How to add panels to a system when existing panel no longer made

PondWater Registered Users Posts: 9 ✭✭
Hi all.

I'm looking to add some panels to an existing off-grid solar system. As you'll see I'm a novice with this stuff.

  • Currently there are 2 REC TwinPeak REC280TP BLK Solar Panel - 280 Watts, in series.
  • 24v rolls battery bank is charged with a Tracer 3210AN series MPPT controller.
  • The Tracer 3210 has a tag that says "Max. PV input power: 390W (12v), 780W(24v).
More details of the system and it's issues are talked about in this previous thread.

The area gets shade, and these panels supposedly work better than others in that regard.
I've heard that it is best to 'match' panels when replacing or installing new panels to the system.
Alas these REC panels are no longer made.

So what are my options? They do make similar REC panels in a higher wattage.
  • I assume I'll have to add panels in increments of 2— adding a parallel strand of 2 in series to the existing 2 in series?
  • Can I add 2 panels of different wattage (REC makes a 360 watt in what I think is the same config: REC360TP2SM72
  • If so, how should they be linked in (as far as series/parallel config of the 2 different types)?
  • OR, should I match wattage (280) from another manufacture? Any tips on what specs/specifics I should be looking for to match to the existing RECs?
  • It looks to me like the MPPT controller cannot take another 560 watts of panel input? It's a bit confusing because other independent current and voltage specs appear to be OK: e.g. Short Circuit Current (ISC) is 9.44A for 1 panel. Double if two parallel strings (?) which is under 20A. MPPT specs 30A, and under 92 V, which is also true with two series panels at 39.2 open. So what am I missing? 
  • If the MPPT is indeed not capable of handling another 2 panels, where do I go (different MPPT or a second one?)
I got some great feedback on my last post. Thanks to all who contribute here. 


  • PondWater
    PondWater Registered Users Posts: 9 ✭✭

    I found a version of the MPPT manual that clarifies the maximum recommended solar array wattage as 1170W.

    So with 280 watts x2 (STC, can I use PTC maybe?) and another 280 x 2 I'm at 1120 w. If I added the REC 360 panel, that puts me at 1280 (over the recommended). If I use PTC (257 and 332) it adds to 1178 w. Which is pretty darn close.

    If I were to wire the new 360w panels so that each one becomes in series with a 280, it sounds like the voltage of the smaller 280 panel will limit that of the 360w panel? Is that correct. Which would reduce the performance of the 360 panels, but would derate the effective array wattage as far as the MPPT is concerned.
  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,044 ✭✭✭✭✭
    Assuming the original 280 watt panels are a 60 cell format you want a similar wattage 60 cell panel. if 72 cells then the same logic applies. Look at Santan solar for quality used panels that may match your needs.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    You need to "match" solar panels in series--Their current to within 10% or better (i.e., 10 vs 11 amps Imp).

    For solar panels in parallel, you need to match Vmp-string (or Vmp-array) within 10% (.e., 30 volts vs 33 volts Vmp).

    Good quality MPPT controllers will take a "larger wattage" array just fine... At noon on a  cool clear day, the MPPT controller will take only the power (wattage) needed to supply the maximum output rated current of 32 amps @ battery voltage...

    And PTC is simply the solar panel/array derated to actual Vmp-hot voltage under normal sun and temperature conditions... We typically use 75% or 77% derating of panel power for normal warm to hot conditions... Using the 75/77% derating factors--Your controller will rarely "clip" output charging current (high noon/cool clear day/discharged battery bank)--i.e., Not that many hours a year, and clipping current by a little bit... (less than 10%?).

    The "optimum" over paneling of 32 amp MPPT controller charging a 24 volt battery bank would be around:
    • 32 amps * 29.5 volts charging * 1/0.77 solar panel+controller deratings = 1,226 Watt array "optimum cost effective"
    You should be able to "go over" the MPPT controller "max panel wattage" rating... But your controller will "clip" the solar array available power to max rated charging current/voltage to the battery bank.

    Over paneling MPPT controllers is common. But if you need to support a larger array, then either getting a new/higher rated output current controller or using 2 or more controllers in parallel charging the same battery bank are the two solutions. Either solution (new controller, or 2 or more controllers in parallel) is fine. And the two controllers do not have to "match"--They can be different size, brand, models. Each will charge the battery bank as it is programmed.

    The details matter. What are the Vmp/Imp ratings for the various panels? Yes you can "mix" different panels on the same array--But there are rules to follow and sometimes, if the specifications are too far apart (i.e., 10% or less miss-match of Imp for series, and Vmp for parallel wired strings), one cannot make an "optimal" match...

    I am not "big" on picking panels with "battery shade" performance... From what I have seen, running a solar panel in shade will, at best, give you about 5% of its normal direct sun output power (for me that was what I saw as blue sky back scatter to my array when shaded by trees towards the end of day)... If you get a "better shade tolerant" panel, maybe 6% harvest instead?

    Shade is a "killer" for solar electric power. Even a little bit of shading on a large solar array (such as utility power line shadows on array) can kill upwards of 50% of harvest until the shade moves off panels (as sun arcs across the sky).

    For a basic first assumption, if your array is "in partial or full shade" for 1/2 the day, then assume the system will only produce 1/2 the expected energy during that day. And as the sun moves across the sky between summer and winter, so will your shading move/charge too.

    For most people with shading issues, as winter approaches, shading from trees/mountains/etc. becomes worse. If you have the choice--Installing the array 100 to 100's of feet away from your cabins is usually the only practical solution. And installing arrays farther away requires larger copper cables (to keep voltage drop down) and usually a higher voltage array (high voltage => lower current and lower voltage drop). But that usually requires purchasing MPPT solar charge controllers with higher input voltage ratings (i.e., a 150 Vpanel-max rating vs 100 Vpanel-max rating... And there are some that have 600 Vpanel-max input rating--All of which usually cost more money).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PondWater
    PondWater Registered Users Posts: 9 ✭✭
    Circling back around to this...

    I'm finding it difficult to find solar suppliers that sell small quantities direct to consumers.
    I am especially having trouble with the half cut cells and REC brand.

    Aside from the voltage and current considerations, do you think I should stick with half cut cell panels since that is what currently exists?

    Any tips on where to shop online for small quantities direct to consumer, ideally with REC panels or some other half cell with acceptable specs?
  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,044 ✭✭✭✭✭
    Look on the host's website They are one of the best places for all quality solar gear.  
    NAZ Solar Electric | Off Grid & Grid-Tied Solar Power Systems (

    You might also look at Santan Solar for, used and sometimes, hard to find panels
    Solar Panels | SanTan Solar

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.