Solar Generation amount differs between PG&E and Telsa app

[RayB]

Our solar panels (originally installed by Solar City about 15 years ago), have been maintenance free since installation. However, I'm not sure if something needs to be looked at now, or if there may be an issue with the PG&E meter. We have a smaller solar system, and use more electricity than we generate (especially since buying a Model Y two years ago).

Our PG&E bill (Jan 5 - Feb 3, 2022) states a net generation of 146.0315 kWh.
The Tesla app (for the full month of January) states we generated 234.8 kWH. Granted, there is some overlap between the two time periods, but not much. Does this look normal? If we generated ~234 kWh, then why does PGE only show we generated ~146 kWh? That's about a 40% difference!

What am I missing??

 

[orbit2020]

Same thing with my system, and it always has been so. My understanding is, when my PV’s are producing, some of it is used by all the stuff that is plugged in and running in the house. It is only the excess over that that is sent to PGE, and it is only that part that I am compensated by PGE. So, If I plug in my EV during the day, or run my AC, when my PV is producing, it goes into my EV or AC. (I am paying Solar City for this as I have a lease). It also means, that it is less expensive to charge only off peak, using the grid. Hope I have understood it correctly.

 

[jjrandorin]

The utility only knows what is sent to them. Energy from a PV system is only sent to the utility if the home doesnt use it at generation. If at the time of PV generation, the home load is higher than the amount of PV that is generated, the result is a lower pull from the utility (thats what the utility sees).

If at the time of PV generation, the home is not consuming all the energy, the surplus is sent to the utility (thats what the utility sees). The tesla app will know how much the solar installation has generated, but the utility will not, unless there is a separate meter on for the solar installation (which is unlikely in a small installation).

 

[spsheridan]

Our PG&E bill (Jan 5 - Feb 3, 2022) states a net generation of 146.0315 kWh.
The Tesla app (for the full month of January) states we generated 234.8 kWH.

What am I missing??

@jjrandorin is correct. The key term in your PG&E bill is “net” generation which is the difference between how much energy your solar panels generate (shown in the Tesla app) minus how much energy you consume in your home. PG&E can only measure the net energy supplied to the home or supplied back to the grid from the home if you're generating more than you consume.

We ended up installing a Sense unit in our main electrical panel which allows us to measure the power our home consumes separately from the power our solar panels generate. When we calculated the difference, it matched our Southern Cal Edison bill fairly accurately (within a kWh per month).

Below is a screenshot from Sense showing the solar power we generated (orange) vs. the power we consumed (red) yesterday.

 

[RayB]

Thanks for the responses. This makes sense now. So, would it be better for me to charge the car and/or run the dishwasher during solar generating hours (sunny days, between 8am and 4pm?) We're currently on the TOU-C plan, with peak hours being between 4pm and 9pm. Or, should we continue with our overnight charging of the car and running the dishwasher overnight. I'm retired, so no need to leave the house everyday. I can easily charge the car between 8am and noon, if it makes sense financially...

 

[jjrandorin]

Thanks for the responses. This makes sense now. So, would it be better for me to charge the car and/or run the dishwasher during solar generating hours (sunny days, between 8am and 4pm?) We're currently on the TOU-C plan, with peak hours being between 4pm and 9pm. Of, should we continue with our overnight charging of the car and running the dishwasher overnight. I'm retired, so no need to leave the house everyday. I can easily charge the car between 8am and noon, if it makes sense financially...

Generally, TOU plans have a off peak from midnight to 6am, to encourage you to use power overnight. it doesnt sound like you have battery storage (making an assumption here but feel fairly comfortable with it), so its kind of up to you as long as you are charging during a rate period that is the same as your overnight rate.

Since you are a net consumer (use more than you generate), I would do what it sounds like you were previously doing, which is set charging in the middle of the night.

 

[spsheridan]

Thanks for the responses. This makes sense now. So, would it be better for me to charge the car and/or run the dishwasher during solar generating hours (sunny days, between 8am and 4pm?) We're currently on the TOU-C plan, with peak hours being between 4pm and 9pm. Of, should we continue with our overnight charging of the car and running the dishwasher overnight. I'm retired, so no need to leave the house everyday. I can easily charge the car between 8am and noon, if it makes sense financially...

My wife and I did some calculations to answer this question and the answer depends. She wrote a blog post on this subject which you can find here.
Dumb Question Corner - Is it Better to Charge Your EV at Night or During the Day if You have Solar? - Podfeet Podcasts
This post assumes that you do not have a battery installed with your solar system. Your results will vary from ours depending on your rates, usage, NEM rules, etc., but you can get an idea of how to determine when to charge from our methodology.

 

[RayB]

Thanks again everyone. You are correct...we have no battery backup. I understand this much better than I did an hour or so ago!
I think I may just play around with charging the car during different times (overnight, or during solar generating hours), since the rate from PG&E is the same from 9pm - 4pm the following day. We bought our solar system outright and it's generated enough power over the past 15 years to pay back the costs. So now, it's just free energy. Eventually, when costs come down further, I'd like to get an upgraded system with battery backup...

 

[jjrandorin]

My wife and I did some calculations to answer this question and the answer depends. She wrote a blog post on this subject which you can find here.
Dumb Question Corner - Is it Better to Charge Your EV at Night or During the Day if You have Solar? - Podfeet Podcasts
This post assumes that you do not have a battery installed with your solar system.

I just took a look at your link in curiosity, and this... is... GREAT! Love it, love it, love it! Very well written, informative, and helpful! If your wife isnt a member here, please pass on my kudos.

@RayB Take a look at the link that was posted by @spsheridan, which actually covers your direct question in an awesome way.

 

[Stumpy_Steve]

Really bummed about this. I always wondered why my PG&E bill was so high.

We have 6.6 kW Solar Panels, 1 Powerwall installed and pay roughly $200/month for financing. We live in Sacramento where it's >90 degrees and sunny from June until September so lots of sun. However, our PG&E bill has always been really high, even still paying in the summer when Tesla says we overproduce.

Today I looked at my app and it said we were producing 3.3 kwh of power, using 0.5 kwh and sending 2.8 kwh to the grid. But when I look at my smart meter, PG&E says we're only sending 1.33 kwh to the grid.

What on earth could be the issue?!

 

[spsheridan]

Really bummed about this. I always wondered why my PG&E bill was so high.

We have 6.6 kW Solar Panels, 1 Powerwall installed and pay roughly $200/month for financing. We live in Sacramento where it's >90 degrees and sunny from June until September so lots of sun. However, our PG&E bill has always been really high, even still paying in the summer when Tesla says we overproduce.

Today I looked at my app and it said we were producing 3.3 kwh of power, using 0.5 kwh and sending 2.8 kwh to the grid. But when I look at my smart meter, PG&E says we're only sending 1.33 kwh to the grid.

What on earth could be the issue?!

The 6.6 kW power you mentioned is probably the DC power coming from your solar panels which is typically what solar installers quote. When DC power is converted to AC power into your electrical panel, there is usually a 20-25% conversion loss. That would bring your 6.6 kW of DC power down to 5.0 - 5.3 kW of AC power. I see from your Sense plot that you are reaching 5.0 kW of peak (AC) power which is consistent with 6.6 kW of DC power. Also, the quoted power is typically the peak power you'd see on the sunniest day at noon in the summer. The lower 3.3 kW of power is likely due to the fact that you are making that measurement at 4:24 (I assume PM) so the sun is not at its peak and as a result, the solar power produced is reduced.

Btw, welcome to the forums!