# How many solar panels

Registered Users Posts: 2
How many 24v 250w poly-crystalline solar panels are required along with 48v 20A MPPT for charging a 48v lead acid battery pack ? please help....
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• Solar Expert Posts: 3,741 ✭✭✭✭
Welcome to the forum,

The answer depends on the size (amphours) of the battery.  The bigger the battery, the more solar panels you need to charge it.

When designing a battery based solar power system, you must have design criteria.  In particular you must have a good description of the load.  After all, the purpose of the system is to power the load.

Once you describe the load, the next step is to pick a battery.  This is the most difficult part of the decision making.

Finally, after you pick a battery, you figure out how to charge the battery... solar, hydro, generator, wind, etc.

Tell us more about the purpose and location of the system.  Location is often an important factor in design... will the batteries freeze? will they overheat? How much sun in the winter?

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
I believe you are in the Deoghar, JH, India area?

For 20 amp MPPT charge controller, the largest "cost effective" solar array would be around:
• 20 amps * 59 volts charging 1/0.77 panel+controller efficiency = 1,532 Watt array "cost effective" maximum
Next, the battery bank--We suggest 5% to 13% rate of charge--For a weekend/emergency/seasonal system, 5% can work OK. If daily use, look at 10% or greater rate of charge. With a 20 Amp charge controller:
• 20 amps * 1/0.05 rate of charge = 400 AH @ 48 volts largest suggested battery bank
• 20 amps * 1/0.10 rate of charge = 200 AH @ 48 volts nominal bank
• 20 amps * 1/0.13 rate of charge = 154 AH @ 48 volts smallest battery bank
Now, you need to look at the Vmp of the solar panel... For a 48 volt lead acid battery bank, you need ~72 volt minimum solar array Vmp voltage for proper charging. A typical 250 Watt solar array, they tend to run around Vmp~30 volts. So you would need a minimum of 3x panels in series (Vmp-array ~ 90 volts). And your array would be:
• 1,532 Watt array / 250 Watts per panel = 6.1 panels ~ 6 panels
So, you would need two strings of 3 panels in series, then connect those two strings in parallel.

Note--You need to read the specifications of the MPPT charge controller, they have both a minimum and maximum Vmp-array voltages. In the US, our "higher end" MPPT controllers typically have a working Vmp-array maximum of around 100 VDC (Vmp rises as temperatures fall, so cold climates you have to "adjust" the maximum Voc voltage open circuit up) for a MPPT controller with a Voc input rating of ~140-150 VDC maximum input voltage.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 2
edit : IF I use 150w 12v polycrystalline panels and size my battery bank to 48v 42Ah, then what will be the case ?