Metering Grid-tie w/ Batteries [RE-wrenches]

Todd Cory, Mt. Shasta Energy Services toddcory at finestplanet.com
Wed May 14 04:48:58 PDT 2008


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Ray,

I am not sure what you are saying below. The OB form 12S meter will 
accurately record power sold to the grid as well as power consumed on 
site. When the grid is down the above arrangement still accurately 
records AC consumption of battery generated power from the inverter. 
This IS the arrangement you want for keeping track of power generated by 
the system. Monitoring the DC will show nothing unless the grid is down.

 From the OB web page:
"This metering configuration will spin the kWh meter forward when power 
is being “sold” back to the utility grid from an RE source connected to 
the battery. The meter will also spin forward when the inverter is 
powering loads from the battery (such as during a power outage). It will 
spin backwards when the battery is being charged (i.e. subtracting the 
power being stored). AC loads which are being powered from the utility 
grid through the inverter will not cause the meter to spin in either 
direction."

You said:
"KWh meters on the AC grid connection only capture the net to the grid
(power generated - power to backup loads)"

this is inaccurate. A properly configured kWh meter will see power sold 
to the grid. Power consumed on site will also be seen (power generated + 
power to back up loads).

Here is what I posted in July of last year about this:

The analog form 12s meter (recommended by Outback) is designed for 
120/208 3 phase so it is expecting to see 120º of phase difference 
between the two legs rather than 180º, and being analog meters use the 
same core for both meter coils, the physics of this would suggest an 
analog meter will not accurately read net power produced (but it will 
probably be only 1 or 2% off).

The best way to accomplish this is with a digital, form 12s meter. Now, 
not any old form 12s meter will work (sigh). These meters come in three 
varieties:

1- The most common default configuration is cumulative read. This will 
_NOT_ work.
2- The other two configurations are net register or
3- dual register

Both of these (2 & 3 above) will work, although net register will do the 
calculations for you and dual register (sometimes called dual function) 
will leave you with separate buy and sell totals that will need to be 
manually added together at the end of each billing cycle.

The proper wiring can be seen at:
http://www.outbackpower.com/pdfs_general/Form%2012S%20Green-Tag%20meter.pdf

I have an electronic meter, GE, Type KV2C, FM12S, 120-480V, remfg. from 
Austin International purchased for $150.00. It works great.

Todd



Ray Walters wrote:

>
>
> Hi Fellow Wrenches;
>
> I am taking another look at how to properly meter an Outback grid tied 
> system. The Trimetric setups for off grid do not really do much in 
> grid tie. KWh meters on the AC grid connection only capture the net to 
> the grid
> (power generated - power to backup loads), and with several backup 
> circuits and a small array, the meter would read zero, or even 
> negative. The best I can figure is to put a DC shunt on the inverter. 
> Even this would only read PV to grid until the grid went down, then it 
> would read battery to backup loads, then when the grid returned it 
> would read minus as it recharged the battery from the grid. If we 
> could forget about the outages, or assume that backup load energy 
> balanced with grid battery charging, I think the shunt on DC side of 
> the inverter would be a more accurate number for REC credits. You 
> could apply a simple efficiency deduction. We've also considered just 
> using the onboard meter on the charge controller, but this doesn't 
> account for the inefficiency of the inverter or the batteries.
> For RECs, if you disregarded losses, you could meter right at the 
> modules, definitely sell more credits that way.
> Any other states paying for RECs yet? PNM gives 13 cents/ kwh on top 
> of net metering.
>
> Thanks,
>
> R. Walters
> ray at solarray.com
> Solar Engineer 


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