Technical Question About Grid-Tied Panels

[J.P.M.]

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.

Actually, at the end of the day, and in a very real and applicable sense, it's all a consequence of entropy. Nothing purist about it, but knowing that leads to the discovery of some few consequential tricks of the trade that not everyone, including most engineers, know about. Fact of life.

[Wy_White_Wolf]

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.

Panels are not connected directly. You're forgetting about the inverter. That stabilizes the voltage from the panels to slightly above the grid voltage.

If you have a 2 meter system then technically you send everything to the grid and receive everything from the grid. But few systems are 2 meter systems today.

Single meter systems the solar is tied into your house after the meter through your panel or something like a line side tap. Then if your house requires more amperage then solar can provide the shortage is made up by the grid. If the amperage requirement is the same all of it is supplied by solar and nothing from the grid. If the house requires less than the solar is producing the excess is sent to the grid.

One basic you seem to miss is electricity can only flow in one direction at a time though a wire. So you can not supply power to the grid at the same time you are taking power from the grid.

WWW

[Charlie W]

Panels are not connected directly. You're forgetting about the inverter. That stabilizes the voltage from the panels to slightly above the grid voltage.

If you have a 2 meter system then technically you send everything to the grid and receive everything from the grid. But few systems are 2 meter systems today.

Single meter systems the solar is tied into your house after the meter through your panel or something like a line side tap. Then if your house requires more amperage then solar can provide the shortage is made up by the grid. If the amperage requirement is the same all of it is supplied by solar and nothing from the grid. If the house requires less than the solar is producing the excess is sent to the grid.

One basic you seem to miss is electricity can only flow in one direction at a time though a wire. So you can not supply power to the grid at the same time you are taking power from the grid.

WWW

Thanks for the answer. It makes complete sense. The question arose in a different discussion, the details of which are too long and boring to recount here. It was really ambiguous, and made no sense to me. But I consider this group to be pretty much the ONLY authoritative, non-Kool Aid solar group on the Internet -- I'm sure there are others, but I haven't found 'em -- so I thought I'd ask here.