I have a 5,000 BTU, 115 volt, 4.8 amp, window-unit air conditioner I'd like to control from my home automation system.

I think easiest thing to do would be to plug the unit into an inline rely module that switches its power on and off.

How do I go about determining the load rating needed for such a relay module?

For example, I have an extra module here that says it's rated for 600W incandescent, 1/2 HP motor load, or 15 Amps resistive. Too small, right?

  • 2
    Does the AC Unit have any kind of remote control? You will be better off hotwiring a spare remote from ebay to turn it on and off instead of trying to put the entire unit on a relay. May 28, 2015 at 12:54
  • Unless your AC has entirely physical controls, this isn't going to work well - it will lose it's settings when it loses power. Many also don't come on automatically after losing power (which helps prevent problems from everyone's AC coming back on at once after a blackout).
    – Grant
    Aug 26, 2015 at 16:25
  • IDK if you're looking for this level of hack, or if it's even doable, but... If the built in thermostat switches low voltage, maybe you could replace the thermostat with the output from the home automation system. That may have some advantages, there may be internal controls to prevent the unit from starting / stopping too frequently when the temperature hovers above and below the setting. Again - just a thought, don't know a thing about window AC controls. Oct 26, 2015 at 9:50

2 Answers 2


@ 80% efficiency is just above 1/2 horsepower. The relay is just slightly too small in my opinion.


That relay should be fine: AC watts = Power Factor x Volts x Amps. In this case we don't know the power factor, but 115 volts x 4.8 amps = 552 watts. Your unit probably draws somewhat less than that in reality. Does the AC unit mention power factor, or watts?

  • 1
    A motor is much harder on a relay than a purely resistive load - they draw far more starting current than when they are running (and can also throw some interesting voltages when you shut them off.)
    – Ecnerwal
    May 28, 2015 at 14:17

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