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Misleading Capabilities of Backup Mode?
- Thread starter BIC1
- Start date
"Solar" might shut down, but your home will still have power, so I am a bit confused on the issue here.
I suspect the problem is that because of the mismatch between solar production and Powerwall capacity, the Powerwall shuts the solar down whenever it is generating more than the 5 kW the Powerwall can absorb, even if the state of charge would allow charging. I haven't heard that anyone has gotten curtailing of solar production to work, which means it's on or off only. This means the Powerwall will turn the solar off until the solar production is low enough that it can charge. This might mean there's not enough solar production left to charge back up again.
Ok, so that makes more sense as to what the specific problem might be, thanks for the explanation. I have 2 powerwalls and an 8.7kWh solar installation, so dont have this problem. The actual issue is too much solar and not enough powerwall, or said another way, for that size PV, there should have been maybe 3 powerwalls?
In a power outage, my original belief was I could run the house normally as long as the sun is out. Solar would go direct to the electrical draw as it does when hooked to the grid (I understand now it doesn't work that way, but at the time of PW purchase....). The battery would just sit there uninvolved till darkness falls and then jump in to save the day.
After testing the system, I see the solar shuts down and the house runs off the battery with the sun producing more than 5kW (even in December, I'm generating 8-9kW). Then, the battery runs down a bit and re-charges. OK, not so bad but for two things.
The (very expensive) battery life will be shortened by cycling the battery repeatedly (minor, but unexpected). Say the solar is off (producing 10kW) and the battery runs down in the late afternoon. Just as the battery needs recharge after hours of full sun, it turns cloudy or dark and I have no overnight power. So, I could be caught with a dead battery after a full day of sun. Maybe I'm misunderstanding all this, but that's how it appears to me. I appreciate all the feedback so far.
After testing the system, I see the solar shuts down and the house runs off the battery with the sun producing more than 5kW (even in December, I'm generating 8-9kW). Then, the battery runs down a bit and re-charges. OK, not so bad but for two things.
The (very expensive) battery life will be shortened by cycling the battery repeatedly (minor, but unexpected). Say the solar is off (producing 10kW) and the battery runs down in the late afternoon. Just as the battery needs recharge after hours of full sun, it turns cloudy or dark and I have no overnight power. So, I could be caught with a dead battery after a full day of sun. Maybe I'm misunderstanding all this, but that's how it appears to me. I appreciate all the feedback so far.
Once the battery drops below about 96% solar will turn on again, so it remains almost full during an outage, provided solar is producing. Cycling the batteries is not harmful, they are designed for that. I use mine for load shifting, and go between 100% and 50% daily.
The first time on backup, mid-day, battery dropped to 80% before charging. Thereafter, it charged at 96%. I thought it didn't charge at 80% because the solar was at 9kW. When the solar dropped to 6kW, it started charging and re-charged later at 96% as the sun was dropping. Hence my concern about getting caught with no battery with max sun out.
You're correct. Because your solar is oversized for 1 PW, it's not going to charge until the solar is below the threshold that 1 PW can accept (minus any house loads). You really should consider taking one of your inverters off of the backup circuit, so that during an outage you can keep the PW topped off.
charlesj
Active Member
The micros he has already is on a 20A breaker or two someplace in the system. Unfortunately it would be a manual process to turn it off or the PW could as needed.
I have 38 x 320kW panels = 12.16kW system. I'll have to confirm this, but believe they are on three circuits as follows:
13 x 320 = 4.2kW
19 x 320 = 6.1kW
06 x 320 = 1.9kW
Tesla says 1 PW per 7.6kW. If I put the 6.1kW circuit on backup, obviously within limit. If I put 6.1kW + 1.9kW = 8.0kW, that would just be slightly over the 7.6 limit.
Everybody feel free to pipe in. Should I take an inverter or two off the backup? If so, which way should I go? Thanks.
I think you're using the DC panel rating figure of 320W per panel. The Tesla figure is AC inverter watts. So, the 8.0kW of the two strings should be fine. I'm guessing you have the 600W dual panel micros, so that's 300 AC Watt per panel, so those 25 panels would be 7.5kW. That should work. Just move the breaker for the 13 panel string outside the Tesla Gateway.
charlesj
Active Member
Curious what your house consumption is during the days especially when you have good sun and solar production.
In my limited thinking if the PW charge is 5 kW + house needs at that time would be the point where the solar would be turned on again if it was over generation beforehand. Perhaps that is not the case although solar can both charge batter and supply house at same time.
Question is at what point would the solar be shut down. I suspect when it is over the needs of both, house and battery.
In my limited thinking if the PW charge is 5 kW + house needs at that time would be the point where the solar would be turned on again if it was over generation beforehand. Perhaps that is not the case although solar can both charge batter and supply house at same time.
Question is at what point would the solar be shut down. I suspect when it is over the needs of both, house and battery.
Only been at this a week or so. House consumption, per Tesla app, seems to range from 1-4kW, I'd say typically about 2-3kW. You are correct, it seems to shut down when the solar production exceeds the needs of the house & battery together.
Tesla suggested increasing consumption in that case. Guess I could invite the neighborhood over to charge their phones.
But you can still run the house in a power outage, correct? To me, where the power goes and how the Powerwall manages the solar and the house loads as well as charging are trivial issues. It sounds like perfect is getting in the way of a good solution. I understand the technical issues and it would be better if the GT inverter could curtail but it can only be on or off. Perhaps more updated inverters could modulate or curtail to make this less of an issue. I agree the Tesla installer did not explain it very well, but the PowerWall is doing what it does.
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As @Ampster notes another solution to this problem is to have your inverters reduce their power during an outage through the microgrid frequency. During an outage your Powerwall will set the local frequency slightly higher which your inverters will notice and "ramp down" production. The Powerwall will dynamically set the frequency - higher than spec when the state of charge is higher or net (production - consumption) is higher, at spec when either are not. This allows your solar production to increase and decrease as the Powerwall charges and discharges with the house load.
@SteveInCal Enphase call it "curtailment" or "ramp down" (https://enphase.com/sites/default/f...Considerations-AC-Coupling-Micros-Battery.pdf, page 6). My IQ6 inverters support this.
SolarEdge inverters all it "P(F) Power Frequency" (https://www.solaredge.com/sites/default/files/application_note_power_control_configuration.pdf, page 6).
@BIC1 I know you said you're maxed on your budget - unfortunately your YC500As don't support this while the newer YC600 model does. It's called "ramp rate" - https://usa.apsystems.com/wp-conten...C600-For-USA-User-manual_Rev1.3_2018-3-15.pdf page 19.
@SteveInCal Enphase call it "curtailment" or "ramp down" (https://enphase.com/sites/default/f...Considerations-AC-Coupling-Micros-Battery.pdf, page 6). My IQ6 inverters support this.
SolarEdge inverters all it "P(F) Power Frequency" (https://www.solaredge.com/sites/default/files/application_note_power_control_configuration.pdf, page 6).
@BIC1 I know you said you're maxed on your budget - unfortunately your YC500As don't support this while the newer YC600 model does. It's called "ramp rate" - https://usa.apsystems.com/wp-conten...C600-For-USA-User-manual_Rev1.3_2018-3-15.pdf page 19.
Aside from getting more Power Walls, another solution would be to break up the solar a bit onto different circuits.
For example, you have the entire system on the "house" side of the Powerwall, but if you were to move much of the solar to a different feed point (on the mains side of the Powerwall) with only the maximum amount of solar still on the "house" side, the feed-in limitation of the Powerwall could be mitigated automatically, without losing any feed-in to the power grid.
Whether or not this could be done legalle (code restrictions?) or if anyone would actually do it for you is another matter as it would certainly require a bit of rewiring and the addition of some more interconnect boxes - but it would probably be cheaper than another Powerwall.
I've seen systems that have two separate feed points where the solar connects to the grid (e.g. the "original" solar - and then additional solar, added later, put on a ConnectDER to avoid running afoul the 125% rule) so it *should* be doable.
CT
For example, you have the entire system on the "house" side of the Powerwall, but if you were to move much of the solar to a different feed point (on the mains side of the Powerwall) with only the maximum amount of solar still on the "house" side, the feed-in limitation of the Powerwall could be mitigated automatically, without losing any feed-in to the power grid.
Whether or not this could be done legalle (code restrictions?) or if anyone would actually do it for you is another matter as it would certainly require a bit of rewiring and the addition of some more interconnect boxes - but it would probably be cheaper than another Powerwall.
I've seen systems that have two separate feed points where the solar connects to the grid (e.g. the "original" solar - and then additional solar, added later, put on a ConnectDER to avoid running afoul the 125% rule) so it *should* be doable.
CT
I think that is the issue. My understanding is that older microinverters do not curtail or ramp down. They are either on or off. It might be less expensive for the OP to upgrade half his micro inverters than buy another PowerWall.
pgrovetom1
Member
I'm exploring designing a battery system to install in my garage that can soak up excess solar production while on backup and then discharge it at night reducing the PW2 discharge.
I'm trying to investigate the issues that are being discussed in this post since the system I want to build might solve the issues here.
Some questions:
What is the per-PW2 charging limit? Is it the 5KW output? If the PW2 has a low charging limit, it creates some problems. Solar Grid Tie inverters are current sources. That means they raises their output voltage until 100% of their capacity is being sunk by the grid. Since the grid is unlimited, a solar inverter can have all its power sunk so everything is ok. It never runs into the situation where the grid is disconnected and it is forced to push its output voltage into dangerous territory well above 240V. But when you add a PW2 into the picture ( or 2-4 of them), things get tricky. If the PW2s cannot sink 100% of the Solar Inverters power, the voltage will rise until the grid tie inverters are in over-voltage and will shut off. But the home usage sinks some of the power but it varies and may not work with all Solar Inverters. For example, Enphase M215 inverters are not split phase. They output 240V so 120V appliances cannot use M215 power directly. But many Solar Inverters do output split phase 120/120 so the home can soak up some power. That complicates the discussion in this thread.
Lets make some simple assumptions to illustrate. Lets say the maximum power a PW2 can soak up for charging is 5KW as in its output specification. Say an application has a real 6KW of Enphase M215 inverters on a single PW2. Lets say the home always draws at least 1KW. In this example, one might expect the PW2 to be able to accept 5KW always and since the home uses at least 1KW, it should be ok right? No because the Enphase inverters cannot power 120V home loads since its not split phase. That means the inverters will keep trying to raise their output voltage to force the grid to sink the power. But there is no grid in backup nor can the home plus PW2 soak it up. If the home plus the PW2 cannot sink 100% of the power produced, the Enphase inverters will shut down.
But if this example uses Solar Inverters that produce split phase, the home plus the PW2 will soak up the power and everything will work correctly. The PW2 will charge to 100% from the solar 6KW minus home 1KW = 5KW until its full and use the 60HZ frequency to shut the split phase capable Inverters down. Then when the PW2 is down to 96% or so it will return the 60HZ to exactly 60HZ and the inverters will come back on and the recharge will occur over and over.The PW2 will remain fully charged when it goes into backup so the user will get the full 13.5KWhr before full shutdown. If the user keeps the home usage under 13.5KWHr overnight, it should be ok.
Its gets more complicated with the new UL1740 inverters that have the capability to be partially throttled back but I doubt the PW2 can support the variable 60HZ or many people have the new spec inverters. So I believe the behavior of the PW system will vary depending on:
1) What can the PW2 sink while charging - 5KW or more?
2) Do the Grid Inverters on the solar have split phase or 240V only?
3) Their maximum solar power on the best sunny day.
I am having 3 PW2s installed:
1) PW2 #1 on one of my SMA Sunny Boy 7000 with about 6KW ( the SMA inverters support split phase)
2) PW2 #2 on another SMA Sunny Boy 7000 with another 6KW ( the SMA inverters support split phase)
3) PW2 #3 on a Enphase M215 system with 18 inverters at about 4KW ( the M215s don't support split phase)
My home always draws more than 1KW which can be powered by SMA split phase support.
Since the 60HZ from the PW2's is common, the PW2's cannot shut down one inverter at a time. If the 5KW per PW2 is a hard limit, I will have a problem if the sum of my 3 inverter outputs exceeds 5KW x 3 minus my home 1KW. So if I hit 6 + 6 + 4 = 16KW minus 1 KW for my home equal 15KW, it should be ok. I would be right on the limit limit if the 5KW charge limit is real. Fortunately my SMA inverters can power my homes 1KW split phase so my M215's shouldn't be a problem. It will be a bummer if on right on the edge and it shuts down at peak power in the summer.
This might explain some of the differences people see. Anybody considered adding say another 5-20KWHr of backup power to supplement the PW2's while in backup for EV charging or other high usage applications?
Thoughts
I'm trying to investigate the issues that are being discussed in this post since the system I want to build might solve the issues here.
Some questions:
What is the per-PW2 charging limit? Is it the 5KW output? If the PW2 has a low charging limit, it creates some problems. Solar Grid Tie inverters are current sources. That means they raises their output voltage until 100% of their capacity is being sunk by the grid. Since the grid is unlimited, a solar inverter can have all its power sunk so everything is ok. It never runs into the situation where the grid is disconnected and it is forced to push its output voltage into dangerous territory well above 240V. But when you add a PW2 into the picture ( or 2-4 of them), things get tricky. If the PW2s cannot sink 100% of the Solar Inverters power, the voltage will rise until the grid tie inverters are in over-voltage and will shut off. But the home usage sinks some of the power but it varies and may not work with all Solar Inverters. For example, Enphase M215 inverters are not split phase. They output 240V so 120V appliances cannot use M215 power directly. But many Solar Inverters do output split phase 120/120 so the home can soak up some power. That complicates the discussion in this thread.
Lets make some simple assumptions to illustrate. Lets say the maximum power a PW2 can soak up for charging is 5KW as in its output specification. Say an application has a real 6KW of Enphase M215 inverters on a single PW2. Lets say the home always draws at least 1KW. In this example, one might expect the PW2 to be able to accept 5KW always and since the home uses at least 1KW, it should be ok right? No because the Enphase inverters cannot power 120V home loads since its not split phase. That means the inverters will keep trying to raise their output voltage to force the grid to sink the power. But there is no grid in backup nor can the home plus PW2 soak it up. If the home plus the PW2 cannot sink 100% of the power produced, the Enphase inverters will shut down.
But if this example uses Solar Inverters that produce split phase, the home plus the PW2 will soak up the power and everything will work correctly. The PW2 will charge to 100% from the solar 6KW minus home 1KW = 5KW until its full and use the 60HZ frequency to shut the split phase capable Inverters down. Then when the PW2 is down to 96% or so it will return the 60HZ to exactly 60HZ and the inverters will come back on and the recharge will occur over and over.The PW2 will remain fully charged when it goes into backup so the user will get the full 13.5KWhr before full shutdown. If the user keeps the home usage under 13.5KWHr overnight, it should be ok.
Its gets more complicated with the new UL1740 inverters that have the capability to be partially throttled back but I doubt the PW2 can support the variable 60HZ or many people have the new spec inverters. So I believe the behavior of the PW system will vary depending on:
1) What can the PW2 sink while charging - 5KW or more?
2) Do the Grid Inverters on the solar have split phase or 240V only?
3) Their maximum solar power on the best sunny day.
I am having 3 PW2s installed:
1) PW2 #1 on one of my SMA Sunny Boy 7000 with about 6KW ( the SMA inverters support split phase)
2) PW2 #2 on another SMA Sunny Boy 7000 with another 6KW ( the SMA inverters support split phase)
3) PW2 #3 on a Enphase M215 system with 18 inverters at about 4KW ( the M215s don't support split phase)
My home always draws more than 1KW which can be powered by SMA split phase support.
Since the 60HZ from the PW2's is common, the PW2's cannot shut down one inverter at a time. If the 5KW per PW2 is a hard limit, I will have a problem if the sum of my 3 inverter outputs exceeds 5KW x 3 minus my home 1KW. So if I hit 6 + 6 + 4 = 16KW minus 1 KW for my home equal 15KW, it should be ok. I would be right on the limit limit if the 5KW charge limit is real. Fortunately my SMA inverters can power my homes 1KW split phase so my M215's shouldn't be a problem. It will be a bummer if on right on the edge and it shuts down at peak power in the summer.
This might explain some of the differences people see. Anybody considered adding say another 5-20KWHr of backup power to supplement the PW2's while in backup for EV charging or other high usage applications?
Thoughts
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https://www.tesla.com/sites/default/files/pdfs/powerwall/Powerwall 2_AC_Datasheet_en_northamerica.pdf
- The Powerwall inverter is rated for 5kw continuous but 7kW peak for up to 10s "off grid".
- The Powerwall inverter can handle 100% split phase imbalance.
pgrovetom1
Member
There is no technical reason the maximum charge rate is tied to the discharge rate. Discharge is the inverter power and should be limited by the heat dissipated by the PW2 inverter driving the home. That is why they say 7KW for 10 seconds as its not a hard limit. Its a heat thing. But when the heat gets too high, the PW2 should shut down its output.
But the maximum charge is based on the heat dissipated in the battery charger. Inverters are more complicated beasts than chargers and should have a little lower efficiency. I would expect the inverters in the PW2 to have an efficiency in the 95-96% range but the charger could more easily be 97-98%. So I would think the charger would be more tolerant. They may just spec them the same since its close. But what is the point where it shuts down on over-charge? If my 3 inverters put out another 1KW or 17KW at mid-day, would the charger overheat and shut down? I have spoken with 2 Tesla engineers neither mentioned this issue. But one put my Enphase inverters on the utility side of the PW2 Gateway and wanted me to use 2PW2s. I then asked to increase it to 3 PW2's and Tesla seems consternated - maybe this is why...
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