Wondering about generator use with Solar Roof and Powerwalls

[GregBallantyne]

I've got a fairly simple system, Solar Roof feeding single Gateway thru two inverters, with Powerwalls. Grid power, inverters, and Powerwalls connect to Gateway, and the Gateway feeds the load center thru the 200A main breaker. It's been in operation since mid June of 2023. Grid power outages are common here, so before getting the solar system I used a generator during outages. The generator puts out a 30A 240V circuit, which feeds my load center bus thru a sub panel from the barn. I was in the habit of manually isolating from the grid to do this.

Yesterday, wind took out our power once more via downed trees interrupting the supply. The initial estimate from BGE was about 3 days for restoration, they had many outages from the large storm. I'm only generation about 12-16 kwh on a good day this time of year, so even tho I had charged my batteries to 100% before the montages I was not certain about how long an outage I could weather. It only ended up a 17 hour outage, and we didn't run the heat pump, just ran backup propane heat. Got decent sun today, and might have made 3 days or more with low use and decent generation.

But.... I began to think about running the generator. I thought of two possible ways of doing this:
1. Wait until the powerwalls go low, throw the main to isolate from the Gateway, and use generator power as before, or
2. Run the generator at night, depending on the Gateway for grid isolation, to power low-level demand and charge Powerwalls

I don't know if that second method is actually feasible. Does anyone have any suggestions or opinions?

 

[CrazyRabbit]

I took easy route, bought 12kw dual fuel generator, 400lbs propane, and installed generator interlock kits on both panels with receptacles. But would much rather have a authorized means of charging powerwalls.
If tesla would make a box that was connected to gateway, with a disconnect switch and CT’s, sinking local grid to generator and then putting a controlled load on generator. That would be perfect.

 

[jhn_]

But... if I throw the main, I can't charge Powerwalls, because I'm also isolated from the Gateway and Powerwalls,. I think that method is feasible - but the ability to charge Powerwalls from generator power is a real advantage. I'd rather charge Powerwalls at night via generator, and run the house during the day with Powerwalls. I can get 90A from them, start heat pump, etc...
Or as you suggest connect the generator to the Gateway as another "grid". Too much modification for what I'm thinking about I suppose, but could certainly be an answer. I think there are installers out there who do exactly that sort of thing. I'm hoping to use what's currently in place. My number 2 method may not be feasible.

Like @nyprepper said, you can’t charge the PWs in this setup. But this is the setup supported by Tesla. I have the same setup.

In four years I have not had to run the generator and all grid down situations have been navigable without just the PWs we have. My generator transfer switch is setup to shift individual breakers loads between “grid” (in this case power from the gateway which could be grid, solar, battery, or a mix) and generator. So in a prolonged grid outage if solar and battery were not enough I would shift some loads to generator while some loads still were gateway fed, essentially creating two isolated systems powering the house.

 

[miimura]

If you have essential loads that you want to selectively power, there are other solutions. One option is a wall mounted transfer switch like the APC UTS-10BI. I have one of these and it’s a good solution for selective loads. It wires in next to an electrical panel and each circuit has a 3-way selector relay that can switch between Grid / UPS / Generator. It can do round robin load shedding so that you can power things like a refrigerator and chest freezer alternately that may overload if run simultaneously. The model I have has a 30A 240 V generator input, a 15A 120V UPS input and can power 8 120V circuits and one 240V circuit.
Since I have PowerWalls I am not using most of the features. I really only use it for central distribution of UPS power to critical lighting, the outlets in my home office and my structured wiring box where my cable modem, router and Ethernet switch are.

 

[miimura]

I've been reading about things called Chargeverter's in the DIY solar forums, they're generally discussed in the context of using a gas/propane generator to provide backup to solar generally in off-grid solar/battery scenarios. They can take modified sine wave output from cheap generators, and are not necessarily that expensive. However, since they're more for off-grid batteries, they're designed more for 12V/24V/48V DC output since that's what DIY battery banks typically use.

So not sure if they provide the proper DC voltage and amps to feed into the OP's solar inverters, which the specs are not specified. And in any case some wiring would need to be modified to decouple the solar roof and couple the generator to the inverter DC inputs.

If you have a 48VDC nominal battery bank there are lots of solutions for generator support. In fact, most hybrid battery inverters have this function built in with automatic generator start, etc.

 

[GregBallantyne]

If you have essential loads that you want to selectively power, there are other solutions. One option is a wall mounted transfer switch like the APC UTS-10BI. I have one of these and it’s a good solution for selective loads. It wires in next to an electrical panel and each circuit has a 3-way selector relay that can switch between Grid / UPS / Generator. It can do round robin load shedding so that you can power things like a refrigerator and chest freezer alternately that may overload if run simultaneously. The model I have has a 30A 240 V generator input, a 15A 120V UPS input and can power 8 120V circuits and one 240V circuit.
Since I have PowerWalls I am not using most of the features. I really only use it for central distribution of UPS power to critical lighting, the outlets in my home office and my structured wiring box where my cable modem, router and Ethernet switch are.

Actually, I'm beginning to wonder about the aging process effects on my brain function more than Powerwalls at the moment.... I should have realized a few things some time ago during this thought process.
There is another fairly obvious operating option available to me, one that requires no modification from my current configuration. During an extended grid outage during which there is insufficient generation, use option1 from my original post, until solar generation charges the Powerwalls. Then switch back to normal operation. During cold weather, heat with propane backup right on thru anyhow. All this is less likely to be required during warm weather due to much greater generation, but just in case, isolate during cooler nights and run off generator power to preserve battery.
Of course, this assumes that Solar Roof operates as usual while isolated from the house load center, and will charge Powerwalls. Should be an easy thing to test, and hopefully risk free as well - but any input on that part would be welcome. Could be an off-grid capable strategy, although not preferred, since it's heavy on the manual action requirement....

 

[CrazyRabbit]

I got two Big Buddy propane heaters that I can use inside. Creating heat inside is much more efficient then running a propane generator to heat the house.

 

[GregBallantyne]

I got two Big Buddy propane heaters that I can use inside. Creating heat inside is much more efficient then running a propane generator to heat the house.

I'm using a propane burner in the house for backup heat. Looks like a fireplace. Mounted in the spot where a fireplace was, before I gave them up (chimneys as well) to maximize Solar Roof. More BTU than my heat pump.... small house, heat pump is 2 ton.
The generator is gas powered - a larger portable, out in the barn. Sends 30A back thru the barn power supply.

 

[shs1]

I believe I started the first Tesla Powerwall with a generator thread in 2017 when we received our first Powerhall 2 and wanted to install it with our existing 18 kW Kohler propane generator. Tesla said, No way, no how, and we and our local installed negotiated with Tesla for many months before they agreed to let the installation go forward. I believe their main concern was that the Powerwall never “see” the generator as the Powerwall or Gateway would have no way of controlling the generator to keep it from overcharging the batteries. The attached diagram was accepted by Tesla. It involves another transfer switch that assures that the Powerwalls or Gateway never “see” the generator. While it is labeled as a manual transfer switch, it does work automatically if the Powerwall runs out of power or has a load that exceeds its capacity and fails. I also have a switch installed in the house with which I can switch or activate the transfer switch and start the generator. With this setup when the generator is powering the house, the generator can still see and be charged by our solar panels (assuming they are not covered in snow). This system has served me well for 6 years now through several outages that have lasted for more than a week. During said outages, I rely mainly on our fireplace for heat and although our Powerwalls (3 now) can run our geothermal heat pump, it will drain the batteries pretty quickly. No problem if our 11 kW of solar are up. Our strategy often is to use the Powerwalls overnight and run the generator for shorter periods of time as needed during the day. It sure would be nice to use the generator to charge the Powerwalls, but for now this works.

 

[h2ofun]

I believe I started the first Tesla Powerwall with a generator thread in 2017 when we received our first Powerhall 2 and wanted to install it with our existing 18 kW Kohler propane generator. Tesla said, No way, no how, and we and our local installed negotiated with Tesla for many months before they agreed to let the installation go forward. I believe their main concern was that the Powerwall never “see” the generator as the Powerwall or Gateway would have no way of controlling the generator to keep it from overcharging the batteries. The attached diagram was accepted by Tesla. It involves another transfer switch that assures that the Powerwalls or Gateway never “see” the generator. While it is labeled as a manual transfer switch, it does work automatically if the Powerwall runs out of power or has a load that exceeds its capacity and fails. I also have a switch installed in the house with which I can switch or activate the transfer switch and start the generator. With this setup when the generator is powering the house, the generator can still see and be charged by our solar panels (assuming they are not covered in snow). This system has served me well for 6 years now through several outages that have lasted for more than a week. During said outages, I rely mainly on our fireplace for heat and although our Powerwalls (3 now) can run our geothermal heat pump, it will drain the batteries pretty quickly. No problem if our 11 kW of solar are up. Our strategy often is to use the Powerwalls overnight and run the generator for shorter periods of time as needed during the day. It sure would be nice to use the generator to charge the Powerwalls, but for now this works.

View attachment 1008898

I do not understand your comment the generator can still see and be charged by the solar panels?

i have a similar setup, but I have 2 200 amp panels. Everything is wired like I have 2 houses. I have a ATS on each panel. So, if I lose power,

and the master ATS loses PW batteries, it turns on both ATS and both panels are run from the generator. At this point, the solar would I guess be dead since the inverters have no power left, since it usually comes from the grid of the PW's.

 

[wwu123]

I do not understand your comment the generator can still see and be charged by the solar panels?

In reading through the whole explanation, I think that was supposed to say "the Powerwall can still see and be charged by the solar panel". I think he's saying when the generator transfer switch is used, the generator is powering the whole house - but the Powerwall and solar can still see each other (but are isolated from the generator and loads) ....

 

[BGbreeder]

In reading through the whole explanation, I think that was supposed to say "the Powerwall can still see and be charged by the solar panel". I think he's saying when the generator transfer switch is used, the generator is powering the whole house - but the Powerwall and solar can still see each other (but are isolated from the generator and loads) ....

Yes. That is the only support configuration.

All the best,

BG

 

[shs1]

In reading through the whole explanation, I think that was supposed to say "the Powerwall can still see and be charged by the solar panel". I think he's saying when the generator transfer switch is used, the generator is powering the whole house - but the Powerwall and solar can still see each other (but are isolated from the generator and loads) ....

That is what I intended to say.

 

[GregBallantyne]

I believe I started the first Tesla Powerwall with a generator thread in 2017 when we received our first Powerhall 2 and wanted to install it with our existing 18 kW Kohler propane generator. Tesla said, No way, no how, and we and our local installed negotiated with Tesla for many months before they agreed to let the installation go forward. I believe their main concern was that the Powerwall never “see” the generator as the Powerwall or Gateway would have no way of controlling the generator to keep it from overcharging the batteries. The attached diagram was accepted by Tesla. It involves another transfer switch that assures that the Powerwalls or Gateway never “see” the generator. While it is labeled as a manual transfer switch, it does work automatically if the Powerwall runs out of power or has a load that exceeds its capacity and fails. I also have a switch installed in the house with which I can switch or activate the transfer switch and start the generator. With this setup when the generator is powering the house, the generator can still see and be charged by our solar panels (assuming they are not covered in snow). This system has served me well for 6 years now through several outages that have lasted for more than a week. During said outages, I rely mainly on our fireplace for heat and although our Powerwalls (3 now) can run our geothermal heat pump, it will drain the batteries pretty quickly. No problem if our 11 kW of solar are up. Our strategy often is to use the Powerwalls overnight and run the generator for shorter periods of time as needed during the day. It sure would be nice to use the generator to charge the Powerwalls, but for now this works.

I think that your setup is much the same as I am considering as a strategy for an extended winter grid outage - it sounds like when you run your generator to power your house, you isolated solar charges the Powerwalls without seeing the house.
Thanks for this confirmation.

 

[shs1]

In the summer I have been able to get through a 10-day outage while we were evacuated during a wildfire, without ever needing the generator. (Keep the refrigerator going!) But in the winter if the solar panels are covered with snow, it can be a problem to keep the Powerwalls charged. Last winter, we got 11 feet of snow in one storm and my panels were not only covered with snow, some of them collapsed! Once I got the snow off of them (just getting to them was a challenge) they were still working, but not a full capacity, so the generator was key to surviving last winter. Getting the propane tank filled during storms is a whole another side to this story. We were more or less without grid power for about 10 days that winter, but fortunately it did come back up briefly several times and I was able to use grid charing to top up my Powerwalls (3). What you don't want to do is to run the generator continuously as even at minimum load it will use a lot of propane. It is really too bad that one can't charge the Powerwalls from the generator as that strategy would make best use of the two technologies, a generator and batteries. However, overall, I am very thankful to have both, even without that ability.

 

[BGbreeder]

In the summer I have been able to get through a 10-day outage while we were evacuated during a wildfire, without ever needing the generator. (Keep the refrigerator going!) But in the winter if the solar panels are covered with snow, it can be a problem to keep the Powerwalls charged. Last winter, we got 11 feet of snow in one storm and my panels were not only covered with snow, some of them collapsed! Once I got the snow off of them (just getting to them was a challenge) they were still working, but not a full capacity, so the generator was key to surviving last winter. Getting the propane tank filled during storms is a whole another side to this story. We were more or less without grid power for about 10 days that winter, but fortunately it did come back up briefly several times and I was able to use grid charing to top up my Powerwalls (3). What you don't want to do is to run the generator continuously as even at minimum load it will use a lot of propane. It is really too bad that one can't charge the Powerwalls from the generator as that strategy would make best use of the two technologies, a generator and batteries. However, overall, I am very thankful to have both, even without that ability.

Wow! Your site took quite the hit. Out of curiosity, how deep did those Sonotube foundations go?

Yes, I do wish Tesla would enable generator charging. It would make much better use of the generator and batteries. I would happily forego the ITC/IRA funding for that feature. The SHTF often enough in California that is a real issue.

All the best,

BG

 

[shs1]

I don't know how deep the Sonotubes went for sure. The guys who put them in used a Bobcat with an auger to get down 3 feet or so from what I saw. We had to remove all the panels and I used my tractor to pull them back up straight. Now all tubes are doubly diagonally braced, so they should be good this winter. And I now has a snowblower that will allow to be cut a trail from the house to the panels and remove the snow from their base so that the snow rake has someplace to put the snow.

 

[wwu123]

I'm not an EE but I can envision a kludge project for someone with more money than brains (and you would still need a lot of both).

Plug in an AC to DC power transformer to the generator capable of simulating the power output of solar panels and handling the maximum output of the generator.

Connect the transformer output to a solar inverter that "clips" at the maximum generator output.

Connect the inverter output to the Gateway just like any other "solar" generation system.

Not elegant or efficient but it might work. But be prepared to let the magic smoke out of something including the powerwalls, solar, or house.

The more I explore battery backup with DIY Powerwall approaches, the more it seems that having a gas/propane generator in the kit is very desirable for multi-day backup, whether the generator is the primary power source for the batteries, or the secondary/supplement to solar panels. The DIY hybrid/off-gridders are increasingly migrating towards this direction, the battery station / "solar generator" folks (e.g. EcoFlow, Anker) are starting to explore it.

The end-state of this thread was that the most popular grid-tie battery solution by far, Tesla Powerwall, does not support generator input, even clean pure sine-wave generators, so the closest compromise is basically a parallel-hybrid approach, a transfer switch to allow either running on solar/Powerall, or gas generator, but not in tandem in any way. Or Enphase supports off-the-shelf generators, but locked into proprietary ecosystem for the solar, battery and gateway portions.

But RKCRLR's statement above is exactly where the diy/hybrid/off-grid crowd has been migrating. Yes, as miimura pointed out, many hybrid inverters already have an AC generator input to make things easy off-grid. But many inverters died because they disliked the somewhat dirty power from gas generators, esp modified sine-wave ones. The key realization of the other communities is - we already have a solar inverter, who said the DC input side MUST come from solar panels? As long as the MPPT control algorithm on the DC "solar" input doesn't get confused, folks are sending in generator via AC-to-DC conversion into the solar inverter DC inputs, while conditioning and cleaning up the dirty generator power to protect their hybrid inverters at the same time. It's not really even the realm of experimentation at this point - you've got 120/240V switched-mode power supplies (SMPS) that output at 48-110V DC, or the latest and greatest, Chargeverters at 60V up to 5000W, people are doing this every day.

So back to this thread - sure, Tesla Powerwall doesn't support generators when off-grid, but they certainly support grid-tie solar inverters, specifically they support MULTIPLE grid-tie solar inverters, whether it is folks with multiple solar/string inverters, or dozens of micro-inverters, all while off-grid. So why NOT wire up another grid-tie solar inverter, totally permitted, but without solar panels?

A key insight is that this is more of a series-hybrid approach, with gas generator feeding/supplementing Powerwall batteries, you don't really need or want a large generator, nor a large inverter, like you would with your rooftop solar or a parallel-backup. You probably want something smaller, running at a higher-efficiency 50-80% load of the generator, that just needs to keep up with the AVERAGE not peak loads on the Powerwalls.

So looking at one of the smaller grid-tie inverters, a $500 3K Growatt, you could do up to 3000W at 93% AC-DC efficiency with:
-$500 3500W 240V modifed sine-wave generator AC > $400 Chargeverter > 3K Growatt > home microgrid > Powerwall

The one mismatch is the Chargeverter (along with most available rectifiers) maxes out around 60V, and the 3K Growatt has min input voltage is around 65-80V. Plus with a 14.3A limit, operating the Growatt in the low voltage range maxes out around 1100W output, though no reason you can't parallel 2 or 3 units. Or completely available off-the shelf, but perhaps only 80% efficiency
3500W generator > 2x SMPS 80-110V DC out (1500W max) > 3K Growatt > home microgrid > Powerwall

Or another approach would be multiple micro-inverters:
3500W generator > $400 Chargeverter 48-60V output > 4-6x microinverters > home microgrid > Powerwall

I don't think there's any "magic smoke" concerns with any of these, just like for the hybrid/off-grid crowd, it's just discovering which grid-tie string and micro-inverters MPPT inputs don't get confused when the DC input doesn't behave like a solar panel IV curve, but rather a constant voltage. And the ones that don't work won't get fried, they just may put out reduced or no power as the MPPT "hunts". We're talking about UL listed solar inverters, installed in intended grid-tie manner, so not like you have to do a run-around of permitting, I would think.

The costs of a series-hybrid kit are far less than installing a whole-home Generac for the parallel-hybrid approach, but even if comparing with a generator interlock and cheap generator with the parallel approach, the fuel-to-AC efficiency of the series-hybrid approach is going to be way, way higher than running the larger generator in parallel. Not to mention the convenience, you wouldn't even need to do any manual transfer switching, you could even just remote start/stop the small portable generator anytime you wanted to top up the Powerwalls when solar falls short, your additional grid-tie "solar" inverter would just automatically feed right into your micro-grid....

 

[sunwarriors]

Powerwall batteries, you don't really need or want a large generator, nor a large inverter, like you would with your rooftop solar or a parallel-backup. You probably want something smaller, running at a higher-efficiency 50-80% load of the generator, that just needs to keep up with the AVERAGE not peak loads on the Powerwalls.

Maybe I am in the minority on this, but my thinking and desire is actually a very large generator running for as short of a timeframe as possible to just charge my batteries back up. I don't want a loud/smelly generator to support constant 50-80% loads if they don't have to so it's purely to recharge batteries to go back to using quiet power.

Folks can go the DIY approach with cables/wires/connections/generator everywhere, but I doubt this will ever catch on. I do think more people will get those portable home batteries coming out like mad, but outside of rare camping or exhibits, etc, I don't think they will really catch on for the majority of home owners.

 

[wwu123]

Maybe I am in the minority on this, but my thinking and desire is actually a very large generator running for as short of a timeframe as possible to just charge my batteries back up. I don't want a loud/smelly generator to support constant 50-80% loads if they don't have to so it's purely to recharge batteries to go back to using quiet power.

Folks can go the DIY approach with cables/wires/connections/generator everywhere, but I doubt this will ever catch on. I do think more people will get those portable home batteries coming out like mad, but outside of rare camping or exhibits, etc, I don't think they will really catch on for the majority of home owners.

I'm with you on not running the generator constantly in a series-hybrid configuration, otherwise there's no point, you'd just run the house off a generator 24x7. Medium, large, I mean in relative terms to your otherwise generator-only sizing.

But when I gave example of a 3500W generator for series-hybrid, I was thinking of say a household with time-average load of 1000W on your Powerwalls, so you'd need to run the generator 1/3 of the time, and not overnight. If average load is 3000W, then maybe a 9000W portable generator. But these would typically be much smaller than what one would be using for whole-house backup, maybe 1/3 the size of the whole-house 11K or 22K Generac, that would be running 24x7 at <10% load just to support the occasional surge load.

I'm not advocating in this thread for people to go the DIY route or the battery station/portable generator route, I was merely applying lessons from them that might help solve the generator-to-Powerwall conundrum for the Powerwall crowd, which I thought was the topic of this thread.... the dead-end here was that you CANNOT use a very large generator to charge your Powerwall back up for even one minute, let alone to back to full.

(And BTW, even in other threads, I'm only exploring the battery station route for those who are looking for a 1/4 to 1/2 Powerwall solution in the next 5-year horizon anyways, above 1/2 Powerwall, it's cheaper and just better to go with one Powerwall. If I had not already installed rooftop solar 10 years ago, I would have gone the route of solar + 1 Powerwall, but my sunk costs (and corresponding sunk returns, which are good) are what they are.)

 

[mikesm]

PS I was wrong about 1 item:

1) The bi-directional inverter does 400V and 800V DC fast changing, not 200V and 400V