I have two air conditioners that are the source of some voltage fluctuation/line noise in my home. The two A/C's running at the same time is definitely and unambiguously at fault. This is primarily a problem because it causes LED lights to flicker.

Rewiring/adding circuits is not an option, nor is disassembly or modification of the A/C's themselves.

I would like to isolate the A/C's with something that plugs in inline to keep the rest of the power clean. I'd prefer to stick something between the A/C's and the wall (because there are only two of them) than have to put lots of filters/conditioners on everything else.

My Google-Fu has failed me - I can only seem to find information on how to protect sensitive devices from EMI/voltage fluctuations (e.g. stick a online UPS between a computer and the wall), and not how to properly mitigate a known dirty device.

I'm in the US with 120VAC single phase power.

What is the proper device to add inline to an air conditioner or other motor-driven device in order to keep the rest of the power clean? Isolation transformer? Line conditioner? UPS (if so, what flavor - battery backup in an outage doesn't matter except as a means to smooth the inrush voltage fluctuations)? Massive ferrite bead?


I have read several proscriptions against using a UPS or AVR, etc on electric motors, primarily because the startup current draw of the motor might exceed the capabilities of the device. (See almost every post on laser printers for reference) Most advice seems to treat this as a blanket prohibition, but in reality there is obviously more nuance to it: https://www.apc.com/us/en/faqs/FA158851/ So to refine the question somewhat - assuming the inline device is properly sized for the motor and providing a natural sinewave, as noted in the APC link above, what is the proper device to use to isolate/smooth a motor driven appliance from its 120V AC power supply?

  • Well 2 window ac’s on the same leg of a us based split 240v supply is a terrible case. Your service type , , location and supply makes a difference, but having 2 120v ac systems ,,, well that means renter ,, and not a professional install,,, with more info maybe we can help.
    – Ed Beal
    Commented Sep 19, 2020 at 22:31
  • Yup, renting. Situation certainly isn't ideal, but it is what it is. What additional info are you looking for exactly? Commented Sep 19, 2020 at 22:38
  • Get a ups uninterruptible power supply for computers on a system with heavy loads like ac units. UPS convert the incoming power to dc and run off of a battery so if spikes that would glitch a computer come through that are common with motor loads the UPS. Will keep the computers running. I have dozens of these units at my current Work location my previous life had them larger than a pickup truck with ion implant machines. I have them in my small farm house to protect big screen tv’s and computer and video surveillance systems.
    – Ed Beal
    Commented Sep 19, 2020 at 22:50
  • My computers are already on UPS anyway for unrelated reasons, and I understand how those work. The question is how to smooth the power usage of the A/Cs (which are the known source of the power fluctuations) so I don't need to stick a UPS or line conditioner on every other circuit powering a LED device in the house. Commented Sep 19, 2020 at 23:00
  • @Jaegermeiste -- are the LEDs dimming when the air conditioners start their compressors, or...? Is there a pattern to which lights are impacted by them? Commented Sep 19, 2020 at 23:42

2 Answers 2


Well after plenty of research the proper device is a online sinewave UPS that is sized to handle the startup load of the motor/compressor. See https://www.apc.com/us/en/faqs/FA158851/ This answers the question above as posed; however, said UPS is prohibitively expensive.

As for my flicker issue (the symptom) I solved it by wiring in inexpensive EMI filters into the affected fixtures.


They make surge suppressors that plug into a socket, and have a socket themselves to plug a protected load into.

Plug the air conditioners into those. They are bi-directional and will suppress surges sourced at the A/C unit.

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