# calculation of solar system

Registered Users Posts: 118✭✭
Hi Informing for a friend

Calculate he is using about 38000 watth hour day with aircon included.

How can I calculate how many and what wattage panels to use and batteries .

Think a 10k kva split info is ok but need how to calculate about panels and batteries.

Bob

edited September 2023 #2
Same island off Venezuela coast? (Curacao, Willemstad)
Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 118✭✭
Yes Bill yes same island think 6 hours per day good sunlight Can you give me an idea about what needed.
appreciate
Bob
My first comment is to look at conservation... Is the home as energy efficient as practical? Lots of insulation, using energy efficient mini-split(s) (heat pump for heating and cooling if needed). It is almost always cheaper to conserve energy than to generate it.

This is just a generic set of calculations based on our rules of thumbs using Lead Acid Batteries... At this point, especially for tropical locations, it may make sense to use a slightly smaller LiFePO4 set of batteries... But that is for another post.

Sizing the battery bank. The typical design is for 2 days of "no sun" (bad weather) and 50% planned discharge (for longer battery life--And it does give a 3rd day of backup power if needed):
• 38,000 WH per day * 1/0.85 AC inverter eff * 2 days of storage * 1/0.50 max planned discharge * 1/48 volt battery bank = 3,725 AH battery bank @ 48 volts (that is not a small battery bank)
Next, the amount of solar panels. Two calculations, one based on minimum rate of charge and second based on hours of sun per day and daily loads (by season).

For an off grid power system, 5% rate of charge (based on battery bank capacity) can be used for emergency/backup power. 10% to 13% is typically the minimum suggested for full time off grid power. Can go to 20-25% rate of charge for areas that may have poor sun or poor winter sun...
• 3,725 AH battery bank * 58 volts charging * 1/0.77 solar panel+controller deratings * 0.05 rate of charge = 14,029 Watt array minimum
• 3,725 AH battery bank * 58 volts charging * 1/0.77 solar panel+controller deratings * 0.10 rate of charge = 28,058 Watt array nominal
• 3,725 AH battery bank * 58 volts charging * 1/0.77 solar panel+controller deratings * 0.13 rate of charge = 36,476 Watt array typical cost effective maximum
And there is sizing based on the daily loads and hours of sun per day. Assuming fixed array, tilted to latitude of 12 degrees, facing south (south of equator) for Curacao, Willemstad:
https://pvwatts.nrel.gov/pvwatts.php

# RESULTS

Print Results

( kWh / m2 / day )
AC Energy
( kWh per month for a 28,000 Watt array )
January5.832,372
February6.082,223
March6.192,489
April5.892,288
May5.832,316
June5.842,269
July5.882,352
August6.262,508
September6.332,443
October5.782,311
November5.462,125
December5.482,200
Annual5.9027,896

Sizing array for 38,000 WH per day, and "worst" month of November (5.46 Hours of sun per day):
• 38,00 WH per day * 1/0.52 off grid AC system eff * 1/5.46 hours of sun per day (November break even) = 13,384 Watt array Nov Break even
And typically, for off grid solar, should plan for using 50% to 65% of daily predicted harvest (some clouds come by, more AC power needed for tools, hot weather A/C, etc.):
• 13,384 Watt array / 0.65 solar fudge factor = 20,591 Watt array suggested for minimum genset usage
Looking at somewhere around 28,058 Watt array (10% minimum rate of charge for battery bank) or even less (20,591 Watt array) because this is a very sunny region.

That is the basics... Could probably get away with a smaller Li Ion battery bank (maybe 60% of Lead Acid?) and less than 20,591 Watt array (Li Ion is more efficient and much better charge acceptance vs Lead Acid).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset