grid tie backfeed question

[ogdento]

Hi, this site is an invaluable resource and I thank all the contributors for their expertise. I've been trying to clarify a question about grid tie back-feed (Enphase iq8 inverters, poco is PGE), and I've found some excellent posts by Mike90250, bcroe, SolTex and others

As I understand it, grid tie inverters that are connected to the house main-panel will supply any house loads before exporting excess energy to the grid, and will "draw energy from the grid if the inverters aren't outputting enough to cover the loads". The last quoted part - if I'm even stating it correctly - is what my question is about:

If my inverters are outputting 3kw, my house load is 0.5kwh, and my air conditioner (4kwh load) suddenly kicks on... assuming the total house load is now 4.5kwh... is the inverter smart enough to only take 1.5kw from the grid or does it shut down completely (i.e. not output anything), causing the entire 4.5kw to come from the grid? I *think* it is smart enough to draw 1.5kw, but I'm just looking for clarification or an explanation if I'm wrong.

[davidcheok]

Load draws from closest source first. Inverters/Batteries etc then grid. Nothing to do with how 'smart' the inverters are. The 'smart' comes when you want to prioritise or control the flow. In short, the house will only draw what the solar cannot provide.

[SolTex]

My simplistic understanding of our grid-tied Enphase system is based on the "voltage is like water pressure" analogy. Certainly seems to jive with the way our system works. The inverters (IQ7+ in our case) adjust their output voltage according to the load they see. The solar panels are always trying to put out as much power as possible.

If our house is consuming more power than our solar panels can supply, then the inverter output Voltage is slightly lower than the mains supply, meaning that outside grid power will continue to flow into the house to make up the short fall.

If the solar panels are supplying more power than the house can use, then the inverter Voltage output rises slightly, and becomes higher than the main supply. This then causes the excess power to flow counter to the normal direction, and the system back feeds the grid.

[davidcheok]

My simplistic understanding of our grid-tied Enphase system is based on the "voltage is like water pressure" analogy. Certainly seems to jive with the way our system works. The inverters (IQ7+ in our case) adjust their output voltage according to the load they see. The inverters are always trying to put out as much power as possible.

If our house is consuming more power than our solar panels can supply, then the inverter output Voltage is slightly lower than the mains supply, meaning that outside grid power will continue to flow into the house to make up the short fall.

If the solar panels are supplying more power than the house can use, then the inverter Voltage output rises slightly, and becomes higher than the main supply. This then causes the excess power to flow counter to the normal direction, and the system back feeds the grid.

Hahah thats exactly it. They call it potential gradient if I remember right.

[Ampster]

My simplistic understanding of our grid-tied Enphase system is based on the "voltage is like water pressure" analogy. Certainly seems to jive with the way our system works. The inverters (IQ7+ in our case) adjust their output voltage according to the load they see. ........
If the solar panels are supplying more power than the house can use, then the inverter Voltage output rises slightly, and becomes higher than the main supply. This then causes the excess power to flow counter to the normal direction, and the system back feeds the grid.

The only clarification I would add it that the grid presents an infinite load to the micros so as you suggest the voltage only needs to increase ever so slightly and the house loads will be served before any excess is sent to the grid. My equipment is not capable of detecting any of those voltage differences so I actually do not see those differences. Physics tells me that it is most likely the case..

[ogdento]

Hey thanks so much for the replies, you've cleared it up for me. Any solar output will be used and the grid supplies the difference between the solar output and the load (or 1.5kw in my example).

[albert436]

Hey thanks so much for the replies, you've cleared it up for me. Any solar output will be used and the grid supplies the difference between the solar output and the load (or 1.5kw in my example).

I wonder if there is a way to somewhat verify that by looking at what your output is, and then comparing against your usage across whatever hours.

[oregon_phil]

I have CT's on my main panel, pole barn sub panel and solar inverter. Then I run home assistant to capture and display data.

[ogdento]

absolutely what oregon_phil said... I don't have my system installed yet, but I'll be using current transformers on every panel as well

[Ampster]

I wonder if there is a way to somewhat verify that by looking at what your output is, and then comparing against your usage across whatever hours.

Yes, as mentioned there are a number of ways to verify that. There are also HAN devices that read the meter and report the data. I have CTs on my Enphase Envoy and also use an Emporia Energy monitor.

[ogdento]

years ago I built a ghetto device that mounted (non-permanently) to my meter and counts the pulses from an infrared emitter on the face to calculate consumption. I wonder what it'll do if I'm exporting vs importing power... unless PGE swaps it for a different one...

https://www.instructables.com/Another-Smart-Meter-Reader-But-With-a-REST-Api/

[albert436]

years ago I built a ghetto device that mounted (non-permanently) to my meter and counts the pulses from an infrared emitter on the face to calculate consumption. I wonder what it'll do if I'm exporting vs importing power... unless PGE swaps it for a different one...

https://www.instructables.com/Anothe...th-a-REST-Api/

That is pretty interesting. I know very little about electronics but I wonder if I could make that with no idea what I am doing just by following the instructions.
Does the smart meter give you a running tab on the LED screen?

Edit: I wonder if there is available one of these devices ready-made? I did a google search but no luck What terms would you suggest I use for better search results? If that is possible.

[Ampster]

That is pretty interesting. I know very little about electronics but I wonder if I could make that with no idea what I am doing just by following the instructions.
Does the smart meter give you a running tab on the LED screen?

Yes my PG&E meter gives me two totals, one for grid use and the other for grid export. Of course those are Net numbers,
Previously, I mentioned HAN devices which pick up those totals from the Zigbee signal which the meter uses. I had previously used a Rainforest device at another home with PG&E. Recently I tried to do that on another home and found that PG&E will not allow HAN devices on meters with NEM agreements. Other users have reported some luck getting another answer but I had no desire to go up the food chain at PG&E.

[albert436]

Yes my PG&E meter gives me two totals, one for grid use and the other for grid export. Of course those are Net numbers,
Previously, I mentioned HAN devices which pick up those totals from the Zigbee signal which the meter uses. I had previously used a Rainforest device at another home with PG&E. Recently I tried to do that on another home and found that PG&E will not allow HAN devices on meters with NEM agreements. Other users have reported some luck getting another answer but I had no desire to go up the food chain at PG&E.

Thanks for that. I have not had time to look up HAN devices and don't know what they are. Maybe if I will be on a NEM it doesn't matter.
But it would be nice to view the info without going out to look at the meter. Say for example if I wanted to see consumption in real time when I turn on an air conditioner. Not sure if that even makes sense in the context of this discussion but I will leave it at that.

[RichardCullip]

I use an Eagle-200 device from Rainforest to see what my SDG&E smart meter is measuring. Once connected to the smart meter and authorized by SDG&E I can get instantaneous electric use data and a history from one day to 1 week to 1 month. It is cool to see the effect of turning on and turning off electrical devices in the house to get an estimate of power usage. It has an iPhone app called EnergyVUE that allows me to check the energy use on the phone. The only drawback is that it is a cloud-based system so there is some vulnerability to nefarious folks.