Technical Question About Grid-Tied Panels

[Charlie W]

You make more electricity than you use during the day, especially in the summer. We all know this. But there's another question I haven't seen discussed anywhere.

I have been under the impression that, when you make more juice than you're using, solar panels sent only the excess production to the grid. But a discussion in an e-mail list I'm part of suggests that, no, you send everything you generate the grid, and get everything you use back from the grid, and that the accounting is done according to (in most cases) a net metering formula that gives you a kWh for each one you send.

I hadn't thought about this detail until now. When you think it through, it would make sense that you'd send everything to the utility and get everything from them, because we could expect that panels might have considerable voltage swings that would make a direct connection to the end user problematic for appliances that require stable voltage.

Does anyone know? Thanks much in advance.

[PNPmacnab]

You send everything to the house. The electrons magically know where to go. Net metering will be a thing of the past. You will soon be billed in 15 minute intervals. Demand pricing will encourage people to start placing their panels facing west. Utilities only agreed to the present metering because they thought it would never take off. Excess noon production is a menace to the utilities.

[bcroe]

You have a lot to learn, and thanks PNP. The panels generate DC voltage which cannot be
connected to the grid. DC is fed to a GRID TIE INVERTER which finds the best panel operating
point (MPPT) at any time, then uses the input to generate AC output power matching up to the grid.

For those of us with only a single meter, the PoCo only sees the DIFFERENCE between what
AC energy you generate, and what you consume. Grid power may flow in either direction
depending on your generation and consumption. Bruce Roe

[sdold]

it would make sense that you'd send everything to the utility and get everything from them,

It makes no sense, in a basic NEM system with a single meter, because if your house is using 6 kW and your panels are generating 4 kW, the grid is delivering 2 kW to your house to make up the shortfall. You can see this on your meter, which tells you how much power is going in which direction. It's easy to understand if you switch everything to DC. DC current can't go in two directions at once.

[Charlie W]

I had thought that only the excess went into the grid, but the post I had seen left some doubt. As it turns out, that post was ambiguously worded and open to varying interpretations, hence my question to this group.

Past that, I am curious as to how the utilities manage the variability of solar, and especially of wind.

By the way, PNP, the sarcasm doesn't help. If I can't ask an honest question here without that kind of crap in return, where do you suggest I go to find knowledgeable people who aren't condescending jerks?

[Mike90250]

.............
By the way, PNP, the sarcasm doesn't help. If I can't ask an honest question here without that kind of crap in return, where do you suggest I go to find knowledgeable people who aren't condescending jerks?

Charlie, wait till you meet Sunking. He just drives the roads up north, needs no snowplow or chains. (snowflakes sublimate in front of him)

This is the internet, and if you were offended by something you imagined PNP wrote (I read it over a couple times, and looked to see if he edited something out) you are going to be in sad shape in a couple days. This forum speaks truth, unlike many forums that are sales driven. This is the straight from the hip shooting , not "well, if you think it's right for you, maybe the laws of physics will accommodate you in this case" style of forum. If you are going to be hands on with solar, you are going to learn about proper connections here (we'll scald you in oil if you think your hammer crimper is going to work on your 1/O cable), the hazards of medium voltage DC, and the dreaded AHJ. (Authority Having Jurisdiction - building code inspector).

[Charlie W]

Charlie, wait till you meet Sunking. He just drives the roads up north, needs no snowplow or chains. (snowflakes sublimate in front of him)

This is the internet, and if you were offended by something you imagined PNP wrote (I read it over a couple times, and looked to see if he edited something out) you are going to be in sad shape in a couple days. This forum speaks truth, unlike many forums that are sales driven. This is the straight from the hip shooting , not "well, if you think it's right for you, maybe the laws of physics will accommodate you in this case" style of forum. If you are going to be hands on with solar, you are going to learn about proper connections here (we'll scald you in oil if you think your hammer crimper is going to work on your 1/O cable), the hazards of medium voltage DC, and the dreaded AHJ. (Authority Having Jurisdiction - building code inspector).

I wasn't arguing with anyone in this thread, and don't think my question deserved the treatment the one poster gave it. I'll certainly live, but I see nothing wrong with calling him out. Trust me, my pushback was mild by my own standards. "Snowflake" isn't something I'm called too often, that's for sure. Other names, yes, but not that one.

[SunEagle]

Basic knowledge is that electricity will take the shortest path to the load. So you house loads get all they want from the solar and then the excess goes to the grid. If there is no excess solar generation to send to the grid then the house loads takes what they need from the grid.

Electricity 101.

[azdave]

...a discussion in an e-mail list I'm part of...

People are still using e-mail lists?

[solarix]

The grid is essentially a voltage controlled power source - it will supply (or absorb) as much current as needed to keep the voltage (240Vac for your house) constant (or at least stable). So now with the voltage fixed, all you need to think about are the current flows. A grid-tied solar inverter is a current source - it will follow the grid voltage and produce as much current as needed at that voltage to expend the DC power it is receiving from the array. The current is injected (or backfed) into your breaker box with the house absorbing as much of it as it wants and the excess then is pushed out into the grid (and measured by the utility's revenue meter). To push the excess power back to the grid, yes it has to boost the voltage a bit (but the grid being very low impedance (resistance), just a tenth of a volt or so).
If you want to read up on this - it is basic electric theory called "Thevenin equivalent" circuits where you model the system as simple elements such as voltage and current sources, loads and resistances etc. Think of your breaker box as a node with the grid as a voltage source, the solar inverter as a current source and your house as a load. Now imagine it with the house loads changing all the time, the solar power adjusting to a cloud going by and your neighbor's A/C kicking on and off.....
Yes, the grid current is ac and is oscillating back and forth, but the power flow (volts times amps) is directional, flowing into your house when the solar power is less than what your house needs and flowing out when there is excess.

[ButchDeal]

You make more electricity than you use during the day, especially in the summer. We all know this.

SOME people make more than they use during the day. You are making an assumption both on generation (a likely assumption) and on consumption (not always true though).

I have been under the impression that, when you make more juice than you're using, solar panels sent only the excess production to the grid.

this is true for MOST NET metering installs.

But a discussion in an e-mail list I'm part of suggests that, no, you send everything you generate the grid, and get everything you use back from the grid, and that the accounting is done according to (in most cases) a net metering formula that gives you a kWh for each one you send.

This is true in a FEW areas and is more commonly known as dual metering. It is much less favorable and make bimodal systems impossible
I have seen it only in coop utility zones and even then only a few of them.

I hadn't thought about this detail until now. When you think it through, it would make sense that you'd send everything to the utility and get everything from them, because we could expect that panels might have considerable voltage swings that would make a direct connection to the end user problematic for appliances that require stable voltage.

Does anyone know? Thanks much in advance.

No this makes zero logic. Yes lots of people know this. Solar Produces DC and the Votage variability is easily handled with MPPT and Inverters, which make nice stable power.
If you understand the word NET in an accounting sense then you would know that net metering means that you use your own power and send excess to the grid.

[PNPmacnab]

What sarcasm? Electrons always know where to go. They flow from a higher potential to a lower. Within the same cycle you could be pulling or sourcing numerous times to the grid. You can only understand the average movement.

[SunEagle]

What sarcasm? Electrons always know where to go. They flow from a higher potential to a lower. Within the same cycle you could be pulling or sourcing numerous times to the grid. You can only understand the average movement.

Good point. Instantaneous movement of electrons is very hard to accurately measure. Even most meters perform an average measurement over a specific time period.

[JSchnee21]

When I was a small boy, maybe 7 or 8 years old, I was down in the basement with my Dad who was brazing a water line for a utility sink. Son he said, "The only thing you need to know about plumbing is that Shxt runs down hill." 35 years later I cannot tell you how many college graduates I've had to teach that basic message to when it comes to electricity -- or any moving fluid or potential energy source. Electrons, like water molecules follow the path of least resistance and thus flow down hill from high potential to low potential. The same is true for heat, solvent and solute gradients for osmolarity and osmolality, ions (like sodium, potassium, and calcium) across cell membranes, etc. Yes, purists and theorists will cringe, but at the end of the day, maybe that little electron does in fact know where he's going.

[bcroe]

Electrons, like water molecules follow the path of least resistance and thus flow
down hill from high potential to low potential.

If you want to be precise, electrons take every path available. However, most of them take the path with
best conductance (least resistance). Bruce Roe