# Metering and load control



## eutecticalloy (Dec 12, 2010)

sprdave said:


> An unusual application, and not sure what's available for this (the locals don't know).
> 
> There is two services, 120/240v single-phase. The main service is 400amp for the load of house and shop (It was existing and don't use near that power). The other is a 10kw grid-tie solar array. Both from the same transformer.
> 
> ...


I need some more info, what kind of heating loads? The net production thing warrants a plc with analog inputs in my opinion


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## wildleg (Apr 12, 2009)

I would think you could easily accomplish this with a CT on the output of the solar to grid. (calibrated output of the CT) AND (thermostadt control for heater) = (contactor input logic for heater contactor).


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## sprdave (Jan 12, 2012)

eutecticalloy said:


> I need some more info, what kind of heating loads? The net production thing warrants a plc with analog inputs in my opinion


Standard electric water heater with resistance elements for now.



> I would think you could easily accomplish this with a CT on the output of the solar to grid. (calibrated output of the CT) AND (thermostadt control for heater) = (contactor input logic for heater contactor).


Where is the main load factored in? The idea is if something is turned on (like the dryer or stove) and they're importing power, the heater should turn off.


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## BBQ (Nov 16, 2010)

sprdave said:


> An unusual application, and not sure what's available for this (the locals don't know).


So we must assume that the heater is not needed, just desired?


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## sprdave (Jan 12, 2012)

BBQ said:


> So we must assume that the heater is not needed, just desired?


Yes it's optional, to make use of cheap power. Heat/pre-heat a store of water. If/when it's depleted the tankless heater would back it up.


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## wildleg (Apr 12, 2009)

BBQ said:


> So we must assume that the heater is not needed, just desired?


that's what he said. But as your question indicates, he may have overlooked something.


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## JRaef (Mar 23, 2009)

Well, the Utility side is easy, but the Solar side poses challenges. Your "solar grid" probably has an inverter, and power from an inverter is inherently full of harmonics that gives headaches to standard metering equipment. But did you check to see if the inverter has a function to provide outputs of power flow data? If so, that could be sent to a simple controller that would make the decision of controlling that contactor.


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## sprdave (Jan 12, 2012)

JRaef said:


> Well, the Utility side is easy, but the Solar side poses challenges. Your "solar grid" probably has an inverter, and power from an inverter is inherently full of harmonics that gives headaches to standard metering equipment. But did you check to see if the inverter has a function to provide outputs of power flow data? If so, that could be sent to a simple controller that would make the decision of controlling that contactor.


First I've heard of power from grid-tie inverters being difficult to meter. I thought it was pretty "clean" and low harmonics to be allowed to grid-connect. I know it's two Aurora inverters and you can plug into a laptop or get remote monitoring equipment to get data. Albeit expensive. And they are a distance from the shop (They are mounted on the rotating trackers). So the logistics are poor and I think it would be easier/cheaper to meter it at the shop.

The bigger issue is "combining" the data from the load and the solar to figure out the surplus (if any) AND then using that net data to operate a relay if there is enough surplus.


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## mbednarik (Oct 10, 2011)

utilities here require separate metering of any customer owned generation and customer load. But they also net meter the billing between the two and pay i do believe energy cost if it is a negative net meter.


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