How does my home know to use the solar power before grid power?

[tlhfirelion]

Self education questions as I do not currently have a solar system. I've looked online but cannot find the answers so here I am with 2 basic questions.

1) How is the inverted power connected to the breaker box? I assume the wires coming from the inverter marry with the power lines coming into the top of my breaker panel to feed the bus, is that correct? Or, is there typically a sub panel set up for specific appliances only? I can't find a pic or diagram of how the physical connections are made.

2) How does my home know to draw from the solar power first before grid power? If all the power is coming in to the breaker box, what makes solar power be the first choice?

Thank you in advance for the feedback.

[Bikerscum]

I'm a little (lot) unclear on that too.

[Mike90250]

power usage

A Grid Tie inverter pushes power onto the grid, by trying to raise it's voltage above grid. Whatever the wire resistance from the inverter to the power grid transformer is, influences the amount of voltage rise.
Think of water in a hose, with a Y and a pump. What comes out of the end, (your house) is a mix of the Grid and the GT inverter. Since there is finite resistance between your GT inverter and the Grid , your house gets first shot at the power, then any excess flows out to the grid.

it's only 97F today, I'm not typing well, gotta cool off.

[inetdog]

An equivalent analysis would be to assume that all of the GTI power goes out toward the grid (through the meter) and some or all of it then comes right back in again, perhaps accompanied by some of POCO's power if you need more.
The effect on your meter reading is exactly the same either way. Currents add.
Whether individual electrons actually turn left before the meter on the way out or go out and back in again is a good theoretical argument, but has no practical consequences.

[carultch]

Self education questions as I do not currently have a solar system. I've looked online but cannot find the answers so here I am with 2 basic questions.

1) How is the inverted power connected to the breaker box? I assume the wires coming from the inverter marry with the power lines coming into the top of my breaker panel to feed the bus, is that correct? Or, is there typically a sub panel set up for specific appliances only? I can't find a pic or diagram of how the physical connections are made.

2) How does my home know to draw from the solar power first before grid power? If all the power is coming in to the breaker box, what makes solar power be the first choice?

Thank you in advance for the feedback.

1) You can either make a line side connection (as you described), or a load side connection. In general, it is most elegant to do a load side connection whereever you can. One less piece of equipment, and you don't need the utility to arrange a shutdown or a separate service/account/meter.

In the case of the load side connection, you connect a breaker sized as required for your inverter(s) on the opposite end of the bus from the main supply. This keeps the current on the busbar as low as possible, since the inverter feeds load circuits before it meets the main supply current. There is a "120%" rule that you need to follow, if you are able to do a load side connection. See NEC705.12(D). Inverter breaker + main supply breaker cannot exceed 120% of the busbar. As an example, a household 200A panel with a 200A main breaker, can only be fed by up to 40A worth of inverter breaker(s) on the opposite end of the bus. The main supply breaker must also be suitable for backfeed.

When you cannot land on the load side, you can land on the line side instead. As close as possible, you need an interconnection fused disconnect or breaker, and then its output wires can "tap" onto the main supply wires. Either at a spare lug position, or with wire connection tap devices. The short tap wires are not properly protected until they meet this device, so there are specific feeder tap rules to follow. Such as 10 ft or 25 ft length limits.


2) This is like asking how your car circuits "know" whether to draw current from the battery or from the alternator (engine powered "generator").

Current flows "down the voltage hill". It takes a voltage difference to push current through the wires. You will have slightly less voltage at your appliances, than you do at the panelboard, slightly less voltage at the panelboard than at the inverter, and slightly less voltage at the panelboard than at your utility transformer's output.

Current is not going to flow "uphill" to the utility transformer, if the voltage is higher there than at your panelboard. And that will be the case, if you are drawing power simultaneously from the inverter and from the utility. If your inverter produces more power than you need at your loads, the voltages will reverse, and the power will flow to the utility transformer, and then to your nearest neighbor.

[tlhfirelion]

Thank you for all the replies, you explained it very well so I have a better understanding to it now.

Have a good day!