I have a wattmeter and motor. I want to know how much this motor consuming the energy(in kWh) by measuring the power (Watt) every minute.

If the wattmeter shows 500 W for 13minutes and then 0 W (it goes off) for the next 47 minutes. Does it means my motor consumes 0.1083 kWh in 1 hour?


1 Answer 1


You've calculated correctly. But, it doesn't use a constant 500W like some light bulb, it greatly depends on the load. For a reasonable estimate, we can use some equations assuming that it uses a constant 500W, like a light bulb. And this is assuming 100% efficiency.

\$\frac{500W × 13min}{60min}=108.3Wh\$

But this is not a fair setup, it will consume \$108.3Wh\$ if it's working as much as it possibly can, full speed and full load for 13 minutes.

If you want a fair setup, then place 2 wattmeters between the power supply and the motor and measure correctly. And if you want the energy consumed, the Wh, then connect some mcrocontroller and integrate the power. That will give you a perfect estimate.

You can get away with only 1 wattmeter if you also got the ground line, but that is also assuming that the coils in the motor are symmetric. aka identic impedances. In real life, that never happens. So with only one wattmeter you might be off by a couple of %.

-- Sorry for the wrong answer I made first, I'm pretty rusty.

  • \$\begingroup\$ Wow, really great answer, thank you so much. I never know it only reads apparent power though \$\endgroup\$
    – Naufal B
    Aug 3, 2017 at 1:08
  • 1
    \$\begingroup\$ A little bit related and little bit off topic -- This is why you shouldn't leave chargers connected to the outlet when they are not charging, current is still flowing through them, reactive power is being drawn and given back (typically around 10VA), and the wires & connections are the only real resistance (when it's not charging), so it will draw a couple of mW. \$\endgroup\$ Aug 3, 2017 at 1:18
  • \$\begingroup\$ Wattmeters measure power, not VA. You don't need to know the pf \$\endgroup\$
    – Chu
    Aug 3, 2017 at 7:01
  • \$\begingroup\$ @Chu ah, I thought wattmeters were less powerful. I thought they just gave U×I, oh well, at least I've learned something. \$\endgroup\$ Aug 3, 2017 at 8:47

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