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Can Powerwall contribute to homes amperage?

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Hi,
Main 3bd/2ba house is 100A main panel. I'm building a detached 2bd/2ba ADU (aka granny unit) in the back yard. Due to underground powerlines, it's super expensive ($20-30k) to upgrade to 200A main panel due to underground trenching to across the street and replacing existing PG&E powerlines)

I'm worried 100A won't be enough for both homes and cause breakers to trip. ADU will have a 60-80A subpanel. If I installed 3 Powerwalls, can it increase the home's amperage to 130-150A? Both homes will have 4kw solar (8kw total) to charge Powerwall during the day and I think the Powerwall can charge via the grid if necessary as well.

Thanks!
 
Hi,
Main 3bd/2ba house is 100A main panel. I'm building a detached 2bd/2ba ADU (aka granny unit) in the back yard. Due to underground powerlines, it's super expensive ($20-30k) to upgrade to 200A main panel due to underground trenching to across the street and replacing existing PG&E powerlines)

I'm worried 100A won't be enough for both homes and cause breakers to trip. ADU will have a 60-80A subpanel. If I installed 3 Powerwalls, can it increase the home's amperage to 130-150A? Both homes will have 4kw solar (8kw total) to charge Powerwall during the day and I think the Powerwall can charge via the grid if necessary as well.

Thanks!
Powerwall will NOT charge from grid. Only time it will do so is in Stormwatch.
 
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More accurately:

Powerwall & the Grid​

When Powerwall is installed without solar, it can charge from the grid if in Backup-only or Time-Based Control to support you during grid outages and to help you save money on your electricity bill. When Powerwall is installed with solar, it will not be able to charge from the grid.

I'm interested in this feature as well. On my boat I had Victron equipment and the term they use for this function is called "Power Assist". When the power demand is greater than the inverter's input amperage it will switch to Power Assist mode and draw power from the batteries to increase the output power.

That was never a problem for me because I had 50A of input shore power and probably not more than about 40A worth of devices on the vessel that I could throw at it. But I tested it by limiting shore power usage to 15A and watched the Power Assist work perfectly. It seems like it could be an excellent feature for older homes with lower supply connections.

All I'm finding online is other people asking the same question.
 
Hi,
Main 3bd/2ba house is 100A main panel. I'm building a detached 2bd/2ba ADU (aka granny unit) in the back yard. Due to underground powerlines, it's super expensive ($20-30k) to upgrade to 200A main panel due to underground trenching to across the street and replacing existing PG&E powerlines)

I'm worried 100A won't be enough for both homes and cause breakers to trip. ADU will have a 60-80A subpanel. If I installed 3 Powerwalls, can it increase the home's amperage to 130-150A? Both homes will have 4kw solar (8kw total) to charge Powerwall during the day and I think the Powerwall can charge via the grid if necessary as well.

Thanks!
What you are asking for is somewhat available from the Powerwalls via Import control but nothing you can count on for your load calculations for the ADU service.

I strongly recommend doing the service upgrade, 100A isn't enough for 2 modern houses, especially once car charging is a thing.

if you are building an ADU you already have high expenses, this will be one of them in order to do it right.
 
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What you are asking for is somewhat available from the Powerwalls via Import control but nothing you can count on for your load calculations for the ADU service.

I strongly recommend doing the service upgrade, 100A isn't enough for 2 modern houses, especially once car charging is a thing.

if you are building an ADU you already have high expenses, this will be one of them in order to do it right.
I might look into the cost difference between going to 400 amp service
 
400A services have a long lead time in my experience. It is certainly worth looking into, but probably not needed for a home plus an ADU unless there is heavy electrification and a need to charge multiple EV's at once. Most homes could charge 2 EV's just fine with a shared 40A circuit.
Well ADUs take a while to get permits for and construct. I wouldn't expect 400A service to take much longer than all of the other stuff that needs to happen, and if you're going to basically run two houses on one property and service line, I'd use a 400A service. Most 400A panels divide it into 2x200A (two separate main breakers) so you could do 200A for each building. Would also make it easy to submeter the ADU.
 
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I agree increasing service to the house is probably the way to go, but you could also look into a non-Tesla solution for power assisting. As I mentioned previously, I did exactly what you're hoping to accomplish with Victron hardware. I'm sure there are other options that could do this as well.
 
Thanks for your input. Will see how it goes with the Powerwalls first If I end up tripping breakers left and right, then will have to upgrade underground powerlines. My initial thinking was some people live off grid so must be doable. I'm shopping around for lower amperage appliances in hopes they will reduce chances of tripping the breakers.
 
Thanks for your input. Will see how it goes with the Powerwalls first If I end up tripping breakers left and right, then will have to upgrade underground powerlines. My initial thinking was some people live off grid so must be doable. I'm shopping around for lower amperage appliances in hopes they will reduce chances of tripping the breakers.
You're getting into a BIG trap here: Commitment Bias (Escalation of commitment) - The Decision Lab

Running a house plus an ADU off of a 100A panel is insane. Expecting Powerwalls to "save" you from overdrawing your main breaker makes the situation highly unpredictable. You do not have any control over when your tenants decide to cook or use the AC. Shopping for lower amperage appliances increases your "commitment" to this strategy further. Constructing an ADU is expensive; a service upgrade is peanuts in comparison. Furthermore, you're going to have plumbers, electricians, and all of the other contractors onsite already just to get the ADU built and plumbed and wired, and to install the solar and all of these Powerwalls. You'd get at least some discount on the upgrade because all of those people had to be on site anyway. If, after all of this effort, you're still tripping breakers, you're not only going to have frustrated tenants but you'll have to do the service upgrade anyway, at higher expense, and you'll also have replaced perfectly good appliances with useful life left in them (another expense). But at that point, you'll likely be in so deep that you'll try other strategies like pre-cooling your house or replacing your HVAC system, etc.

For the record, I believe a Powerwall is warrantied for 37800 kWh of energy cycled through it, and costs around $11k. That's 29¢/kWh in expenses just for the storage! I get that PG&E is a ripoff but the cost delta between off-peak and peak rates (ETOU-D) is only 4¢ most of the year and 12¢ during "summer" (June-September) and peak rates are only in effect for 3 hours/day 5 days a week..15 hours out of 168. Assuming the Powerwall assists you in avoiding peak costs, you'd have to consume 100,000 kWh during that time to have it pay for itself (except you can't, because it's likely that it won't last that long). If we assume that you do this over 10 years, there are roughly 7800 peak hours over 10 years on ETOU-D. Are you really going to be consuming 12.8 kW whenever peak hours roll around because that's what you'd have to do in order to hit that target. But wait, it gets better...the Powerwall can only supply 5kW max, so you'll need to extend that time frame to 25 years. Oops wait...that assumes you're drawing 5kW for 3 hours a day on weekdays. Can't do that either, as the Powerwall doesn't have 15kWh of usable energy in it. Gotta extend that timeline to closer to 30 years. But wait...we know that if batteries are cycled close to 100% every day, they won't last very long, so there's no way I'd expect the thing to even last 10 years under those usage conditions.

In addition, I expect storage costs to decline dramatically in the future in terms of what you're paying per kWh of storage while service upgrade costs will probably increase. In other words, I think you'd be best holding off on the solar and storage and spending the money on a service upgrade instead.

Look, I get that solar and storage is cool. It's the future. I get that people want to do the right thing for the environment. There's some value in that to a lot of people. But if you're a finances/numbers type of person, I just don't see how this stuff even remotely makes sense right now. The Powerwall is way, way too expensive for the amount of storage you get. The economics on EVs make a lot of sense, but the economics on Powerwalls, at their current prices, do not. As a comparison, the Tesla Model 3 long range has a usable battery capacity of ~75 kWh and costs about $55k. And you don't just get batteries for that amount of money, you get an entire car! In the world of Powerwalls, you'd be paying over $60k just to get 75 kWh worth of usable storage and that doesn't include the solar.
 
You're getting into a BIG trap here: Commitment Bias (Escalation of commitment) - The Decision Lab

Running a house plus an ADU off of a 100A panel is insane. Expecting Powerwalls to "save" you from overdrawing your main breaker makes the situation highly unpredictable. You do not have any control over when your tenants decide to cook or use the AC. Shopping for lower amperage appliances increases your "commitment" to this strategy further. Constructing an ADU is expensive; a service upgrade is peanuts in comparison. Furthermore, you're going to have plumbers, electricians, and all of the other contractors onsite already just to get the ADU built and plumbed and wired, and to install the solar and all of these Powerwalls. You'd get at least some discount on the upgrade because all of those people had to be on site anyway. If, after all of this effort, you're still tripping breakers, you're not only going to have frustrated tenants but you'll have to do the service upgrade anyway, at higher expense, and you'll also have replaced perfectly good appliances with useful life left in them (another expense). But at that point, you'll likely be in so deep that you'll try other strategies like pre-cooling your house or replacing your HVAC system, etc.

For the record, I believe a Powerwall is warrantied for 37800 kWh of energy cycled through it, and costs around $11k. That's 29¢/kWh in expenses just for the storage! I get that PG&E is a ripoff but the cost delta between off-peak and peak rates (ETOU-D) is only 4¢ most of the year and 12¢ during "summer" (June-September) and peak rates are only in effect for 3 hours/day 5 days a week..15 hours out of 168. Assuming the Powerwall assists you in avoiding peak costs, you'd have to consume 100,000 kWh during that time to have it pay for itself (except you can't, because it's likely that it won't last that long). If we assume that you do this over 10 years, there are roughly 7800 peak hours over 10 years on ETOU-D. Are you really going to be consuming 12.8 kW whenever peak hours roll around because that's what you'd have to do in order to hit that target. But wait, it gets better...the Powerwall can only supply 5kW max, so you'll need to extend that time frame to 25 years. Oops wait...that assumes you're drawing 5kW for 3 hours a day on weekdays. Can't do that either, as the Powerwall doesn't have 15kWh of usable energy in it. Gotta extend that timeline to closer to 30 years. But wait...we know that if batteries are cycled close to 100% every day, they won't last very long, so there's no way I'd expect the thing to even last 10 years under those usage conditions.

In addition, I expect storage costs to decline dramatically in the future in terms of what you're paying per kWh of storage while service upgrade costs will probably increase. In other words, I think you'd be best holding off on the solar and storage and spending the money on a service upgrade instead.

Look, I get that solar and storage is cool. It's the future. I get that people want to do the right thing for the environment. There's some value in that to a lot of people. But if you're a finances/numbers type of person, I just don't see how this stuff even remotely makes sense right now. The Powerwall is way, way too expensive for the amount of storage you get. The economics on EVs make a lot of sense, but the economics on Powerwalls, at their current prices, do not. As a comparison, the Tesla Model 3 long range has a usable battery capacity of ~75 kWh and costs about $55k. And you don't just get batteries for that amount of money, you get an entire car! In the world of Powerwalls, you'd be paying over $60k just to get 75 kWh worth of usable storage and that doesn't include the solar.
Great write up!
 
Great write up!
Just realized that the numbers are even worse than I estimated. The Powerwall's cost is $11-12k, but the 12¢/kWh peak to off-peak cost delta is only in effect for 4 months, or 1/3 of the year. The rest of the year, your savings are only 4¢/kWh, so this probably increases the payback time to well over 60 years. You can get tax breaks on solar systems in some cases, and PG&E might raise the rates and with them, raise the cost delta between peak and off-peak. But when the back of the envelope numbers look this bad, I just don't see how this ever reaches a point where you're getting a positive return on investment. Not to mention that you'll definitely be able to get more storage that doesn't degrade as quickly for a lot cheaper 5-10 years from now.
 
Just realized that the numbers are even worse than I estimated. The Powerwall's cost is $11-12k, but the 12¢/kWh peak to off-peak cost delta is only in effect for 4 months, or 1/3 of the year. The rest of the year, your savings are only 4¢/kWh, so this probably increases the payback time to well over 60 years. You can get tax breaks on solar systems in some cases, and PG&E might raise the rates and with them, raise the cost delta between peak and off-peak. But when the back of the envelope numbers look this bad, I just don't see how this ever reaches a point where you're getting a positive return on investment. Not to mention that you'll definitely be able to get more storage that doesn't degrade as quickly for a lot cheaper 5-10 years from now.
i tell folks all the time, that batteries are a total waste of money, unless you just have money to spend. Get as much solar as you want. And if the possibility of power outages freak you out, just get a generator. For 99% of folks, this would be fine.
 
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i tell folks all the time, that batteries are a total waste of money, unless you just have money to spend. Get as much solar as you want. And if the possibility of power outages freak you out, just get a generator. For 99% of folks, this would be fine.
Yeah, I have a small 2.4 kW, 120V generator. It's old and loud as hell. If the power goes out, it can power the fridge to keep my food from spoiling, it can power the toaster oven and air fryer and microwave so I can cook stuff, it will keep my internet service and computers operating, and it will keep me from being in the dark. Been thinking about replacing it with a quieter one, or getting a 240V split phase generator and transfer switch, but I typically use this thing 0-3 times per year for 1-6 hours each time. It just doesn't make sense to spend money on something I'll barely ever use when I already have basic backup power.

I never really thought about how out of line the pricing on Powerwall storage is vs. cars with batteries in them, but I can't blame Tesla for selling these things for what people are apparently willing to pay for them. When there aren't any more takers at $11k, I'm sure they'll lower the price to $10k, and when all of those folks have bought, they'll lower it to $9k and so on. They can still make a hefty profit given that they can make a car with 75 kWh of usable battery capacity and sell it for $55k after also manufacturing the car body, seats, computers, motors, etc.

The thing is, OP was talking about how a service upgrade that cost $20-30k is "super expensive" so it seems like the OP doesn't have money to spend. But then the OP talks about buying 3 Powerwalls which would cost about $33k, more than the high end of the service upgrade estimate. Except that service upgrade is good for as long as the OP owns the property, and the Powerwalls will likely wear out and have to be replaced in 10-15 years max, well before they produce a positive return on investment, and to make matters worse, they don't solve the actual problem. Plus, OP should be able to get a way better storage product for cheaper by waiting a few more years.
 
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You're getting into a BIG trap here: Commitment Bias (Escalation of commitment) - The Decision Lab

Running a house plus an ADU off of a 100A panel is insane. Expecting Powerwalls to "save" you from overdrawing your main breaker makes the situation highly unpredictable. You do not have any control over when your tenants decide to cook or use the AC. Shopping for lower amperage appliances increases your "commitment" to this strategy further. Constructing an ADU is expensive; a service upgrade is peanuts in comparison. Furthermore, you're going to have plumbers, electricians, and all of the other contractors onsite already just to get the ADU built and plumbed and wired, and to install the solar and all of these Powerwalls. You'd get at least some discount on the upgrade because all of those people had to be on site anyway. If, after all of this effort, you're still tripping breakers, you're not only going to have frustrated tenants but you'll have to do the service upgrade anyway, at higher expense, and you'll also have replaced perfectly good appliances with useful life left in them (another expense). But at that point, you'll likely be in so deep that you'll try other strategies like pre-cooling your house or replacing your HVAC system, etc.

For the record, I believe a Powerwall is warrantied for 37800 kWh of energy cycled through it, and costs around $11k. That's 29¢/kWh in expenses just for the storage! I get that PG&E is a ripoff but the cost delta between off-peak and peak rates (ETOU-D) is only 4¢ most of the year and 12¢ during "summer" (June-September) and peak rates are only in effect for 3 hours/day 5 days a week..15 hours out of 168. Assuming the Powerwall assists you in avoiding peak costs, you'd have to consume 100,000 kWh during that time to have it pay for itself (except you can't, because it's likely that it won't last that long). If we assume that you do this over 10 years, there are roughly 7800 peak hours over 10 years on ETOU-D. Are you really going to be consuming 12.8 kW whenever peak hours roll around because that's what you'd have to do in order to hit that target. But wait, it gets better...the Powerwall can only supply 5kW max, so you'll need to extend that time frame to 25 years. Oops wait...that assumes you're drawing 5kW for 3 hours a day on weekdays. Can't do that either, as the Powerwall doesn't have 15kWh of usable energy in it. Gotta extend that timeline to closer to 30 years. But wait...we know that if batteries are cycled close to 100% every day, they won't last very long, so there's no way I'd expect the thing to even last 10 years under those usage conditions.

In addition, I expect storage costs to decline dramatically in the future in terms of what you're paying per kWh of storage while service upgrade costs will probably increase. In other words, I think you'd be best holding off on the solar and storage and spending the money on a service upgrade instead.

Look, I get that solar and storage is cool. It's the future. I get that people want to do the right thing for the environment. There's some value in that to a lot of people. But if you're a finances/numbers type of person, I just don't see how this stuff even remotely makes sense right now. The Powerwall is way, way too expensive for the amount of storage you get. The economics on EVs make a lot of sense, but the economics on Powerwalls, at their current prices, do not. As a comparison, the Tesla Model 3 long range has a usable battery capacity of ~75 kWh and costs about $55k. And you don't just get batteries for that amount of money, you get an entire car! In the world of Powerwalls, you'd be paying over $60k just to get 75 kWh worth of usable storage and that doesn't include the solar.

Powerwall only makes sense for me by switching to EV2-A instead of TOU-C or TOU-D. there is a bigger delta between off peak and mid peak/peak. there is not enough difference between off peak and peak for TOU-C and TOU-D work due to the 90% roundtrip efficiency. a system without power wall would need to be sized bigger to offset peak rates, rates that that will continually be manipulated by PG&E to reduce solar benefits, so with a power wall one can get away with a smaller sized system. I'm still expecting a payback period of 6-7 years with power wall so I think it works for my situation. plus the power wall is just so cool to "complete" the system. I'm happy when I look at the Tesla app!
 
Powerwall only makes sense for me by switching to EV2-A instead of TOU-C or TOU-D. there is a bigger delta between off peak and mid peak/peak. there is not enough difference between off peak and peak for TOU-C and TOU-D work due to the 90% roundtrip efficiency. a system without power wall would need to be sized bigger to offset peak rates, rates that that will continually be manipulated by PG&E to reduce solar benefits, so with a power wall one can get away with a smaller sized system. I'm still expecting a payback period of 6-7 years with power wall so I think it works for my situation. plus the power wall is just so cool to "complete" the system. I'm happy when I look at the Tesla app!
Re-running the numbers with EV2-A (31.251¢ price delta between peak and off-peak): need to run $11000/$0.31251= 35199 kWh through the unit for break-even. Except that only applies during June-September. During the rest of the year, the price delta is just 18.54¢/kWh, and you'd have to run 59331 kWh through it. Realistically, it's probably about 2/3 of the way between the two, or around 50,000 kWh. But remember, the warranty is only good for 37800 kWh. Let's take the high end of your estimate, 7 years, which is about 2557 days (if there's two leap years in there, not that it really matters for this analysis though). 50000 kWh/2557 days=19.55 kWh/day. Wait a minute...the PW only holds 13.5 kWh of usable energy (plus if you use 100% of its capacity daily, it'll degrade rapidly and won't even be able to supply 13.5 kWh). So I'm not sure how you get a payback period of 6-7 years.

This analysis assumes, of course, that you have 19.55 kWh/day of load that you absolutely cannot shift to other times and must use the Powerwall to provide. If you are capable of shifting your usage, the payback time gets even longer.
 
I'm not too worried about the warranty, most likely the pw along with the panels will continue to work past the warranty period albeit diminished capacity plus there is also intrinsic value when selling the property down the road.

Your numbers are fine, but they don't account for escalating and unpredictable time of use rates that PG&E and other utilities will impose on consumers in the future. your numbers are a good starting point but in reality the math improves due to increasing rates. off peak rates alone have doubled in the past several years. Besides oversizing the system which is also costly, battery storage is the only way to counter time of use rates. my system (panels + pw) is expected to pay for itself in 6-7 years since I replaced all my gas appliances and it's offsetting $2.5k-$3k annual electricity bill and gas bill is poof. Had I didn't get a pw, I would have needed to install more panels to offset peak rates. to me it was worth it since I wanted to "complete" the system with panels and power wall :) plus, we haven't even talked about power outages and for that, the value placed on it is dependent on the individual and can't be assigned a value with math.
 
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I'm not too worried about the warranty, most likely the pw along with the panels will continue to work past the warranty period albeit diminished capacity plus there is also intrinsic value when selling the property down the road.

Your numbers are fine, but they don't account for escalating and unpredictable time of use rates that PG&E and other utilities will impose on consumers in the future. your numbers are a good starting point but in reality the math improves due to increasing rates. off peak rates alone have doubled in the past several years. Besides oversizing the system which is also costly, battery storage is the only way to counter time of use rates. my system (panels + pw) is expected to pay for itself in 6-7 years since I replaced all my gas appliances and it's offsetting $2.5k-$3k annual electricity bill and gas bill there is no longer there. Had I didn't get a pw, I would have needed to install more panels to offset peak rates so the difference was several thousand dollars. to me it was worth it since I wanted to "complete" the system with panels and power wall :)
IMO, I just did it brut force. I have so much solar, I could care less about rates, peak, non peak, etc. I just enjoy my nice warm house in the winter, and cool in the summer
 
IMO, I just did it brut force. I have so much solar, I could care less about rates, peak, non peak, etc. I just enjoy my nice warm house in the winter, and cool in the summer

unfortunately I have a limited budget so my system is smaller than I would have wanted, but it should be enough for 1 EV plus all home usage for now :). we don't have AC but we may add it in the future...

great setup you got there!!