Question about Mixing Panels


11 year old grid-tied system

2 paralleled mppt inverters: Xantrex STRX2500, Xantrex STRX1500

STRX2500 has a total of 20 AP-100 100W Astropower panels. 5 sets of 4 panels in series, then paralled into the inverter
STRX1500 has same AP-100 panels, total of 12 panels, 3 sets paralled into inverter.

AP-100 monocrystalline Panel Specs:

Vmp 16.1 V
Imp 6.2 A
Voc 20.1 V
Isc 7.2 A
Max. Series Fuse Amps 15

I have access to 8 AP-130 Astropower 130W monocrystalline panels and would like to use them in current system:

Vmp 18.9 V
Imp 6.9 A
Voc 24.6 V
Isc 7.6 A
Series Fuse: 15 Amps
Bypass Diode: 8 Amps


1) Can I use these 8 extra panels? 1 set of 4 panels in series into each inverter?

2) Can you explain the "within 10%" rules for mixing panels?

3) I understand that there may be some inefficiencies introduced by mixing these panels. What might I expect?

4) Do I only have to worry about inefficiency issues or could the panels be damaged if not within this "10% rule?"

Additional piece of information:

After 11 years, the mppt voltage on the inverter display appears to be decreasing....mppt voltage is now around 58V,
it used to be much higher. I assume this is do to losing ~1% of power output per year?


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Question about Mixing Panels

    The mixing panels rule-of-thumb is this: for parallel connections the Vmp of the panels should be within 10% of each other. For serial connections the Imp of the panels should be within 10% of each other.
    For the two panels given: Vmp 16.1 * 1.10 = 17.71 < 18.9 (no good for parallel connections). Imp 6.2 * 1.10 = 6.82 < 6.9 (no good for serial connections).
    The further apart the difference, the more power will be lost as the "lower" panel pulls the "higher" one down. It won't damage them unless you have a combination without fuses where the higher current panels can force power back through the lower ones under certain conditions.

    The idea that panels automatically lose 1% of their power per year has proven to be a myth. They can, however, lose power over time or more likely suffer damage/corrosion resulting in a drop or cessation of output. You can test them individually to detect any that may have failed, but it means disconnecting everything.
  • balee123balee123 Solar Expert Posts: 86 ✭✭
    Re: Question about Mixing Panels

    Thanks for your expertise. Much appreciated.
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