• some devices can work on less power than they naturally pull. Examples: resistance loads like coil hot plates, simple coffeemakers, normal electric motors²⁾
• a TRIAC³⁾ is a particular circuit component used to do this magic. They are often found in , but also refers to a controller device that uses that internal component to adjust the power a device consumes. They are often sold as “router speed control” or similar (“speed control” hereafter.
• the load can typically be reduced to ~50% of its normal draw
• running the load at a low percentage (50% of normal) will cause less heat in the speed control than running it at a high percentage (90% of normal).⁴⁾

It will take the same amount of power to do the same amount of work (heat water, etc) so why reduce it? Because it can help us stay under the limits of our power setup. What if we have a 1500w hair dryer or hot plate but only a 1000w inverter? What if we have a corded drill that will run on our inverter but not start due to surge? What if we have a heating pad that is too hot even on its lowest setting? Or a crockpot we wish we could set somewhere between HI and LO for better temperature control?

Or we might be on shore power but have multiple 120vac devices that we want to run simultaneously but would trip a 15A shore power outlet.

• analog⁵⁾ resistance loads like heating pads, crock pots, hot plates, immersion heaters, rice cookers, bento “lunchboxes”, coffee grinders, etc. Warning: anything with electronics like displays, etc, will not work right and could be damaged.
• simple devices with motors like fans, routers, etc. Warning: fancy “brushless” motors will not work right and could be damaged

There is little clear information on the topic since running them off inverters is an extreme minority position, but it appears that speed controls work best on PSW inverters. They may run on MSW but seem unable to reduce power less effectively (see below) and may run hotter.

While resistive loads don't care about MSW vs PSW, motors will run hotter and “notchier” on MSW. So you might want to run the latter off PSW whether or not a controller is used.

One helpful Amazon reviewer⁶⁾ shared these results:

I used it this morning to control the heater element on my coffee maker - the coffee maker element is rated at 1440 W at 120V, and a Kill-a-watt registers 1420 from it. If set to non-variable mode the kill-a-watt registered 1410. When switched to variable, the output was as follows: at the lowest setting it output 687 W, at the middle of L it output 710, at the top of L: 750, at mid-M 812, at low H 955, and at the max the Kill-a-watt showed 1374 W. My desire was to reduce the output of the coffee maker to slow the brewing and it worked perfectly for that. The unit heats up whether in variable or non-variable mode, in variable mode it does get hotter, but not excessively in my opinion. Your mileage will likely vary…

The above reviewer was able to vary the power from 48% to 97% of the normal measured power by adjusting the controller's knob. Note that at ~50% power it will take about 2x as long to brew the coffee. Maybe you want that, or maybe you are willing to put up with it to be able to squeak through at 710w instead of 1420w.

One RVer noted he tried the controller with a grinder on both PSW and MSW:⁷⁾

The grinder did not go as slow as it does on psw, but slow enough.

He didn't give numbers on the difference.

Note from secessus:

when I ran a measured 928w hotplate at the lowest controller setting the power was ~550w, or about 60% of normal full power.

So until further information is forthcoming we might ballpark speed controls on PSW reducing power to ~50% of normal and MSW to ~60% of normal. But another Amazon reviewer⁸⁾ says:

…there is a hidden adjustment potentiometer inside for minimum power setting. Wall dimmer switches have these as well. I was able to adjust it, but I’m waiting for my tachometer to get it where it needs to be. By the sound of the motor, I can tell it will go down enough. They really should disclose this adjustment in the instructions… Note: The hidden adjustment is soldered onto the top side of the PCB inside but you need to use a small flathead screwdriver to adjust it from the back after removing the back cover.

See this video to see the pot adjusted. After adjustment secessus was able to hold 33% of rated power stably; any further adjustment caused the circuit to drop out.

Note that the Amp rating on these is at 120vac. 8A is 1000w, 9A is 1120w, 15A is 1800w.

• harbor freight sells an 8A version
• Amazon has many in different Amp ratings

• TRIAC switching circuit and applications
• YT: TRIAC AC Dimmer Circuit - How to dim AC Power for Motors and More, especially the oscilloscope section that starts at 1:50.