I have the old school electric meter with the (far too rapidly) spinning disc. I already know how to read five little clock faces for cumulative KWH.

I want to know the instantaneous kilowatts my house is drawing right now. How can I figure that out by timing how fast the disc spins?

  • I am curious as to why you want to know the instantaneous KWH your house is using at any particular time. What problem are you trying to solve or how will this information be used. – d.george Jun 25 '17 at 11:30

Yes. You can determine your instant watts or kilowatts by using a stopwatch to time how long the disc takes to go around. Then you need to do a little bit of math. Not too much, promise :)

First, look for a figure somewhere on your electric meter called Kh. 7.2 is a common number, so you'd see a marking like 7.2 Kh or perhaps a different number. **

enter image description here

Take the Kh number and multiply it by 3600. You can call this number Ks. You might as well write this number on the meter, you'll be using it a lot. ***

Use a stopwatch/phone to count the number of seconds per 1 revolution of the disc. (If it's moving too fast or you want more precision, time 10 revolutions and divide by 10.)

Start with the Ks number (Kh x 3600) and now divide by the number of seconds. That is the number of watts being used right now. **** For instance if Ks is 25920 and it took 6 seconds, 25920/6 is 4320 watts.

If you want kilowatts, divide watts by 1000 (4.320 kw).

** gory details: Kh is the number of watt-hours per revolution. A watt-hour is 1 watt for 1 hour. *If you had only a 1-watt LED nightlight burning, it would take Kh hours to turn.*

*** gory details: multiplying Kh by 3600 gives the number of watt-seconds per revolution, since there are 3600 seconds in an hour. By the way, for a 7.2Kh meter, this number will be 25920.

**** gory details: Ks is the watt-seconds per revolution. Divide by the number of seconds per revolution, the result is watts!

***** gory details: another example picture.

enter image description here

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