Oversized System - Best Routine?

[HardHitter]

I got PTO activated last week and since turning it on, I've just monitoring our usage pattern. Setup is 12kW System + 2 PW. Tesla settings is self-powered mode with 80/20 reserve split. Average house consumption is ~18.7kWh with solar being ~37.2kWh (28% to home, 25% to PW, 47% to grid).

My usage pattern seems to be as follows:

- Solar starts generating ~8AM
- Solar exceeds house needs ~9AM starts charging batteries
- Batteries fully recharged by ~11AM-12PM
- From there, all excess is sent to grid
- ~6PM Solar generation ends
- From 6PM to 8AM next day, I am running on battery
- Battery usually reaches ~65-70% capacity by 8AM next day when solar starts generating again.

- We have 1 EV which I charge during the day and has no effect on batteries (8-9kW needed when charging vs ~9-10kW from system)
- I can run AC during the day with the same effect (5-6kW needed when running AC vs ~9-10kW from system).

We plan on adding another EV in 3 years and perhaps switching to electric water heater but other than that, we have no need to do anything with the grid but are we maximizing our return?

Should I leave it in self-powered mode or switch to time-based control? I'm on EV-A2 for PG&E so rate schedule is

- Off-peak 12:00 AM - 3:00 PM - $0.25kWh
- Mid-Peak 3:00 PM - 4:00 PM - $0.41kWh
- Peak 4:00 PM - 9:00 PM - $0.43kWh
- Mid-Peak 9:00 PM - 12:00 AM - $0.41kWh

Do I have any reason/need to pull from the grid and incur NBCs? If I for some reason do pull from the grid, does it just net against the kWh I've exported and I just pay the NBC? (e.g. I've exported say 30kWh of peak power and I consumed 5kWh today. PG&E wouldn't charge me $0.43kWh, just the NBC of the 5kWh and my 30kWh credit goes down to 25kWh?

 

[holeydonut]

@Redhill_qik incoming with his NEM-knowledge

Lucky for you you’re under NEM2… the answer to your question under NEM3 will be “you get rekt”

 

[sunwarriors]

Aren't NBCs always there? Someone correct me if I'm wrong, but I thought/think if you're a net exporter even if you pull nothing, you still have a minimum charge for being connected to the grid on NEM2.0. I tihnk it's like ~$10-$11/month in SDG&E land.

I've now gone the route of setting my reserve higher on my battery (to preserve the charge/discharge cycles) and even though it won't make much difference probably for warranry, it's nicer to have a higher reserve in case of a power outage (main benefit of a battery I feel) and the IOUs pay very little anyways for excess generation ($0.03 - $0.06 for most folks?).

I got my true-up recently and got like $130? or something credit which sorta wipes out all my NBCs throughout the year so at this point, don't see a real benefit for me to be fully self powered even though I mostly could be during the winter months (no AC).

Trying to make money on excess seems low on payout and you can sign up for virtual power wall or that other program, but I just don't want anyone else to access my battery storage (trust issues).

 

[NathanielHrnblwr]

Yes, you'll pay $120/yr if you use grid power or not. Might as well use $10/mo of power somewhere.

 

[RKCRLR]

Yes, you'll pay $120/yr if you use grid power or not. Might as well use $10/mo of power somewhere.

Yes and no. Your minimum monthly payments can be used to offset NBCs.

 

[Redhill_qik]

I got PTO activated last week and since turning it on, I've just monitoring our usage pattern. Setup is 12kW System + 2 PW. Tesla settings is self-powered mode with 80/20 reserve split. Average house consumption is ~18.7kWh with solar being ~37.2kWh (28% to home, 25% to PW, 47% to grid).

My usage pattern seems to be as follows:

- Solar starts generating ~8AM
- Solar exceeds house needs ~9AM starts charging batteries
- Batteries fully recharged by ~11AM-12PM
- From there, all excess is sent to grid
- ~6PM Solar generation ends
- From 6PM to 8AM next day, I am running on battery
- Battery usually reaches ~65-70% capacity by 8AM next day when solar starts generating again.

- We have 1 EV which I charge during the day and has no effect on batteries (8-9kW needed when charging vs ~9-10kW from system)
- I can run AC during the day with the same effect (5-6kW needed when running AC vs ~9-10kW from system).

We plan on adding another EV in 3 years and perhaps switching to electric water heater but other than that, we have no need to do anything with the grid but are we maximizing our return?

Should I leave it in self-powered mode or switch to time-based control? I'm on EV-A2 for PG&E so rate schedule is

- Off-peak 12:00 AM - 3:00 PM - $0.25kWh
- Mid-Peak 3:00 PM - 4:00 PM - $0.41kWh
- Peak 4:00 PM - 9:00 PM - $0.43kWh
- Mid-Peak 9:00 PM - 12:00 AM - $0.41kWh

Do I have any reason/need to pull from the grid and incur NBCs? If I for some reason do pull from the grid, does it just net against the kWh I've exported and I just pay the NBC? (e.g. I've exported say 30kWh of peak power and I consumed 5kWh today. PG&E wouldn't charge me $0.43kWh, just the NBC of the 5kWh and my 30kWh credit goes down to 25kWh?

It appears that you will be a net generator on an annual basis and maybe even with a second EV, BTW get a hybrid heat pump electric water heater and not a direct electric when you convert from gas. Your profile indicates that you are in San Jose, so you even if you are with the SJCE CCA for electric generation your net generation compensation will be the same as with PG&E or around $0.05/kWh, so there isn't a lot to be gained there and you are better off using the excess yourself through charging your EVs and switching from gas to electric heat pumps for your water heater and eventually HVAC.

To minimize your PG&E payments keep an eye on the amount of kWh that you import per day/month. The current MDC, Minimum Daily Charge, is $0.34810/day and the current NBC, Non-Bypassable Charge, is $0.02666/kWh which means that you can import on average 13.06 kWh/day or 4,765 kWh/year without exceeding the annual MDCs of $127.06.

Based on your reported usage and generation that gives you a lot of flexibility, but you might exceed that if you are charging your EVs at night from the grid instead of during the day from solar. If you did exceed your MDCs then just think of it as paying $0.02666/kWh for EV charging which is a great deal. So personally, I would be on Time Based Control in your situation. With your batteries just being used during Mid-Peak and Peak.

 

[taphil]

To minimize your PG&E payments keep an eye on the amount of kWh that you import per day/month. The current MDC, Minimum Daily Charge, is $0.34810/day and the current NBC, Non-Bypassable Charge, is $0.02666/kWh which means that you can import on average 13.06 kWh/day or 4,765 kWh/year without exceeding the annual MDCs of $127.06.

Thanks for the explanation, I didn't know MDC counts towards NBC (I thought it MDC was a charge on top of NBC). Is that how it works for SCE too?

That would therefore incentivize running in time-based control mode rather than self-powered mode. With a yearly energy rebate and net generator/true-up credits, I haven't paid SCE anything for most of this year running TBC mode.

 

[Redhill_qik]

Thanks for the explanation, I didn't know MDC counts towards NBC (I thought it MDC was a charge on top of NBC). Is that how it works for SCE too?

That would therefore incentivize running in time-based control mode rather than self-powered mode. With a yearly energy rebate and net generator/true-up credits, I haven't paid SCE anything for most of this year running TBC mode.

As far as I know the NEM 2.0 rules are the same across all IOUs, but the numbers are different. You will need to look at the SCE tariff schedules to determine what the MDC and NBC rates are to recalculate how much you can import before paying additional amounts beyond the MDCs.

 

[HardHitter]

It appears that you will be a net generator on an annual basis and maybe even with a second EV, BTW get a hybrid heat pump electric water heater and not a direct electric when you convert from gas. Your profile indicates that you are in San Jose, so you even if you are with the SJCE CCA for electric generation your net generation compensation will be the same as with PG&E or around $0.05/kWh, so there isn't a lot to be gained there and you are better off using the excess yourself through charging your EVs and switching from gas to electric heat pumps for your water heater and eventually HVAC.

To minimize your PG&E payments keep an eye on the amount of kWh that you import per day/month. The current MDC, Minimum Daily Charge, is $0.34810/day and the current NBC, Non-Bypassable Charge, is $0.02666/kWh which means that you can import on average 13.06 kWh/day or 4,765 kWh/year without exceeding the annual MDCs of $127.06.

Based on your reported usage and generation that gives you a lot of flexibility, but you might exceed that if you are charging your EVs at night from the grid instead of during the day from solar. If you did exceed your MDCs then just think of it as paying $0.02666/kWh for EV charging which is a great deal. So personally, I would be on Time Based Control in your situation. With your batteries just being used during Mid-Peak and Peak.

Thank you for this breakdown. So is the end goal to import enough by year end to offset the MDC? What if you are importing more? Yes you will get a "payout" but it is at a fraction of what it should be so you are better off having it consumed right? Since PTO, it shows that I am exporting an average of ~19.332kWh/day.

Also, how exactly does the NEM work? For example, you recommended that I switch to Time Based Control where the batteries are being used during Mid-Peak and Peak times. If I pull from the grid during off-peak and say I consume 30kWh/day, then I need to ensure my PV system exports 30kWh/day to avoid any charge that day? Or will the charges for that day be $0.7998? (30kWh*$0.02666/kWh). How does any exported kWh during the mid-peak and peak times get handled and impact this example?

 

[RKCRLR]

Someone else can probably explain this better but I'll do it at a top level. Under NEM you get credited at the rate during the time period you are exporting. So if you are exporting during peak you will get credited in dollars at the peak rate. This is tallied monthly. During true-up at the end of the year you need to have both a dollar credit and a kWh credit to get net surplus compensation. I.e., if you have a credit in dollars since you did most of your exporting during peak rates but you actually consumed more in kWh than you exported you won't get a net surplus credit.

 

[taphil]

You bank production credits with daily overproduction to cover deficits for other days or times of the year, like when it’s raining or cloudy for multiple days in a row, or in the summer when running AC exceeds daily production.

So in my case, I bank enough over the spring, fall, and winter seasons to have enough credit to keep the house cool throughout the summer. Even after this, I still have excess production that I get some credit for at true-up, and this credit is enough to pay for several months of NBC charges for the next few months.

 

[Redhill_qik]

Thank you for this breakdown. So is the end goal to import enough by year end to offset the MDC? What if you are importing more? Yes you will get a "payout" but it is at a fraction of what it should be so you are better off having it consumed right? Since PTO, it shows that I am exporting an average of ~19.332kWh/day.

Also, how exactly does the NEM work? For example, you recommended that I switch to Time Based Control where the batteries are being used during Mid-Peak and Peak times. If I pull from the grid during off-peak and say I consume 30kWh/day, then I need to ensure my PV system exports 30kWh/day to avoid any charge that day? Or will the charges for that day be $0.7998? (30kWh*$0.02666/kWh). How does any exported kWh during the mid-peak and peak times get handled and impact this example?

My end goal is to have the lowest possible electric and gas costs on an annual basis, some want to minimize their cost while also minimizing any draw from the grid and others want to minimize their costs while keeping their house at 68F during the summer. How you achieve the goal you set is very dependent on if you are a net importer/consumer (you import more kWh than you export on an annual basis) or a net exporter/generator/producer (you export more kWh than you export and an annual basis).

NEM 2.0 works primarily based on TOU charges with a MDCs and NBCs as a secondary consideration. You get a credit for every kWh that you exported based on the TOU price at the time that you exported and you get a charge for every kWh that you exported based on the TOU price at the time that you imported. The credits and charges offset each other on 1:1 basis and once a year on your PTO anniversary there is an annual True-Up and if you balance is positive then you owe and if it is negative the it is reset to zero. Your MDCs and NBCs factor also factor into what you owe and you can see the sticky thread NEM-PS Annual True-Up Calculation [PG&E example] for more information. Your question on the 30kWh import/export would have NBCs of $0.7988 added to the cumulative total for the year and the energy charges would be negative as you

When you are a net importer then you need to pay close attention to the TOU tariff rates as you need to minimize your importing and maximize your exporting during the higher cost Peak and Partial Peak periods to decrease your charges and increase your credits. The Powerwall is advantageous in this situation as it can reduce your imports to zero and if you have and use the Export Everything option you can increase your exports at the higher rate during this period.

When you are a net exporter then the TOU tariff rates have little impact unless you have a very skewed usage with little usage during the Off-Peak and very high usage during Peak as your export credits more than offset your import charges. At your annual true-up your credit negative balance gets reset to zero and you simply pay the difference between the cumulative NBCs for your imports and the cumulative MDCs and if this negative (NBCs < MDCs) then you owe nothing extra.

The other aspect of a net exporter is that you get paid for the excess kWh. For PG&E and SJCE (if you are with the CCA) the payment for your net exported kWh does not factor in the TOU period, but there are a small number of CCAs that compensate based on the TOU period.

If you want to minimize your electric costs then your want to makes sure that your cumulative NBCs ($0.02666/kWh on imports) doesn't exceed your annual MDCs ($0.3841/day or $127.06/year). It doesn't matter how much less this, just that it is less so that you don't owe anything more than what you have already paid monthly in MDCs. You can import a lot (4,765 kWh/year currently) before your NBCs exceed your MDCs.

If you want to maximum your excessive export credits then you want to maximize your export kWh and minimize your consumption (note: consumption not imports). Your Powerwalls have 90% efficiency for charge/discharge so when you discharge 10 kWh from the Powerwalls then you need to recharge the Powerwalls with 11.1 kWh and that is 1.1 kWh that is not exported to the grid. As a net exporter with two Powerwalls, I am running in Time Based Control with a 80% reserve level and discharging only 5.4kWh during the Peak period which reduces my exports by 0.54kWh (plus the daily loss of ~0.250kWh/PW). I would be better off it I had this set to 100%, but I want exercise the Powerwalls regularly and it makes me feel a bit better to reduce the grid during Peak.

Pay attention to your regular PG&E blue bill and the ESS/Powerwall PG&E black bill to see how you are trending on annual basis and then adjust accordingly.

 

[Redhill_qik]

You bank production credits with daily overproduction to cover deficits for other days or times of the year, like when it’s raining or cloudy for multiple days in a row, or in the summer when running AC exceeds daily production.

So in my case, I bank enough over the spring, fall, and winter seasons to have enough credit to keep the house cool throughout the summer. Even after this, I still have excess production that I get some credit for at true-up, and this credit is enough to pay for several months of NBC charges for the next few months.

Export/production energy credits do not offset NBCs these credits only offset your import energy charges. Your paid MDCs do offset NBCs and if your have a positive balance your import energy charges. If you have a positive energy charge balance then some of your NBCs are offset because the tariff rate already includes the NBC components, see the sticky thread NEM-PS Annual True-Up Calculation [PG&E example]

 

[holeydonut]

Pay attention to your regular PG&E blue bill and the ESS/Powerwall PG&E black bill to see how you are trending on annual basis and then adjust accordingly.

But if you do this and realize you're using much more electricity than your solar produces... you will be super sad once you try and add more solar because there's too much red tape and Powerwalls are too scary

 

[Redhill_qik]

But if you do this and realize you're using much more electricity than your solar produces... you will be super sad once you try and add more solar because there's too much red tape and Powerwalls are too scary

Not everyone is handcuffed by PG&E , but my comment was really intended to point out that the system behavior during the first month in October might not be true year round.

 

[HardHitter]

My end goal is to have the lowest possible electric and gas costs on an annual basis, some want to minimize their cost while also minimizing any draw from the grid and others want to minimize their costs while keeping their house at 68F during the summer. How you achieve the goal you set is very dependent on if you are a net importer/consumer (you import more kWh than you export on an annual basis) or a net exporter/generator/producer (you export more kWh than you export and an annual basis).

NEM 2.0 works primarily based on TOU charges with a MDCs and NBCs as a secondary consideration. You get a credit for every kWh that you exported based on the TOU price at the time that you exported and you get a charge for every kWh that you exported based on the TOU price at the time that you imported. The credits and charges offset each other on 1:1 basis and once a year on your PTO anniversary there is an annual True-Up and if you balance is positive then you owe and if it is negative the it is reset to zero. Your MDCs and NBCs factor also factor into what you owe and you can see the sticky thread NEM-PS Annual True-Up Calculation [PG&E example] for more information. Your question on the 30kWh import/export would have NBCs of $0.7988 added to the cumulative total for the year and the energy charges would be negative as you

When you are a net importer then you need to pay close attention to the TOU tariff rates as you need to minimize your importing and maximize your exporting during the higher cost Peak and Partial Peak periods to decrease your charges and increase your credits. The Powerwall is advantageous in this situation as it can reduce your imports to zero and if you have and use the Export Everything option you can increase your exports at the higher rate during this period.

When you are a net exporter then the TOU tariff rates have little impact unless you have a very skewed usage with little usage during the Off-Peak and very high usage during Peak as your export credits more than offset your import charges. At your annual true-up your credit negative balance gets reset to zero and you simply pay the difference between the cumulative NBCs for your imports and the cumulative MDCs and if this negative (NBCs < MDCs) then you owe nothing extra.

The other aspect of a net exporter is that you get paid for the excess kWh. For PG&E and SJCE (if you are with the CCA) the payment for your net exported kWh does not factor in the TOU period, but there are a small number of CCAs that compensate based on the TOU period.

If you want to minimize your electric costs then your want to makes sure that your cumulative NBCs ($0.02666/kWh on imports) doesn't exceed your annual MDCs ($0.3841/day or $127.06/year). It doesn't matter how much less this, just that it is less so that you don't owe anything more than what you have already paid monthly in MDCs. You can import a lot (4,765 kWh/year currently) before your NBCs exceed your MDCs.

If you want to maximum your excessive export credits then you want to maximize your export kWh and minimize your consumption (note: consumption not imports). Your Powerwalls have 90% efficiency for charge/discharge so when you discharge 10 kWh from the Powerwalls then you need to recharge the Powerwalls with 11.1 kWh and that is 1.1 kWh that is not exported to the grid. As a net exporter with two Powerwalls, I am running in Time Based Control with a 80% reserve level and discharging only 5.4kWh during the Peak period which reduces my exports by 0.54kWh (plus the daily loss of ~0.250kWh/PW). I would be better off it I had this set to 100%, but I want exercise the Powerwalls regularly and it makes me feel a bit better to reduce the grid during Peak.

Pay attention to your regular PG&E blue bill and the ESS/Powerwall PG&E black bill to see how you are trending on annual basis and then adjust accordingly.

Thank you so much for this detailed response.

Because I have an oversized system at the moment, I expect to be a net exporter come my annual true up. Since my PTO date of October 7 through October 20, per PG&E's Energy Usage Report, it shows a credit of ~304kWh and a monetary credit of ~$83. My settings have been set as "Self-Powered" with 80/20 split.

Reviewing my Powerwall graphs over the last couple of days, it shows that starting around 5:00 PM - 5:30 PM, the Powerwalls start to discharge and by the time solar starts generating the next morning, they've discharged to around 65-70%. They will typically hit 100% again around 10:00 AM - 11:00 AM depending upon a few factors (discharge status that morning, PV generation, if I'm charging my EV that day, etc.)

So the real question for me is does it make sense to stay on "Self-Powered" setting or to switch to "Time-Based Control". If my understanding is correct, if I switch the "Time-Based Control" the negatives are that I am going to start generating NBCs since I'll actually be importing from the grid again. I don't see what the beneficial "trade off" would be to switching?

Also, is there any kind of hardware degradation to the batteries' lifetime by operating the way I have been doing? (Batteries charge during day, batteries will discharge during day only during instances when house consumption is greater than PV generation (example of this is when I am charging my EV during the day) batteries provide for house consumption from 5:00 PM onward until charging starts again next morning)

 

[Redhill_qik]

Thank you so much for this detailed response.

Because I have an oversized system at the moment, I expect to be a net exporter come my annual true up. Since my PTO date of October 7 through October 20, per PG&E's Energy Usage Report, it shows a credit of ~304kWh and a monetary credit of ~$83. My settings have been set as "Self-Powered" with 80/20 split.

Reviewing my Powerwall graphs over the last couple of days, it shows that starting around 5:00 PM - 5:30 PM, the Powerwalls start to discharge and by the time solar starts generating the next morning, they've discharged to around 65-70%. They will typically hit 100% again around 10:00 AM - 11:00 AM depending upon a few factors (discharge status that morning, PV generation, if I'm charging my EV that day, etc.)

So the real question for me is does it make sense to stay on "Self-Powered" setting or to switch to "Time-Based Control". If my understanding is correct, if I switch the "Time-Based Control" the negatives are that I am going to start generating NBCs since I'll actually be importing from the grid again. I don't see what the beneficial "trade off" would be to switching?

Also, is there any kind of hardware degradation to the batteries' lifetime by operating the way I have been doing? (Batteries charge during day, batteries will discharge during day only during instances when house consumption is greater than PV generation (example of this is when I am charging my EV during the day) batteries provide for house consumption from 5:00 PM onward until charging starts again next morning)

Switching to TBC will increase your NBCs, but the amount is highly unlikely to exceed your MDCs so it doesn't matter. You will lose less kWh recharging the Powerwalls that will be exported instead which is a benefit.

The Powerwalls are guaranteed to have at least 70% of the original capacity at 10 years. There are several threads here about pros/cons of staying at 100% for long periods and deep cycling with no definitive conclusion. A few people have reported significant capacity losses here, but those seemed to have been more likely marginal battery cells on day 1 than due to usage conditions.

My two Powerwalls are both above 13.5kWh after two years as reported by the REST API that you can access by logging into the gateway. You can do a search here for the URL to get this info.

 

[cwied]

My two Powerwalls are both above 13.5kWh after two years as reported by the REST API that you can access by logging into the gateway. You can do a search here for the URL to get this info.

I'll just add a comment that I don't think this number is necessarily reliable. I have the first generation Powerwall 2's and if I discharge to 20%, I seem to lose capacity every day. If I discharge only to 33%, the capacity slowly comes back. Here's an example (I was on vacation 10/1 to 10/9, so reserve was set at 20%). It's unclear if this reflects a real capacity change or just an estimation issue.

 

[HardHitter]

Switching to TBC will increase your NBCs, but the amount is highly unlikely to exceed your MDCs so it doesn't matter. You will lose less kWh recharging the Powerwalls that will be exported instead which is a benefit.

The Powerwalls are guaranteed to have at least 70% of the original capacity at 10 years. There are several threads here about pros/cons of staying at 100% for long periods and deep cycling with no definitive conclusion. A few people have reported significant capacity losses here, but those seemed to have been more likely marginal battery cells on day 1 than due to usage conditions.

My two Powerwalls are both above 13.5kWh after two years as reported by the REST API that you can access by logging into the gateway. You can do a search here for the URL to get this info.

I switched to TBC and trying to figure out what % split I should be using for backup vs time-based control. Also, how does the system work compared to Self-Powered?

When I switched to TBC, it looks like all the PV was sent straight to the battery and whatever the house needed, it pulls it from the grid but I would sometimes see it flip to where the PV was covering the house, sending some back to the grid and then the majority going to charging the battery. Does the system do this to keep the grid consumption at net 0 until the batteries are full and then it will export it all to the grid? What will happen when PV system is gone? it will always pull from the batteries or is it going to pull from the grid?

 

[Redhill_qik]

I'll just add a comment that I don't think this number is necessarily reliable. I have the first generation Powerwall 2's and if I discharge to 20%, I seem to lose capacity every day. If I discharge only to 33%, the capacity slowly comes back. Here's an example (I was on vacation 10/1 to 10/9, so reserve was set at 20%). It's unclear if this reflects a real capacity change or just an estimation issue.
View attachment 866554

I manually check my data once a week when the Powerwalls are 100% and I haven't seen the variation like you are seeing. I have seen a few hundred Wh variation across seasons (lower in Winter, higher in Summer), with 10-20 Wh week-to-week. I was did check when I discharged down to 5% as part of VPP and it was <10Wh change day-to-day.