Help please!

yiannis5 · January 2017

I live in Greece off grid and currently my setup is:

24v 450ah battery bank
4 x 235w 24v solarworld poly panels = 940 watts

Can please someone let me know if the current panels give enough power to my power bank? I would like to add 2 more panels but I cannot find the same make and wattage. Shall I go for higher wattage (lets say 250w) of the same brand or find the same wattage from another brand?

Many thanks in advance!!!

Estragon · January 2017

Are your panels getting the batteries fully charged at least once or twice per week? Are you able to get at least to absorb most days? Do you have a generator? Depending on loads, location, etc. your panels might be okay or not.

Assuming you have a MPPT charge controller capable of handling the extra current, you will want to add panels close in Vmp to your existing ones. Brand isn't all that important. Panel specs are.

mcgivor · January 2017

yiannis5 said:

I live in Greece off grid and currently my setup is:

24v 450ah battery bank
4 x 235w 24v solarworld poly panels = 940 watts

Can please someone let me know if the current panels give enough power to my power bank? I would like to add 2 more panels but I cannot find the same make and wattage. Shall I go for higher wattage (lets say 250w) of the same brand or find the same wattage from another brand?

Many thanks in advance!!!

Firstly, what type of charge controller are you using? This is important because the panels may not be compatible. Answer this question first.

yiannis5 · January 2017

mcgivor said:

yiannis5 said:

I live in Greece off grid and currently my setup is:

24v 450ah battery bank
4 x 235w 24v solarworld poly panels = 940 watts

Can please someone let me know if the current panels give enough power to my power bank? I would like to add 2 more panels but I cannot find the same make and wattage. Shall I go for higher wattage (lets say 250w) of the same brand or find the same wattage from another brand?

Many thanks in advance!!!
Firstly, what type of charge controller are you using? This is important because the panels may not be compatible. Answer this question first.

Hello, thanks I have the Outback Flexmax 60 mppt controller and my panels are solarworld sw235 poly

yiannis5 · January 2017

Estragon said:

Are your panels getting the batteries fully charged at least once or twice per week? Are you able to get at least to absorb most days? Do you have a generator? Depending on loads, location, etc. your panels might be okay or not.

Assuming you have a MPPT charge controller capable of handling the extra current, you will want to add panels close in Vmp to your existing ones. Brand isn't all that important. Panel specs are.

Yes, the batteries are fully charged on a daily basis in the summer and twice per week in the worst winter months. So if I wish to add 2 more panels you suggest to find the ones with the closest Vmp value of the current ones?

BB. · January 2017

SW 235
Maximum power Pmax 235 Wp
Open circuit voltage Voc 37.0 V
Maximum power point voltage Vmpp 30.0 V
Short circuit current Isc 8.35 A
Maximum power point current Impp 7.85 A

OK... You have a 24 volt battery bank. I hope you have the array setup as 2x series by 2x parallel strings?

The normal range for charging your battery bank:

450 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 847 watt array minimum
450 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,695 watt array nominal
450 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 2,203 watt array "cost effective" maximum

So, at 940 Watts, your array is about the smallest I would recommend (summer/sunny weather use, weekend use, etc.). If you are full time off grid and using a fair amount of power, the 10%+ rate of charge is really nice.

Vmp~30 volts is common. Get any other panels (mono or poly crystalline, any good brand, "60 cell" panels) that you can put 2 in series (Vmp-array~60 volts), and you will be fine.

With 3 or more parallel strings, you should have a fuse/circuit breaker per string to reduce the chances of fire if one of the strings fails (broken panel, shorted wiring).

-Bill

yiannis5 · January 2017

BB. said:

SW 235
Maximum power Pmax 235 Wp
Open circuit voltage Voc 37.0 V
Maximum power point voltage Vmpp 30.0 V
Short circuit current Isc 8.35 A
Maximum power point current Impp 7.85 A

OK... You have a 24 volt battery bank. I hope you have the array setup as 2x series by 2x parallel strings?

The normal range for charging your battery bank:
450 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 847 watt array minimum
450 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,695 watt array nominal
450 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 2,203 watt array "cost effective" maximum
So, at 940 Watts, your array is about the smallest I would recommend (summer/sunny weather use, weekend use, etc.). If you are full time off grid and using a fair amount of power, the 10%+ rate of charge is really nice.

Vmp~30 volts is common. Get any other panels (mono or poly crystalline, any good brand, "60 cell" panels) that you can put 2 in series (Vmp-array~60 volts), and you will be fine.

With 3 or more parallel strings, you should have a fuse/circuit breaker per string to reduce the chances of fire if one of the strings fails (broken panel, shorted wiring).

-Bill

Thanks Bill. The battery array is setup asyou suggested. Regarding the Outback Flexmax 60, will it be happy handling the extra panels - I cannot find 235 watt ones in Greece only 250 watts.

BB. · January 2017

As long as the parallel voltages are similar (within 5% good, within 10% will work), you can mix any wattage panels.

Roughly, the maximum array for a single 60 Amp MPPT controller:

60 amps * 29 volts charging * 1/0.77 panel+controller derating = 2,260 Watt array max "cost effective" for 60 amp MPPT controller @ 24 volt battery bank

Keep the charge controller in a cool/well ventilated location and shaded from direct sunlight. Keeping electronics cool is always a good thing. The Outback fans tend to be noisy--As you add panels, the fans will run longer (and faster?) in the middle of the day.

-Bill

yiannis5 · January 2017

BB. said:

As long as the parallel voltages are similar (within 5% good, within 10% will work), you can mix any wattage panels.

Roughly, the maximum array for a single 60 Amp MPPT controller:
60 amps * 29 volts charging * 1/0.77 panel+controller derating = 2,260 Watt array max "cost effective" for 60 amp MPPT controller @ 24 volt battery bank
Keep the charge controller in a cool/well ventilated location and shaded from direct sunlight. Keeping electronics cool is always a good thing. The Outback fans tend to be noisy--As you add panels, the fans will run longer (and faster?) in the middle of the day.

-Bill

Many thanks, so by looking at the outback spec sheet the NEC recommended maximum array wattage of 1500w is not the 'real' ceiling?

BB. · January 2017

Generally, solar arrays (plus charge controller losses) output only (on average) a typical maximum power of ~77% of the array's rating. Vmp falls with hot solar panel temperatures--So in hot climates, the derating is more "real". In very cold climates/sub freezing, the Vmp rises, and can output more power through the MPPT controller (power = Voltage * Current, rising Vpanel, rising power).

Good quality MPPT controllers will limit their output current to the rated amount (60 amps), even if more power is available. No damage, just normal operation.

For most people, the charge controller will "clip" or limit output only a little bit on cool/clear days. Not usually enough to worry about.

Of course, if you want, you can always purchase another charge controller (or run the battery bank at 48 VDC).

1,500 Watts / 60 amps = 25.0 volts...

That is what is usually done (use derated solar array by 77%). If the manual says 1,500 Watts, then you will not go wrong either.

-Bill

Estragon · January 2017

Just to expand a bit on Bill's good advice - as you get close to the amp output limit of a charge controller it may make sense to consider a second controller for two reasons; first because working a single controller near max generates more heat, which can shorten electronics life, and second because adding a second controller makes life a lot easier when (not if) one controller dies.

It sounds like your batteries are fairly happy with the setup you have though, given your location and loads.

Help please!

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