ratcheting cyclometers [RE-wrenches]

[Robert Nuese]

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Greetings!

I read with great interest the recent "ratcheting cyclometers" posts. I 
too, need to install output meters on 3 SW systems I'm currently 
installing. Here in California, since April, we are required to install 
a "performance meter" on each on-grid PV system which will receive a 
state California Energy Commission rebate, so that "…customer can 
determine the amount of energy produced by the system."  Seems 
reasonable to me, and it would be of value to any customer, even 
without the bureaucratic requirement. But how to best do this?

It certainly looked to me like Doug Pratt had solved the problem of how 
"to distinguish between grid feed and PV feed to the subpanel" in his 
posting of Dec, 11, 2003:


> -----Original Message-----
> From: Doug Pratt [mailto: dmpratt at sbcglobal.net]
> Subject: RE: ratcheting cyclometers [RE-wrenches]
>
> …Simple. Using a standard 240v utility meter, wire the 120v leg from 
> the
> SW output to the subpanel so that power passing to the subpanel spins
> the meter forward. Wire the other 120v leg so that power passing from
> the main panel to the SW input spins the meter backwards.
>
> If the SW is simply passing utility power straight thru to the 
> subpanel,
> you'll have no meter action. The two currents cancel each other out. If
> the SW is pushing solar power to the subpanel, the meter spins forward.
> If the SW is pushing solar power to the subpanel *and* the utility, the
> meter spins forward faster.
>
> This ought to still work just fine with a ratcheted meter so that night
> time battery charging isn't deducted.
>
> *Caution*, I must admit I haven't DONE this, but I can't see any 
> problem
> with it. We run meters forward and backward like this all the time. If
> I'm missing something obvious, I'm sure we'll all hear about it. :-)
>
> Cheers,
> Doug Pratt


Well, I hope Doug is not missing anything, and it certainly wasn't 
immediately obvious to me - But on close inspection, I'm uncertain. I 
did some research on the Internet to learn more about how watthour 
meters work. I have yet to find any complete internal circuit diagrams, 
or thorough texts about meters, but the information I've found so far 
suggests that, in effect, the meter multiplies the sensed current by 
the sensed voltage difference between the two line terminals. Some 240v 
meters can be adjusted for use on 120v systems by changing the wire 
routing and installing a jumper to neutral, as has currently been 
discussed. In the case Doug describes, we seem to be doing a 240v 
installation where the direction of one leg is reversed to allow in and 
out currents to cancel one another under some circumstances. My problem 
is that the voltage differential the meter will sense across the two 
line terminals will actually be zero. We are not talking about 2 
different 120v legs of a 240v, 1-phase system, that are 180 degrees out 
of phase and thus would have 240v difference between them. We are 
talking about 2 different parallel branches from the same 120v inverter 
output, which have 0v difference between them. I'm afraid this means 
the proposed setup won't work.

I'm hoping others will show me I'm wrong, because it looked like a 
mighty good system otherwise.

IF Doug's system doesn't work, then I'm afraid we'll need two meters. I 
think the one for the dedicated circuit can be any type, forward only 
or both ways, ratcheting or not, since it will only measure output. I 
think the one for the line to the grid MUST be calibrated for forward 
and backward use, and CANNOT be ratcheting, because then power from the 
grid to the dedicated circuits, at night for instance, would be 
measured by the dedicated circuit meter as if it was production by the 
solar system, without being offset by backwards motion of the other 
meter.

If my doubts are valid, this is not a very pleasing arrangement because:
1) It requires 2 meters.
2) It requires user to do addition of amounts on two meters to find 
total PV production. On some occasions, this will be addition of a 
negative number. On each meter, this is done AFTER doing a subtraction 
of a previous reading, to find the amount for the time period. These 
calculations will NOT be entirely intuitive for the less technically 
inclined customers.
3) The client can only determine total electricity used on premises by 
adding to the above calculation the amount shown on the electric 
utility meter (adding a negative number during periods when the meter 
has run backwards).
4) Even with these three meters, the client has no way of finding how 
much of the electricity used on the dedicated circuit was produced by 
the PV, or how much used in the rest of the house was produced by the 
PV - only the combined PV use. So far as I can tell, finding these 
separately would require having 2 meters on the line from inverter to 
grid alone (we're now up to 4 meters!), each one set to ratchet in 
opposite direction to the other.

So, I'm hoping someone on this list will please tell me I'm wrong, and 
explain why. Barring that, does anyone have a simpler solution?

Looking forward to your help,
And Wishing You All a Good 2004,
Robert Nuese

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