Mixing panel wattage

System

I know that the answer is in here some where, I just can't find it. Please explain it again.

I have 4 SolarWorld 175watt panels paired as 48vdc. I ordered 2 more, planning on changing to 72vdc. When they arrived, they were 165watt. NON-RETURNABLE!

Question: Is it best to hook them up 175+175+165 each string? Or would I have less of a loss of power if I went two strings ie: 175+175+175 and 175+165+165? System is FX3048, MX60, 8 golf batteries and a combiner box. If it's best, I could go with three strings ie: 175+175 175+175 165+165 and add another 15 amp breaker to the combiner box.
I live in Hawaii, 5 miles from the most active volcano in the world. Low temperature is not a problem for the MX60.

crewzer

Re: Mixing panel wattage

Where do you plan to install the system (~city and state)? A "72 V" array (three 24 V modules in series) would probably be a problem in a cold environment.

More later,
Jim / crewzer

mike95490

Re: Mixing panel wattage

What are you using for a charge controller, that's what the voltages need to match.

crewzer

Re: Mixing panel wattage

Cryobear,

The answer you’re looking for probably goes something like this:

Modules wired in series will operate at the lowest module current and at the sum of the module voltages. Modules (individually or in series strings) wired in parallel will operate at the lowest module (or string) voltage and at the sum of the module (or string) currents.

Module specs ( http://lib.store.yahoo.net/lib/wind-sun/SW-175-Mono.pdf ):

SW175
Voc: 44.4 V
Vmp: 35.8 V
Isc: 5.30 A
Imp: 4.89 A

SW165
Voc: 44.0 V
Vmp: 35.3 V
Isc: 5.30 A
Imp: 4.68 A

You have three practical options for your module inventory:

Option A, two series strings wired in parallel:

Series string 1: 175 + 175 +165
Series string 2: 175 + 175 +165

Array spec:
Vmp = 35.8 V + 35.8 V + 35.3 V = 106.9 V
Imp = 4.68 A x 2
Array STC Power = 106.9 V x 4.68 A x 2 = 1000.6 W


Option B, three series strings wired in parallel:

Series string 1: 175 + 175
Series string 2: 175 + 175
Series string 3: 165 + 165

Array spec:
Vmp = 35.3 V + 35.3 V = 70.6 V
Imp = 4.89 A + 4.89 A + 4.68 A = 14.46 A
Array STC Power = 70.6 V x 14.46 A = 1020.9 W


Option C, three series strings wired in parallel:

Series string 1: 175 + 175
Series string 2: 175 + 165
Series string 3: 175 + 165

Array spec:
Vmp = 35.8 V + 35.3 V = 71.1 V
Imp = 4.89 A + 4.68 A + 4.68 A = 13.85 A
Array STC Power = 71.1 V x 13.85 A = 984.7 W

Option B will deliver the highest power. I do not typically recommend “48 V” arrays for 48 V nominal flooded-cell battery systems. However, the combination of the relatively high array voltage (70.7 Vmp STC) plus the mild average daytime high temp (Honolulu = 89 F) may allow for an operational array voltage that’ll be high enough to charge and EQ the batteries.

If Option B’s summer operational Vmp is not high enough, you’ll have to consider Option A. Its winter Voc should be just below the MX60’s operational limit of ~141 V.

HTH,
Jim Goodnight
“crewzer”
Eastern Regional Sales Manager
OutBack Power Systems