How many disc revolutions make 1kWh on this kWh-meter?

Question

This a kWh meter from a TV show. I'd like to know how many disc revolutions make 1 kWh. I think it's somehow coded in the whole data on the panel or could be derived from that information:

I found a formula

P = (3600 * Kh) / t

(t = time in seconds taken by the disc to complete one revolution, P = power in watts)

which gives me 138,(8) revolutions per 1 kWh but I am not sure I used the formula correctly.

Answer 1

The Kh is (indeed) what you're after, and the formula you give seems to be correct, according to wikipedia, 'Electromechanical' section.

P = (3600 * Kh) / t         (for one rotation)

If we call the amount of rotations n, we get (P / n) = (3600 * Kh) / t

Or: P = (3600 * Kh) / (t * n)

Where:

    t = Time in seconds
    P = Power in Watts

That means the unit of Kh is:

• (W / n) = (3600 * Kh) / s
• (3600 * Kh) = W / (n * s)
• Kh = W / (n * s * 3600)
• And because s * 3600 = 1 hour: Kh = W / (h * n)

So if Kh = 7.2, and it takes 1 hour for the dial to rotate 1 time, you've used 7.2 Watts.

You want (n * kWh). Let's change that into (n * Wh) for now.

• Divide everything on the right side through W: Kh = 1 / (Wh * n)
• (n * Wh) = 1 / Kh

Now, we need to change Wh to kWh, so we multiply both sides with 1000:

• (n * kWh) = 1000 / Kh

So for Kh = 7.2 you get 138.89 n*kWh or 138.89 rotations per kWh.

Your calculation is correct. :-)