Can I stack inverters with different wattage?

SystemSystem Posts: 2,511 admin
Hello,
I am thinking about alternative energy and I have questions. First of all, I see that I can get used inverters a lot cheaper than new and that is great. But finding two same wattage inverters are different. So can I stack different wattage inverters?

Secondly, I am curious about grid tie systems. How does current chooses where to come from? From the grid or from inverter? I saw system diagrams and inverters are connected straight to the grid system.

Also, how does inverters synchronise them self then stacked?

Thats all for now.

Best regards,
Robert

Comments

  • CariboocootCariboocoot Posts: 17,615Banned
    Re: Can I stack inverters with different wattage?

    Stacked inverters must be units designed for that application; you can't tie together any two inverters and get anything other than smoke, sparks, and fire. So two Outback 3524's, yes. One Samlex and one Magnum no. Likewise they should be of the same Wattage. In theory two different Watt but compatible units could be connected, but when the load goes up the smaller unit will be over-taxed in comparison to the larger one. Stack-compatible inverters synchronize by having one "Master" inverter from which the "Slave" inverters take their cue for frequency synchronization and power output. Mis-matched units would probably result in confused synch info. I don't know exactly because I don't build the things and I'm not going to try any such experiment! :p

    Don't know if I'm explaining this well.

    Grid-tie inverters function in a similar fashion, viewing the grid as a "stacked inverter".

    There are experts in this field here who can give you quite a detailed explanation, if you wish. :D
  • BB.BB. Posts: 24,674Super Moderators admin
    Re: Can I stack inverters with different wattage?

    I agree with Marc--For various reasons, you really should only "stack" identical brand/model of off-grid inverters.

    And there are two types of stacking--one where you can add power (2x 2000 watt inverters for 4,000 watts at 120 VAC) and another where you can take two 120 VAC inverters and get 120/240 VAC split phase. They are not the same.

    If you are asking about "stacking" Grid Tied inverters of different capacities--Yes you can do that. The inverters synchronize with the Grid / Utility Power--And they output power based on how much solar energy is available and each inverter's own limits.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dwhdwh Posts: 1,332Solar Expert
    Re: Can I stack inverters with different wattage?

    "Stacking" basically means tying the inverters together (as already described).

    If you are asking about "tapping" a battery bank with multiple inverters - then yes, you can certainly do that.

    An example would be to tap off the battery with say a 300w pure sine wave inverter to feed sensitive electronics, and also tapping off a 2000w modified square wave inverter to feed power tools. That's a relatively common thing to do.
  • VanDeusenVanDeusen Posts: 129Solar Expert ✭✭✭✭✭
    Re: Can I stack inverters with different wattage?

    ok i will bite.. WHY buy 2 seperate inverters to be stacked and lets say that both ate 2500 watt inverters to achieve a 5000 watt capacity. Why not just buy a 5000 watt inverter?
  • mike95490mike95490 Posts: 6,347Solar Expert ✭✭✭✭
    Re: Can I stack inverters with different wattage?
    VanDeusen wrote: »
    ok i will bite.. WHY buy 2 seperate inverters to be stacked and lets say that both ate 2500 watt inverters to achieve a 5000 watt capacity. Why not just buy a 5000 watt inverter?

    Because some stacking kits let inverter # 2 sleep till inverter #1 is at 75% capacity. That saves you the standby losses of 2 inverters, till the 2nd one is actually needed. Not burning 30 or 40 watts 24/7 becomes a big thing in winter (short sun)
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • VanDeusenVanDeusen Posts: 129Solar Expert ✭✭✭✭✭
    Re: Can I stack inverters with different wattage?
    mike90045 wrote: »
    Because some stacking kits let inverter # 2 sleep till inverter #1 is at 75% capacity. That saves you the standby losses of 2 inverters, till the 2nd one is actually needed. Not burning 30 or 40 watts 24/7 becomes a big thing in winter (short sun)


    GOOD answer, I also have ONE 2500 msw stackable inveter that just sits in my basement BRAND NEW and collecting dust. I bought it because at one time I was going to stack and thought why bother when I can just get a 5k. Now I know I will never use the thing and it will probably be listed for sale to get some of my 300 bucks back.
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: Can I stack inverters with different wattage?

    you may sell it here if you wish to, but leave the bulk of correspondence via pm. you should specify some things online like make, model, when bought, etc.
  • System2System2 Posts: 6,290 admin
    Re: Can I stack inverters with different wattage?

    Thanks for replays,

    You mentioned main and slave inverter, does this mean, that there is one main grid off inverter and one grid tie inverter? Also maybe there is place to read about inverter connections and everything. I am searching web but found only basic information.
  • mike95490mike95490 Posts: 6,347Solar Expert ✭✭✭✭
    Re: Can I stack inverters with different wattage?

    Stacking inverters is specific to each model. You can't mix 'n match, they have to be designed to work stacked with each other, and sometimes need a controller module added.

    Stacking for 240V , both inverters would be on all the time.

    Stacking in parallel @ 120V, for additional load capacity, one is master, and one is slave, to save standby losses, or both could be on 100%.

    Grid Tie does not need to stack, each inverter just feeds the grid, monitoring the grid health on its own.
    Battery based, the inverters communicate somehow with each other, either directly, or via a control module.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • BB.BB. Posts: 24,674Super Moderators admin
    Re: Can I stack inverters with different wattage?
    robertas9 wrote: »
    You mentioned main and slave inverter,

    In many cases, there is one inverter that is the Master (sets frequency and voltage levels) and it outputs a signal to the other inverters.

    The slaves are configured to follow this signal for proper frequency (and phase--you don't want the AC output to be at +170 volts and the other at =170 volts when they are tied together).
    does this mean, that there is one main grid off inverter and one grid tie inverter?

    This is another way to setup inverters... A grid tied inverter is always "slave" to the utility power. A GT inverter follows the AC voltage and outputs current to follow the AC voltage wave form, frequency, phase, etc.

    It does this without any special "master/slave" input control lines.

    For what you mentioned specifically--For some Off-Grid Sine Wave type inverters... You can simulate the "Grid" with the Off-Grid inverter and a GT inverter will behave exactly like it is on the grid and output current to the shared AC output of the off-grid inverter. Remember, the GT inverter output is only based on the amount of solar energy available from the array--Voltage and frequency is controlled by the Utility or Off-Grid inverter (in this example).

    Now, here is were it gets complicated and interesting... Say you have your Off-Grid inverter running along supplying 1,000 watts to your load. Now, connect your GT inverter with a 500 watt array (or partial sun) and the GT inverter will help power the load=--500 watts from the Off-Grid inverter and 500 watts from the GT inverter.

    Now, the sun comes out full, and there is 1,500 watts of solar power available to the GT inverter... It outputs 1,500 watts of current into the shared AC circuit. However, the load is only able to take 500 watts--and now (at least a few Off-grid sine wave inverters can) the extra 1,000 watts of energy will be "back fed" through the Off-Grid inverter and actually begin to charge the battery bank.

    Cool, huh? The issues:
    1. The Off-Grid inverter has not been tested and rated for this type of operation. While it may work fine--you are on your own if anything bad happens.
    2. There is no "battery charger/controller" and it is possible to overcharge the battery bank in this setup (damage/distroy battery bank, or worse).
    3. The Off-Grid inverter Watt/VA rating has to be at least as large as the GT inverter to prevent overheating/overloads during back feeding of the Off-Grid inverter.
    While all of the above issues are "manageable"--This is something that you would be assuming the risk on and developing your own solutions for.

    There are lots of possible solutions out there--but some require work on your part, or work on the part of the inverter designers.

    There is one shipping system that shares the AC power among devices--but it is not cheap.

    Sunny Island 5048U 5000 Watt Battery Based
    Also maybe there is place to read about inverter connections and everything. I am searching web but found only basic information.

    Do you have a specific application that you want to address? Many times, reading the manuals is the best way to start. Each Manufacturer has its own solutions and they don't always "map well" across product lines and between different companies or specific applications.

    Here is an interesting paper written by "Solar Guppy" (poster here) back in 2003 that talks about some of the issues and resolutions in one inverter design.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • VanDeusenVanDeusen Posts: 129Solar Expert ✭✭✭✭✭
    Re: Can I stack inverters with different wattage?
    niel wrote: »
    you may sell it here if you wish to, but leave the bulk of correspondence via pm. you should specify some things online like make, model, when bought, etc.



    Would I create a new thread or just put it here?
  • BB.BB. Posts: 24,674Super Moderators admin
    Re: Can I stack inverters with different wattage?

    Either is fine.

    You might prefer to have separate thread to not distract from the discussion about your system in this thread.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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