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Solar roof, powerwalls... and generators/reverse charging...

I will be building a house this coming spring.

All of our utilities will be electric (geothermal HVAC, heat pump water heater, etc).

My intention is to use a Tesla Solar Roof and 3-4 powerwalls as backup.

When the grid is online, I'd like to run off batteries from 2-6pm (peak pricing). Ideally, perfect world, they'd recharge off the grid form 12-6am (off peak pricing), and any solar I generate would either be sold to the grid or used.

When the grid is offline, obviously the batteries/solar would power everything. In an extended, low sun period (very possible in PA), I'd like to have two levels of backup:
1) a generator input, from my old 7kw propane generator TO RECHARGE THE BATTERIES (not power the house-- so I can run the generator at the most efficient load and just drain the propane tank into the battery bank.
2) reverse charging off a non Tesla EV.

... How much of what I'm hoping ot do will be possible with the Tesla solar roof/powerwalls?

Thanks!
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Moderator
Nov 28, 2018
15,063
19,452
Riverside Co. CA
Ideally, perfect world, they'd recharge off the grid form 12-6am (off peak pricing), and any solar I generate would either be sold to the grid or used.

Nope, not happening with a tesla system in the US.

1) a generator input, from my old 7kw propane generator TO RECHARGE THE BATTERIES (

Nope, not happening with a tesla system in the US that is grid tied at all (meaning connected to the grid in any way)

2) reverse charging off a non Tesla EV.

If by "reverse charging" you mean the powerwalls... not happening.

... How much of what I'm hoping ot do will be possible with the Tesla solar roof/powerwalls?

Almost none of it (with tesla equipment), based on your desires.

You likely need to look at some other vendors equipment.
 
Enphase has a generator tie in that's live now actually:
https://www.reddit.com/r/solar/comments/qr0n0u
Since I'm on Enphase batteries already and have the full smart switch, this is my plan if I ever decide to get a generator hooked up.

If you take the tax credit, I don't think batteries are allowed to charge from the grid outside of storm watch. Also, installers probably never set it up that way and would be very confused to do that.

Charging from an EV isn't really that common/possible yet. I'd wait to see how the F150 EV is implemented.
 
1) a generator input, from my old 7kw propane generator TO RECHARGE THE BATTERIES (not power the house-- so I can run the generator at the most efficient load and just drain the propane tank into the battery bank.

You may be able to do this one - however it would have to be on you to bring out another electrical contractor, and it is going to be a bit expensive.

What the Tesla Power Walls, Inverters and Gateway HAVE to see for them to connect to mains power, is clean 60Hz 240v coming in (the 60hz sign wave has to be stable). What you would have to do, is install a whole house transfer switch (200a Transfer Switch ~$700) , that would then switch mains power to an Inverter Type of Generator (99% sure your 7kW generator is not clean at all - I am thinking of a Honda EU7000iS - MSRP $5549) - so you would need one of those as well.

As long as Tesla/Power Walls see clean 240v coming in, it would then be able to bring up its inverters and sync to the generator output. From there it would be just like any other day that Tesla see's utility power coming in (but in your case it would be locally generated power). Your generator can then power up your batteries, or power your house, while your solar roof charges your batteries.

So I am guessing about $6500 in parts, with $x in labor for you to meet #1 above.
 
In addition to not being able to "do" charging of powerwalls from a generator, I don't know why you would want to do that.

Depending on the size of system, and time of year, you are not going to draw any energy from the grid from 2 - 6 as the sun is usually out from 2 - 6.

Four powerwalls (I have three) will not only make it from whenever the sun goes down until 6 (end of peak) but all night with a 20% reserve left over in the morning, especially for a new, efficient, build.

You may well be able to add a non-tesla ev as essentially an additional battery back up.

But the following things don't seem to add up to me.

1. Charging powerwalls from any source other than the sun (or the grid during stormwatch events). There is an energy loss running any electricity to the powerwalls. Running a car battery through a PW is like a 20% loss for no reason, maybe 10% each battery. Just use the non Tesla EV to power house loads.
Same thing for charging a battery with a generator. Why? You are just exchanging one form of storage (propane) for another (the PW). Maybe I don't understand how these large generators work.

2. Using PWs for utility rate arbitrage has been discussed alot here. I don't think the loss of the tax credit (even assuming you get Tesla to install it that way, which they don't) is made up for by rate arbitrage. Especially, again, because if you have a large enough system the 4 PWs will not need to recharge at night.

I suggest the following analytical structure.

A. Get the largest system you can.
B. The largest system you can get will overproduce like crazy, which is where you get 4 PWs to charge them from the sun (best, cheapest source) and use them during peak or all night.
C. Next best source of energy is probably middle of the night early morning low peak from the grid. Your large system will likely generate credits to use against this.
D. A generator.
E. The Grid.

The plan is to see if you can power the whole house for a much of the year as possible from A and B. I don't think you want to even use the generator.
 
  • Disagree
Reactions: buckets0fun
Rather than try to make the Tesla Powerwall system do what you want, I suggest one of the other vendors who build this into their systems.

Mojave
https://enphase.com/installers/storage/generator
FranklinWH - Whole Home Energy Solution

Typically the generator is used to cover those times when there is just too much cloud cover to get good PV generation. Also most people are tolerant of a generator during the day for some limited time, it's a nuisance overnight. You can't just run a 7k W generator all night with a few hundred watts of load on it, that makes no sense either.

If possible it would be nice to run the 7 kW generator full tilt and recharge 2-4 Powerwalls in something less than 6 hours during the day. Then the home can draw a few hundred watts from batteries all night, with spikes as needed for hot water, well pump HVAC etc.

There is no current Tesla-approved way to do this with Powerwalls while on a grid-tied property.
 

h2ofun

Active Member
Aug 11, 2020
3,740
903
auburn, ca
In addition to not being able to "do" charging of powerwalls from a generator, I don't know why you would want to do that.

Depending on the size of system, and time of year, you are not going to draw any energy from the grid from 2 - 6 as the sun is usually out from 2 - 6.

Four powerwalls (I have three) will not only make it from whenever the sun goes down until 6 (end of peak) but all night with a 20% reserve left over in the morning, especially for a new, efficient, build.

You may well be able to add a non-tesla ev as essentially an additional battery back up.

But the following things don't seem to add up to me.

1. Charging powerwalls from any source other than the sun (or the grid during stormwatch events). There is an energy loss running any electricity to the powerwalls. Running a car battery through a PW is like a 20% loss for no reason, maybe 10% each battery. Just use the non Tesla EV to power house loads.
Same thing for charging a battery with a generator. Why? You are just exchanging one form of storage (propane) for another (the PW). Maybe I don't understand how these large generators work.

2. Using PWs for utility rate arbitrage has been discussed alot here. I don't think the loss of the tax credit (even assuming you get Tesla to install it that way, which they don't) is made up for by rate arbitrage. Especially, again, because if you have a large enough system the 4 PWs will not need to recharge at night.

I suggest the following analytical structure.

A. Get the largest system you can.
B. The largest system you can get will overproduce like crazy, which is where you get 4 PWs to charge them from the sun (best, cheapest source) and use them during peak or all night.
C. Next best source of energy is probably middle of the night early morning low peak from the grid. Your large system will likely generate credits to use against this.
D. A generator.
E. The Grid.

The plan is to see if you can power the whole house for a much of the year as possible from A and B. I don't think you want to even use the generator.
A, yep. B. yep, c yep. D. yep. E. yep :)
 

h2ofun

Active Member
Aug 11, 2020
3,740
903
auburn, ca
Rather than try to make the Tesla Powerwall system do what you want, I suggest one of the other vendors who build this into their systems.

Mojave
https://enphase.com/installers/storage/generator
FranklinWH - Whole Home Energy Solution

Typically the generator is used to cover those times when there is just too much cloud cover to get good PV generation. Also most people are tolerant of a generator during the day for some limited time, it's a nuisance overnight. You can't just run a 7k W generator all night with a few hundred watts of load on it, that makes no sense either.

If possible it would be nice to run the 7 kW generator full tilt and recharge 2-4 Powerwalls in something less than 6 hours during the day. Then the home can draw a few hundred watts from batteries all night, with spikes as needed for hot water, well pump HVAC etc.

There is no current Tesla-approved way to do this with Powerwalls while on a grid-tied property.
When I had no batteries, and ran my generator, would run from like 4am to 6am, then 11 am to 1pm, and like 5pm to 7pm. Did all I needed for my house, and keep the refrig cold. I have no idea why anyone would run more than that, let alone all night long
 
Maybe they have a person in the house with medical needs that requires power 24/7
Of course, but the question was not “whether to have a generator in addition to a large system so there would be no risk of ever being without power,” the question, is about power/rate arbitrage including whether to charge PWs from the grid, the generator, or even a car.

Hence, the answers focusing on technical capability and how to structure use.

There have been posts about people with full systems and generator back up, inckuding the guy with ten Pws and windmills and a generator in Wyoming i think. I can’t recall one about actual experience during an outage where a generator backed up the system.
 
I have a 6.6kw array and two Powerwalls and also have a 5kw Yamaha gas genset that I can plug into a sub-panel that has essential circuits for my well pump, fridge, LED lights, and few outlets. The genset scenario for me comes into play with 3+ day grid outage with zero solar production due to immovable ice/snow/ice on the panels. I wish I could charge the Powerwalls with the genset in this scenario, but alas, it's not possible.
 
Does any manufacturer make a AC to AC "inverter"? What I mean is there a way to clean a non-inverter generator output to mimic inverter output from the solar panels?
Interesting question, I know products exist that do this, such as a Cyberpower Inverter UPS. In practice however I don't know of a large and economical version of this tech. It likely would need it's own battery or other small-scale storage to be able to perfectly clean up a dirty AC load such as a generator. They tend to be a bit wild on frequency, and the Gateway is pretty sensitive to a dirty grid.

This is probably better answered by a more classically trained EE.
 

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