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I tried searching the internet, and everything I found was about using inverters in parallel seemed to be for coordinating multiple inverters supplying to a single bus. I'm curious about using multiple inverters with different power ratings for circuits that will have different loads.

I'm building an all-electric, off-grid system for a tiny house. There are a few things that draw a lot of power intermittently: Induction burner, oven, on-demand water heater, etc. Everything else is low-power.

With a large inverter the draw would be far below the efficient range of operation the vast majority of the time. At least, I've found this image in multiple places on the internet, such as here:

Curve showing inverter efficiency as a function of percentage capacity used.

Rather than a using a single 6000W inverter and only drawing enough power for lights, laptop, and refrigerator 90% of the time, I was wondering if I could, instead, use multiple smaller inverters in parallel (as shown below)? Also, since each inverter would have its own switch and fuse, I wouldn't really need a breaker panel.

This would not require coordination between the inverters. I don't see why it would, in principle, create an excessive load on the batteries compared to a single inverter at the maximum draw.

Am I missing something?

            +---------+                                                                   
      ------|         |---+------------------+                                            
 2.5A       |  300W   |   |                  |                                            
      ------|  120VAC |---------+            |                                            
            +---------+   |     |            |                                            
                          |     |            |                                            
                          |     |            |                                            
                          |     |            |                                            
            +---------+   |     |            |                                            
      ------|         |---+     |            |                                            
 2.5A       | 300W    |   |     |            |                                            
      ------| 120VAC  |---------+           ----                                          
            +---------+   |     |         ---------                                       
                          |     |           ----                                          
                          |     |         --------- 12 V                                   
                          |     |           ----                                          
            +---------+   |     |         ---------                                       
      ------|         |---+     |             |                                           
  15A       |  2000W  |   |     |             |                                           
      ------|  120VAC |---------+             |                                           
            +---------+   |     |             |                                           
                          |     |             |                                           
                          |     |             |                                           
                          |     |             |                                           
            +---------+   |     |             |                                           
      ------|         |---+     |             |                                           
  30A       | 4000W   |         |             |                                           
      ------| 120VAC  |---------+-------------+                                           
            +---------+                                                                                                                                                                                                                           

Note: This is NOT the same question as this: Can I connect ten 100W invertors (each with separate battery) parallel to power one 1kW consumer?

3 Answers 3

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Off-grid designs loosely like this are common, because most people who are off-grid get used to thinking about power and redundancy. The simple version is close to what you've drawn and is compatible with it. You have one inverter for each specific load or combinations of loads. You just change which switch you use to turn things on - inverter instead of wall switch. The redundancy part comes in here: if one inverter fails you grumble and re-arrange things and get it fixed or replaced when you can. If you only have one inverter and it fails now you have a fridge or freezer full of stuff and a limited time to get AC going into it again.

Typically there's the "basic something is on" inverter that powers small appliances, or lights and fans if you don't have DC versions of those. Typically you rarely to never turn this one off, and it'll be the expensive/quality low power inverter (if you have an expensive low power one at all).

Then there's one (or more) high power inverters that you turn on to run the washing machine or whatever. Depending on exactly what the big loads are you might have more than one, or a three phase setup (can be three single phase inverters or one multiphase one).

This is also where dump loads sometimes get used. You confgure your setup either explicitly to send excess power to a heater (if it has that feature) or you manually turn the load on when the battery is full but it's still sunny.

IME people only do the exact setup you propose either right at ther start like you, or once they get sick of trying to find DC versions of their small loads. Commonly the latter have 12V or 24V LED lighting and fans already, but they have a Dell or Lenovo laptop that won't run on just any old 19V, they need the special name brand Ac to DC adaptor sold by the manufacturer. If there's a 12V to 19V adapter from that brand it'll be expensive and hard to find. Just to name one specific pain point. It might be for any other small appliance.

Note that fridges and other heap pumps are a special case because their start current can be extreme, so a small inverter will not work. I had a cheap chest freezer that used 60W when it was running... and more than 2.4kW for one cycle when it started the compressor. But my house fridge draws 90W when running/300W starting so a 650W UPS is quite happy to run it. It can be worth buying a more expensive "inverter fridge" or "soft start fridge" because you save that money back by buying a smaller inverter (you may find that your washing machine resets when the fridge starts up even with a 6000W inverter, for example).

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  • thanks for the thoughtful answer! I've had a number of people tell me that it doesn't make sense. And I've never run across a practical discussion of using multiple inverters such as you presented. I must be looking in the wrong places. Where should I be looking?
    – abalter
    Commented Apr 4 at 18:46
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    I'm not entirely sure, Will Prowse's forums are a current hotspot, but I mostly deal with this stuff in offline contexts. The most common "several inverters" case is probably people who upgrade and keep the old one connected "just in case". Also, at 6kW a 12V battery is expensive because you need such thick wire. Normally people go 48V at about 2.5-3kW.
    – Móż
    Commented Apr 4 at 22:37
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Your design is theoretically viable but in practice, it might do more bad than good. So, what I think you are missing here is the combined/cumulative load on the battery that is related to both the efficiency (rating and other parameters) of battery and individual inverter.

Assuming all the inverters are 90% efficient, then the said inverters would collectively draw (333.33 + 333.33 + 2222.22 + 4444.44) = 7333.32 W from the battery in contrast to a 6000W (90% efficient) inverter that would draw 6666.67 W. This energy is dissipated as thermal energy meaning your suggested setup would be hotter compared to one big inverter would dissipate.

Other factors to consider in mind are overall cost, wiring complexity, short circuit protection and so on.

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  • However, the small inverters would be operating AT capacity while a large inverter would be operating far below capacity. How does that figure in?
    – abalter
    Commented Apr 4 at 18:42
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Inverters can be paralleled, but they must be parallel capable. The reason being that if two inverters try to create alternating current with different phase angle, they would essentially be short circuited and cause major issues.

The paralleled inverters need to be in sync. This means that either one of the inverter is first turned on, doesn't detect signal on its output and decides to be the master. Then another inverter is turned on, detects signal on its output and decides to be a slave following the master. This happens with many parallel capable inverter generators.

Another possibility is that the inverters are not parallel capable but can be modified to be. If you have two inverters that are identical or from same series but different power rating, you can open both of them and find the location that drives the MOSFETs. Then you disconnect the MOSFET driving signal on one inverter and supply the same signal to that inverter from the other inverter. Then they are guaranteed to be in sync.

Some instructions.

The problem in your plan is that if the high loads can turn on at any time, like being thermostat driven fridges with high starting current, you need all inverters on at all times. Two 1500W inverters typically have higher idle power consumption than one 3000W inverter, so the plan does not make sense.

If you have the possibility to decide when the high loads are turned on, then it's of course possible to have a basic low power inverter on at all times and then turn on a bigger inverter in parallel to supply the needed high current.

Also note that most inverters have 2X the starting capability. So a 2000W battery inverter will most likely withstand a device needing 4000W starting current.

The same is not true of inverter generators, since the 4000W starting current would exceed the power rating of the engine, and would stop the engine.

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  • Please correct me if I'm wrong about this. I think inverters only need to be coordinated if their output is being combined. For instance, if I have two 10A inverters supplying 20A together. Then they need to be in sync. But I don't think they need to be synchronized if they are supplying different loads.
    – abalter
    Commented Apr 8 at 19:22
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    Yes, if each inverter has their different loads. Oh, indeed, I didn't read the question carefully enough.
    – juhist
    Commented Apr 9 at 16:52
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    Thank you for clarifying. I want to make sure I understand this correctly.
    – abalter
    Commented Apr 9 at 18:23

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