Can solar only be used to charge powerwalls during a power outage?

[voip-ninja]

I have an existing 13 year old 7kw system and am in discussions to expand the system with another 3-4kw of capacity, installation of Tesla string inverter for the new smaller system and one or more power walls.

I plan to use the powerwalls for time of day load shifting as well as for emergency backup power during a power outage. We don't get a lot of power outages but we've had ones run for over 24 hrs.

I've read that the solar connected to the powerwall can charge the battery during an outage, but can that power also be used to feed the in-home grid? I'm a bit confused about this point.

With my current system as soon as the grid power is interrupted the system is disconnected, to protect the grid from being hot when it is being worked on.

Thanks

 

[bmah]

I have an existing 13 year old 7kw system and am in discussions to expand the system with another 3-4kw of capacity, installation of Tesla string inverter for the new smaller system and one or more power walls.

I plan to use the powerwalls for time of day load shifting as well as for emergency backup power during a power outage. We don't get a lot of power outages but we've had ones run for over 24 hrs.

I've read that the solar connected to the powerwall can charge the battery during an outage, but can that power also be used to feed the in-home grid? I'm a bit confused about this point.

With my current system as soon as the grid power is interrupted the system is disconnected, to protect the grid from being hot when it is being worked on.

Yes if you have Powerwalls, then during an outage, power from your solar system will first go to run your house loads...whatever is left after that will go to the batteries.

Our Powerwalls and solar (details in sig) have run our house throughout a 60-hour outage, despite the fact that they weren't particularly designed to do that.

Bruce.

 

[voip-ninja]

Yes if you have Powerwalls, then during an outage, power from your solar system will first go to run your house loads...whatever is left after that will go to the batteries.

Our Powerwalls and solar (details in sig) have run our house throughout a 60-hour outage, despite the fact that they weren't particularly designed to do that.

Bruce.

Thanks for the info Bruce, that's good news, because I would probably have enough solar capacity most of the time (non winter) to run things in a limited fashion indefinitely with that being the case.... solar during the day charging batteries and running my stuff and then powerwall keeping things going at night.

Only would really be a problem in winter when the solar panels might not be producing after a storm unless I was willing to go clear them off.

 

[jjrandorin]

Thanks for the info Bruce, that's good news, because I would probably have enough solar capacity most of the time (non winter) to run things in a limited fashion indefinitely with that being the case.... solar during the day charging batteries and running my stuff and then powerwall keeping things going at night.

Only would really be a problem in winter when the solar panels might not be producing after a storm unless I was willing to go clear them off.

I think @MorrisonHiker is in your state, and they have a large system. I believe they also had one system and then added onto it so might have some input on how that went, etc.

 

[daniel]

I have solar and Powerwalls. During power outages, everything seems to operate normally. As far as I know, the solar panels are running the house and charging the batteries until the batteries are full. At night the batteries are running the house. In fact, the only way I know there's an outage is at night the street lights are out. Also, before I got fiber optic for internet, the cable internet would go out if there was no grid power to the block.

CAVEAT: I don't understand how any of this stuff works. I just know that when the grid is down, I have power. I think that the solar panels shut off for several minutes when the grid goes down, before turning back on again. I have an over-sized system because it was more important to me to minimize carbon-based grid power usage than to minimize cost. This may (?) affect how my system operates.

 

[charlesj]

Thanks for the info Bruce, that's good news, because I would probably have enough solar capacity most of the time (non winter) to run things in a limited fashion indefinitely with that being the case.... solar during the day charging batteries and running my stuff and then powerwall keeping things going at night.

Only would really be a problem in winter when the solar panels might not be producing after a storm unless I was willing to go clear them off.

One thing to remember is that you should monitor what the house is using that you may not really need. Hard to know how long the outage and what the weather will be tomorrow.

 

[voip-ninja]

One thing to remember is that you should monitor what the house is using that you may not really need. Hard to know how long the outage and what the weather will be tomorrow.

Yes, I was advised I would need several (probably 3-4) power walls to be able to run everything for a day or two during a long term outage. Run everything means using major electric appliances like electric oven, induction cooktop, clothes dryer, etc.

Because I work from home and do network security I have a pretty large continued usage (say 200-300 watts), another thing I could do during a longer term outage is disconnect/shutdown some of that equipment to further stretch my runtime on batteries.

With two power walls we should be able to limp around for a day or two with no solar production if we very much limited our electrical usage... or if I was willing to clear snow from the panels if the outage occurred during the winter time and after a storm when the panels were covered and not generating power.

 

[voip-ninja]

Another interesting thing I've learned in my research is that Xcel Energy will provide a rebate that covers about 30% of the cost of power walls but in return Xcel gets the ability to control the PowerWalls ... so that during peak usage they can tell your PowerWalls to start delivering output power to feed the grid to help with load.

They won't allow your PowerWall to go below 40% when this is done but I'm not sure how I feel about it... as peak demand could also coincide with an outage occurring after your batteries have been partially discharged.

Thousands of dollars in rebates though.

 

[Matt-FL]

Tesla recommended two when I was building out the system - based on my utility usage. I went with four, and honestly now that I've been using them for the past nine months, two would most definitely not have cut it. Four is about the right amount to live normally during an outage (for those rare times that the AC, water heater, stove are all running at the same time. If I did have an extended outage, I'd reduce circuit loads to compensate. Under normal circumstances, if I have two cloudy days in a row, I'm out of juice with four powerwalls and start pulling from the grid.

 

[jgleigh]

Tesla recommended two when I was building out the system - based on my utility usage.

Tesla's recommendations only seem to work with zero clouds and spring solar loads. Real-world usage will drain your batteries quickly with a few cloudy days.

 

[daniel]

Tesla recommended two when I was building out the system - based on my utility usage. I went with four, and honestly now that I've been using them for the past nine months, two would most definitely not have cut it. Four is about the right amount to live normally during an outage (for those rare times that the AC, water heater, stove are all running at the same time. If I did have an extended outage, I'd reduce circuit loads to compensate. Under normal circumstances, if I have two cloudy days in a row, I'm out of juice with four powerwalls and start pulling from the grid.

Another downside to allowing your Powerwalls to feed the grid during peak times is that the batteries will be discharged lower every day, shortening their useful life. Whether the 30% subsidy compensates for that is beyond me to estimate.

 

[charlesj]

Yes, I was advised I would need several (probably 3-4) power walls to be able to run everything for a day or two during a long term outage. Run everything means using major electric appliances like electric oven, induction cooktop, clothes dryer, etc.

...

Then you may need more panels to get it charged enough to matter having 4.

 

[voip-ninja]

Then you may need more panels to get it charged enough to matter having 4.

I already have nearly 7kw on my existing system but I would need to replace my existing Fronius string inverter with a Tesla inverter

 

[ArizonaJon]

I already have nearly 7kw on my existing system but I would need to replace my existing Fronius string inverter with a Tesla inverter

I am not sure you need to replace your Fronius string with Tesla. I have 3 year old system with 2 PWs, a standard Tesla gateway, etc. But my inverter (not string) is Solaredge. The PWs take the same 240v AC via an AC to DC inverter that powers the house. You should be able to add PWs with the gateway, etc. to your current system.
In a power outage, my solar continues to run as long as the house and battery can take the energy. If the solar energy is more than the house and the battery can use, the Powerwall will turn off the solar until the energy can be used again.

 

[voip-ninja]

I am not sure you need to replace your Fronius string with Tesla. I have 3 year old system with 2 PWs, a standard Tesla gateway, etc. But my inverter (not string) is Solaredge. The PWs take the same 240v AC via an AC to DC inverter that powers the house. You should be able to add PWs with the gateway, etc. to your current system.
In a power outage, my solar continues to run as long as the house and battery can take the energy. If the solar energy is more than the house and the battery can use, the Powerwall will turn off the solar until the energy can be used again.

Ok interesting. Does the Tesla gateway have a circuit interrupter to shut the power off? My installer seems pretty up to speed so they can probably fill me in on options for doing this.

 

[bmah]

Ok interesting. Does the Tesla gateway have a circuit interrupter to shut the power off? My installer seems pretty up to speed so they can probably fill me in on options for doing this.

Gateway v1 does this by increasing the AC line frequency away from 60Hz as the battery approaches full during a grid-out scenario. The solar inverter will shut down past a certain threshold (seems to vary by manufacturer). I think newer systems (i.e. Gateway v2) do the same thing, someone else can probably speak to that better than I can.

Based on my experience during multi-day outages, to run indefinitely during a grid outage, you basically need enough daily solar generating capacity to run your house, and you need enough battery storage to run the house when there's no (or not enough) solar power. Given that our solar and Powerwalls weren't even sized with multi-day outages in mind, I'm surprised it all worked as well as it did. I agree and reiterate that if you're faced with an outage of unknown length, you decide or learn pretty quickly what loads you can do without (for us the priorities were the refrigerator, minimal lighting, minimal HVAC, and partial IT infrastructure...YMMV obviously).

Bruce.

 

[charlesj]

As an addendum, not that long ago, certainly this year, we had power out for 76 hours.
We started with 60% state of charge, unfortunately. Then cable went out as well, no internet, nothing.
Had to log into Gateway frequently to see how much battery we had left, recharged so we didn't run the battery into nothingness.
It was a close call.
Obviously, weather didn't cooperate enough so we didn't have to monitor it closely.
Next time may be worse.
I have 2 batteries with an almost 4kW system. Cannot install more panels, not because of roof real-estate.

 

[Jeff Hudson]

e sure to factor the cost of any rewiring into your decision to go with one or two Powerwalls.

Thank you, that is a good observation that I will take into consideration and it is helping me to make this decision.

 

[Matt-FL]

I already have nearly 7kw on my existing system but I would need to replace my existing Fronius string inverter with a Tesla inverter

I don't think 7kw will be enough to recharge four powerwalls and provide sufficient power for your house. If you had full sun generating 7kw for 8 hours, you are just filling 56kWh of 4 powerwalls, without leaving any extra for house loads.

 

[bmah]

I don't think 7kw will be enough to recharge four powerwalls and provide sufficient power for your house. If you had full sun generating 7kw for 8 hours, you are just filling 56kWh of 4 powerwalls, without leaving any extra for house loads.

While your math is correct, I would look at things differently. The whole point is to be able to run the house off of solar. The solar power first goes to satisfy the house's instantaneous needs. Whatever's left goes into the Powerwalls. But the goal isn't to fill the Powerwalls all the way from empty to full, the idea is to fill them up enough to run the house when there's no sun on the panels, which might be less than that.

I'd start with whether the solar system can generate enough energy in a day to run the house. Then see if the Powerwalls can hold enough to run the house overnight. Extra solar or extra batteries help (in different ways) by giving you some reserve for cloudy days, unforeseen load, etc.

Bruce.