A tech here in the Appliantology forums ran into a surprising customer complaint about a microwave: it's doing its job too well! They say that it's cooking food way faster and way hotter than it used to.

(The above picture is a slight dramatization.)

So, our tech took a reasonable step and measured the amps going to the microwave. After all, if the magnetron is producing more watts than expected, it will have to be drawing more amps.

Measuring Power Consumption

His measurement showed 17 amps during operation. Since we're talking about power here, we need to convert those amps to watts. To do that, we'll use one of our trusty Ohm's law equations.

P = E x I will do the trick! P = watts, E = volts, and I = amps.

A microwave is a 120 volt machine, so by simply multiplying 120 by 17, we get our watts: 2040 watts.

And hold on -- our microwave's label says it's only a 1200 watt microwave. So is the customer right? Are we really putting out 800 more watts than the machine is designed for? 

Maybe in a perfect world that would be the case, but we live in a world governed by pesky rules like the laws of thermodynamics. No conversion of one form of energy to another will be 100% efficient. In fact, it's usually not even close to 100% efficient.

The magnetron of a microwave converts electrical energy into microwave radiation, which is what heats the contents of the microwave.  Through that process, something from 35-40% of the energy is lost, mostly as heat.

What's the upshot of all this? Well, that ~2000 watts is the power consumption of the microwave, not the power output. And power output is what the microwave's label is talking about.

If we take that ~2000 watts and take 60% of it, you get 1200 watts -- exactly what you would expect as your output from this microwave. So based on our power consumption, it doesn't seem like there's anything amiss here.

Measuring Power Output

It's all well and good to assume a 40% power loss, but is there a way to directly test how many watts of power the microwave is putting out?

Turns out there is, and it doesn't require any fancy equipment. In fact, all you need is a glass of water and a calculator.

Our fellow tech FD7 gave us the following formula in the forum topic where we found this scenario:

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Standard wattage output test. One liter of water. Measure the temp in C. Run the microwave for 62 seconds. Measure the temp again in C. Every degree in temp rise = 70 watts.

Sure enough, our tech ran this test in the microwave and got the following results:

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So after doing your test, it started at 27.3 C and ended at 44.8 C
 

Doing the math, That’s 17.5 C rise

17.5 x 70 =1,225 

That seems normal, it’s a 1200 watts microwave 

So where does this leave us?

Well, best we can determine, the microwave is operating within specs. This may very well be a case of a customer who was unwittingly using their microwave at a lower setting for years and recently switched it to its full power setting by accident. Time to educate the customer -- and we learned some things about power consumption along the way, too!

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