaps

Dave Angelini

Also, how many people?
Does the system need to grow later?
How do you get water?
Do you need to cool the living space?

JAYMIN

dear sir i have a daily load 4500kwh , days of autonomy : 2 days , no i do not want to grow system in later or sooner , i am living in warm place average 5.5 sunshine hour/day in a year

BB.

A quick off grid system design, using our standard rules of thumbs as a starting point. Assume flooded cell lead acid batteries, 50% maximum discharge (for longer battery life):

• 4,500 WH per day * 1/0.85 AC inverter eff * 2 days of storage * 1/0.50 maximum discharge * 1/48 volt battery bank = 441 AH @ 48 volt battery bank

Note that you can almost get away with a 24 volt battery bank (~882 AH @ 24 volts--exactly the same amount of stored energy--Just change to series/parallel connections of battery bank) if your equipment choices are more limited at 48 volts (will require heavier copper wiring on DC side, probably two solar charge controllers @ 24 volts vs one controller @ 48 volts).

Next, size of solar array, two calculations needed. One based on size of battery bank. The second based on hours of sun per day and your loads.

First size of battery bank--5% to 13% rate of charge is typical recommendation. 5% can be OK for weekend/sunny season usage. 10%+ is recommended for full time off grid usage (9+ months a year):

• 441 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,690 Watt array minimum
  
• 441 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 3,379 Watt array nominal
  
• 441 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 4,393 Watt array "cost effective" maximum

And then based on your hours of sun per day... Assuming you are around Surat India:
http://solarelectricityhandbook.com/solar-irradiance.html

Surat
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 69° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
5.57	6.16	6.54	6.35	6.02	5.38
Jul	Aug	Sep	Oct	Nov	Dec
4.12	4.15	5.12	5.81	5.70	5.18

Toss bottom three months (assume use genset/less power), December has 5.18 hours of sun (on average) per day:

• 4,500 WH per day * 1/0.52 off grid system eff * 1/5.18 hours of sun per day = 1,671 Watt array minimum (break even for December)

Note that you should not plan on using 100% of your predicted solar power per day--Some days more, some days less... If you want to use 4,500 WH minimum per day, then you should assume 65-75% system derating:

• 1,671 Watt array * 1/0.75 derating = 2,228 Watt array  (75% of rated power)
  
• 1,671 Watt array * 1/0.65 derating = 2,571 Watt array (65% of rated power)
  

If this is a full time off grid system and significant power used during the day (loads share solar array power with battery charging), then I would humbly suggest that a 10% minimum array would be much more satisfactory (less day to day battery/load management) or 3,379 Watt array minimum. Such an array would produce for December (on average):

• 3,379 Watt array * 0.52 off grid system eff * 5.18 hours "average" December day = 9,102 Watt*Hours per day
  

Such a system will work very nicely with daytime or a mix of daytime/nighttime loads. And quickly recharge the battery bank after a few days of bad weather.

A 441 AH @ 48 volt battery bank will support a useful maximum of ~4.5 kWatt AC inverter nicely (technically 1kW per 100 AH @ 48 volts or 4.41 kW inverter) and a ~4,410 Watt array maximum (too much charging current for battery bank can cause overheating of battery bank).

The above is a rules of thumbs design for a full time off grid home/cabin. If your needs are different (say running an off grid business with nearly 100% of loads during the day)--Then may need to adjust array size (and possibly could use a smaller AH battery bank). If you have large (for example) well pump with high starting surge current--That could make other design changes.

-Bill

JAYMIN

BB. said:

A quick off grid system design, using our standard rules of thumbs as a starting point. Assume flooded cell lead acid batteries, 50% maximum discharge (for longer battery life):

• 4,500 WH per day * 1/0.85 AC inverter eff * 2 days of storage * 1/0.50 maximum discharge * 1/48 volt battery bank = 441 AH @ 48 volt battery bank

Note that you can almost get away with a 24 volt battery bank (~882 AH @ 24 volts--exactly the same amount of stored energy--Just change to series/parallel connections of battery bank) if your equipment choices are more limited at 48 volts (will require heavier copper wiring on DC side, probably two solar charge controllers @ 24 volts vs one controller @ 48 volts).

Next, size of solar array, two calculations needed. One based on size of battery bank. The second based on hours of sun per day and your loads.

First size of battery bank--5% to 13% rate of charge is typical recommendation. 5% can be OK for weekend/sunny season usage. 10%+ is recommended for full time off grid usage (9+ months a year):

• 441 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,690 Watt array minimum
  
• 441 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 3,379 Watt array nominal
  
• 441 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 4,393 Watt array "cost effective" maximum

And then based on your hours of sun per day... Assuming you are around Surat India:
http://solarelectricityhandbook.com/solar-irradiance.html

Surat
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 69° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
5.57	6.16	6.54	6.35	6.02	5.38
Jul	Aug	Sep	Oct	Nov	Dec
4.12	4.15	5.12	5.81	5.70	5.18

Toss bottom three months (assume use genset/less power), December has 5.18 hours of sun (on average) per day:

• 4,500 WH per day * 1/0.52 off grid system eff * 1/5.18 hours of sun per day = 1,671 Watt array minimum (break even for December)

Note that you should not plan on using 100% of your predicted solar power per day--Some days more, some days less... If you want to use 4,500 WH minimum per day, then you should assume 65-75% system derating:

• 1,671 Watt array * 1/0.75 derating = 2,228 Watt array  (75% of rated power)
  
• 1,671 Watt array * 1/0.65 derating = 2,571 Watt array (65% of rated power)
  

If this is a full time off grid system and significant power used during the day (loads share solar array power with battery charging), then I would humbly suggest that a 10% minimum array would be much more satisfactory (less day to day battery/load management) or 3,379 Watt array minimum. Such an array would produce for December (on average):

• 3,379 Watt array * 0.52 off grid system eff * 5.18 hours "average" December day = 9,102 Watt*Hours per day
  

Such a system will work very nicely with daytime or a mix of daytime/nighttime loads. And quickly recharge the battery bank after a few days of bad weather.

A 441 AH @ 48 volt battery bank will support a useful maximum of ~4.5 kWatt AC inverter nicely (technically 1kW per 100 AH @ 48 volts or 4.41 kW inverter) and a ~4,410 Watt array maximum (too much charging current for battery bank can cause overheating of battery bank).

The above is a rules of thumbs design for a full time off grid home/cabin. If your needs are different (say running an off grid business with nearly 100% of loads during the day)--Then may need to adjust array size (and possibly could use a smaller AH battery bank). If you have large (for example) well pump with high starting surge current--That could make other design changes.

-Bill

Thanks bill for reply and OFF GRID SYSTEM HAS EFFICIENCY ONLY 52% ! while on grid system has efficiency of 80%.

BB.

The efficiencies (roughly) work out as:

81% of rated actual solar output power at nominal operating temperatures (typical summer day--Vmp falls as solar cells get hot)
95% for charge controller
80% typical for flooded cell battery
90% typical for AGM/Sealed battery
85% typical for DC to AC inverter

• 0.81 panel derating * 0.95 GT inverter = 0.77 typical efficiency (from solar panel STD ratings to useful AC power)
• 0.81 panel derating * 0.95 charge controller eff * 0.80 flooded cell batt * 0.85 AC inverter = 0.52 typical end to end off grid AC efficiency

Yep--That is the "conservative worst case" system efficiency. For flooded cell battery systems, how you operate them can affect battery efficiency (80% for older batteries, charging at >80% state of charge, etc.). You can get >90% eff for flooded cell and >98% for AGM with careful battery charging cycles (tends to be more efficient with deeper cycling, less efficient when shallow cycling).

-Bill