Should I upgrade to 6 Powerwalls?

[Gu$ G]

I have an order in to add 12 solar panels and 4 PW to my existing 44 panels and 2 PW. That is about 4.4 KW added to a ~17 KW spec'd 18 month old install. My goal is to increase energy storage capacity to help with retail arbitrage / cost-savings over a 10 year horizon. Off-grid resiliency for storms and outages as well as reduced wear and tear would be nice perks but not my main motivation.

We live in Oahu, Hawaii and have seen a huge savings from initial Tesla solar system installed about 16-19 months ago. I've calculated the original $50k system will pay for itself after less than 5 years based on our current driving habits with two Teslas.

In 2022 the Tesla app reports we generated 20.94 MWh and consumed 20.32 MWh. From my electric bill I can see that we consumed 1.9 MWh from the grid and sent 1.8MWh to the grid. In Oahu we currently pay $0.41/kwh and NEM pays us back only about $0.10/kwh. The cost of HECO rates have risen 30-40% over the past year alone.

We're looking for a cost effective, relatively simple way to make us of our solar before we give it away to HECO for such a discount. Right now we're on track to owe HECO about $2k for 2023 and we're still pretty restrictive in how we conserve / use energy!

Options include having a third Tesla (we currently drive two to work during daylight hours), adding up to 4 PW, or exploring the path of using a Tesla vehicle battery pack as a DIY PW. The estimate for the solar upgrade from Tesla is another $50k or so. At today's prices, I could buy brand new M3 for significantly less... Let alone build a DIY PW out of salvage Tesla packs...

 

[Jeremy3292]

You get a 30% federal tax credit on the powerwalls; you likely don’t get anything on a new M3. Of course I’m assuming your income is too high for any tax credit on the M3, which may not exist for the M3 after March anyways. Additionally, a DIY solution seems incredibly difficult and there’s zero warranty on that and may also void the warranties on your existing powerwalls and solar panels if you tie it into them. If your payback period is currently five years, the powerwalls and solar panels are warranted for far longer than that also. Sounds like the price of electricity in Hawaii is astronomical, so it seems like more solar and powerwalls is the right answer.

 

[dareed1]

I think you need to identify the nature of your energy flows in order to see if more storage is actually useful. I ran PVWatts on Honolulu and it shows that winter (3 months) generation is about 11% below the yearly average, while summer (4 months) is about 9% higher. The other months are quite close to average. Perhaps your actual generation pattern is quite different, but taking the PVWatts numbers suggests that the much of the observed 1.9MW import/1.8MW export difference is driven by seasonality. If so, then more PWs won't help much.

Just looking at the financial return, it seems that at current rates and $0.31 differential rate, the annual cost is about $600. Even assuming substantial rate increases, I think it will be difficult to justify a $50K investment if money were the only reason.

On the other hand, adding 25% more solar generating capacity without PWs would plausibly be $10K, but the increased capacity might be enough to minimize your consumption from the grid - again under the assumption that seasonality is the primary cause of importing power. Of course, you'll probably resent over-producing and not getting much compensation for it.

This is a large investment, so if it were me considering it, I would want to look carefully at the timing of importing power. Is the import every day or in short bursts of bad weather? In that case PWs can help. If the import is mostly in winter, then I suspect the PWs won't have much effect.

 

[sunwarriors]

I'm guessing charging the EVs is the main culprit. San Diego has higher energy rates than Hawaii now (we're at $0.519/kWh ($0.636 over baseline for winter)). We'll be seeing over $0.70/kWh - $0.80 this summer. Since you got your install recently, your terms were probably not as good as earlier solar adopters in Hawaii.

Downside for you is it sounds like you are already in the 1 : 0.25 rate. Similar to what's happening in NEM3.0, I don't think adding more PWs will help too much if the EV charging requires always tapping from the grid. Charging EVs just takes too much power and you'll just be constantly draining your powerwalls if you added more. Doesn't seem like PWs are the solution. Looking for ROI with ESS when there is very little is probably not the way to go I think. It's mostly to try to zero out your bill. Hawaii does have all sorts of incentives though.

Your goal should probably be to avoid using any energy at peak and enough PWs to cover on-peak timing.

If I was you, I would really look into when I charge the cars or where you are using the energy now. The bad thing in your case is you can't charge your Tesla's during the day due to having to drive to work. A lot of that excess generation is just wasted selling to the grid at $0.10 when it could be used to charge your cars. Maybe not helpful, but looking for a work from home job or simply using less or find a way to charge the cars during the day should save some $$ vs. getting low export credits.

In CA, drawing any grid power means in NEM2.0 means you're forced to pay non-bypassable charges (if that's a thing in Hawaii) as you'll always be charging at night during low peak rates now, but there are still fees.

I wouldn't bank on using another Tesla EV as a DIY project personally. Seems like a hassle/waste of $$/time and you have insurance costs with another car.

With Hawaii being so close even in traffic, maybe only charge on weekends during the day when you are at home may help if you're not doing that already.

 

[Powderkeg]

Your numbers to me would question whether you will generate enough power to run your house, cars, and then have residual to charge six batteries. I am running a 21Kw system with six powerwalls in Florida. With sunlight 100% of the daytime, I barely charge the six batteries to capacity each day. As expensive as Powerwalls are they are useless if not charged.

Each morning when I wake up the Powerwalls are between 20 and 30% (floor is set at 20%) and despite having a 21Kw system it is a struggle to fully charge them before the sun goes down. Although the days get longer during the summer months so does my home consumption since the two 5-ton air conditioners run almost constantly as the daily temperature increases.

Many on this forum assume that a 21Kw system provides 21Kw of capacity. That is simply not true. I have a solar roof (not panels) and they are mounted facing all four directions. Therefore, the most I can generate is about 15.5Kw peak since the sun cannot beat on all four sides at once.

I have a Tesla MS with FUSC so I rarely charge it at home.

 

[h2ofun]

Your numbers to me would question whether you will generate enough power to run your house, cars, and then have residual to charge six batteries. I am running a 21Kw system with six powerwalls in Florida. With sunlight 100% of the daytime, I barely charge the six batteries to capacity each day. As expensive as Powerwalls are they are useless if not charged.

Each morning when I wake up the Powerwalls are between 20 and 30% (floor is set at 20%) and despite having a 21Kw system it is a struggle to fully charge them before the sun goes down. Although the days get longer during the summer months so does my home consumption since the two 5-ton air conditioners run almost constantly as the daily temperature increases.

Many on this forum assume that a 21Kw system provides 21Kw of capacity. That is simply not true. I have a solar roof (not panels) and they are mounted facing all four directions. Therefore, the most I can generate is about 15.5Kw peak since the sun cannot beat on all four sides at once.

I have a Tesla MS with FUSC so I rarely charge it at home.

Yep, I have 7 PW's, on a 30KW system. Inverters limit the max to 22kw, but the most I have seen is 21kw. And that is in the summer.
I have been down to 5kwh for entire days with the storms, (185kwh in summer), and batteries still on storm watch. I have them like at 70 to 90% since they do not get charged back up now.

 

[MorrisonHiker]

Many on this forum assume that a 21Kw system provides 21Kw of capacity. That is simply not true. I have a solar roof (not panels) and they are mounted facing all four directions. Therefore, the most I can generate is about 15.5Kw peak since the sun cannot beat on all four sides at once.

It depends on your layout, hardware, location, etc. We have 20.5 kW and are already seeing peak generation of 18.8 kW in February. In May/June, at solar noon our system often exceeds 22 kW.

 

[Jeremy3292]

Would a 10-11 kW system (7.6 kW inverter) facing south be enough to charge 3 PW's or only 2 PW's?

 

[h2ofun]

Would a 10-11 kW system (7.6 kW inverter) facing south be enough to charge 3 PW's or only 2 PW's?

Nothing wrong with having more, but IMO, having the most solar you can get is better than more batteries

 

[Jeremy3292]

Nothing wrong with having more, but IMO, having the most solar you can get is better than more batteries

With TOU rates like I have, it is a delicate balance between production vs. storage as generating solar during off peak time doesn't get me that much unless I store it for use later. My utility has monthly netting for each TOU period with a sell price of $0.03/kWh if you produce more than you use for each TOU period. I'm also all electric with 4 ton AC/heat pump & water heater. The winter months dual peak is the real killer I think.

 

[Gu$ G]

Thanks for everyone's contributions. I think 6 PW is overkill. Does anyone know if there are technical limitations to upgrading from 2 to 6? I had one Tesla designer tell me they would probably need to do an extra inspection and consider extra equipment.

Also, it seems they are planning on installing this as a second system. I had preferred they tied both Tesla systems together so I have an integrated experience. Anyone have insight on this?

 

[Gu$ G]

As for the number of PW I could benefit from, here's a few thoughts I've had after reading some responses:

1. I charge almost exclusively at home. I was considering increasing PW storage to help recharge from the mileage we're driving during the day.

2. Other than the EV, our energy consumption is pretty low at night. 2 fully charged PW are adequate, 3 would be great.

3. I'm pretty convinced I need to reduce the number of PW I'm adding. I agree that increasing solar generation will help. I also agree that fully charging that many PW will be very infrequent/difficult, especially to justify their cost.

4. I'm considering a Ford F-150 with it's V2H solution. That would be a nice truck to have as a third vehicle and might be an option instead of adding up to 4 batteries I barely have room for.

5. A final reason I was looking at adding 4 has to do with Hawaii's current battery bonus program. They are reimbursing half the cost of a PW if you commit it's capacity to be consumed during a peak two hour window (6-8pm). The program is designed that this energy would be added to the grid. However, it also appears if you consume the energy yourself you meet the intent, get to use your energy for free, and get considerable rebates. I had made another post about this program for anyone interested in reading more.

 

[Jeremy3292]

If you have 6 PW's I imagine you would definitely need some sort of panel upgrade. Each Powerwall needs a 50 amp breaker per Tesla documents.

 

[jgleigh]

If you have 6 PW's I imagine you would definitely need some sort of panel upgrade. Each Powerwall needs a 50 amp breaker per Tesla documents.

Powerwall+ need a 50A breaker (battery + inverter). Normal Powerwall 2s only need a 30A breaker.

 

[Jeremy3292]

Powerwall+ need a 50A breaker (battery + inverter). Normal Powerwall 2s only need a 30A breaker.

Either way the point remains the same with 6 of them. You also will need more than one 7.6 kW inverter with the OP's solar size.

This does drive an interesting question though. Let's say you have 2 PW's: 1 Powerwall+ with the 7.6 kW inverter but no inverter on the 2nd Powerwall bc you don't need it for your solar size. Does that mean the 2nd Powerwall's power is less than than the one with the inverter? And if so, why would that be the case? The battery remains the same; one just has an inverter on top. The specs on Tesla's website for the Powerwall+ vs the Powerwall 2 lend me believe that is the case but I have no idea why...

 

[jjrandorin]

1. I charge almost exclusively at home. I was considering increasing PW storage to help recharge from the mileage we're driving during the day.

Planning on using energy in powerwalls to charge EVs is sort of like planning on using AA batteries to charge a D battery. The situation in Hawaii is a bit different since I think there are net metering concerns, but it doesnt make very much sense to me to PLAN to use energy from powerwalls to charge an EV, especially with dealing with conversion losses on top of the fact that most EVs are going to have the equivalent of 4-6 powerwalls worth of storage themselves.

4. I'm considering a Ford F-150 with it's V2H solution. That would be a nice truck to have as a third vehicle and might be an option instead of adding up to 4 batteries I barely have room for.

I would suggest you look very, very carefully if you decide to go that direction and expect to be able to actually use the V2H capability of that truck. It requires a special charging station that is installed by a solar provider, and given that you already have powerwalls, I suspect that most companies would look at your existing solar + powerwalls, and your request to integrade V2H on a Ford lightning into that and say "umm... yeah.... nahhh".

 

[jgleigh]

This does drive an interesting question though. Let's say you have 2 PW's: 1 Powerwall+ with the 7.6 kW inverter but no inverter on the 2nd Powerwall bc you don't need it for your solar size. Does that mean the 2nd Powerwall's power is less than than the one with the inverter? And if so, why would that be the case? The battery remains the same; one just has an inverter on top. The specs on Tesla's website for the Powerwall+ vs the Powerwall 2 lend me believe that is the case but I have no idea why...

The two datasheets don't list the exact same set of numbers, so it's hard to compare. I think they are identical and the Powerwall+ datasheet is combining the battery outputs and the inverter outputs together.

They both list 5.8 kVA max continuous power w/o sun.

 

[Jeremy3292]

The two datasheets don't list the exact same set of numbers, so it's hard to compare. I think they are identical and the Powerwall+ datasheet is combining the battery outputs and the inverter outputs together.

They both list 5.8 kVA max continuous power w/o sun.

I do wonder if you have the Powerwall+ with the inverter and another Powerwall (2 total), which specs the 2 batteries would take on? Bc they are tied together when installed, no? So they would have to have the same specs to be interoperable for load capabilities IMO.

My guess is if you have a Powerwall+ then all other Powerwalls you have are the same specs as the Powerwall+. Likewise if you have another brand solar inverter or no solar at all the specs are that of the Powerwall.

I think this sentence also tells the story too:

• "Powerwall+ is able to intelligently power heavy equipment, and direct solar integration improves solar recharging during power outages, resulting in a better off-grid experience than ever before."

So because of the seamless integration with the inverter, Tesla is able to "control the environment" better and thus confidently "unleash" more power safely. Otherwise Tesla essentially "throttles" the Powerwall due to the unknowns of how it is being used potentially.

How Powerwall Works | Tesla Support

www.tesla.com

 

[Wahiawahi]

Planning on using energy in powerwalls to charge EVs is sort of like planning on using AA batteries to charge a D battery. The situation in Hawaii is a bit different since I think there are net metering concerns, but it doesnt make very much sense to me to PLAN to use energy from powerwalls to charge an EV, especially with dealing with conversion losses on top of the fact that most EVs are going to have the equivalent of 4-6 powerwalls worth of storage themselves.

I would suggest you look very, very carefully if you decide to go that direction and expect to be able to actually use the V2H capability of that truck. It requires a special charging station that is installed by a solar provider, and given that you already have powerwalls, I suspect that most companies would look at your existing solar + powerwalls, and your request to integrade V2H on a Ford lightning into that and say "umm... yeah.... nahhh".

As someone who lives in Hawaii this is exactly what we are doing to avoid paying HECO $0.43 per kWh. We got our solar panels and 2 PW’s October 2022. A couple of weeks later we decided to order a M3. We received the M3 last month, and since then we have been charging it with our PW’s. I’ve been researching adding additional panels and PW’s to our system and signing up for the powerwall program HECO is offering. HECO has raised rates every month since July 2021, and their plan is to continue to increase rates to force everyone to get solar. The last thing we want to do is pay HECO hundreds of dollars a month to charge a car. Of course now that we have our first EV, we are looking at getting another one, which will require even more solar panels and PW’s.
Living in Hawaii forces you to use your PW’s every single day.

 

[sunwarriors]

As someone who lives in Hawaii this is exactly what we are doing to avoid paying HECO $0.43 per kWh. We got our solar panels and 2 PW’s October 2022. A couple of weeks later we decided to order a M3. We received the M3 last month, and since then we have been charging it with our PW’s. I’ve been researching adding additional panels and PW’s to our system and signing up for the powerwall program HECO is offering. HECO has raised rates every month since July 2021, and their plan is to continue to increase rates to force everyone to get solar. The last thing we want to do is pay HECO hundreds of dollars a month to charge a car. Of course now that we have our first EV, we are looking at getting another one, which will require even more solar panels and PW’s.
Living in Hawaii forces you to use your PW’s every single day.

I have batteries and don't see how people are charging EVs heavily with them. The math doesn't work. If you are empty and drive a ton in Hawaii (not common I'd guess), charging means you drain your PWs on your EV now, but you now need enough solar during sun hours to recharge your PWs to full to avoid peak. It's simple math where if you use too much energy, you won't have enough.

It's like that AA battery and D battery example. The M3 has 75/82 kWh? You will drain your 2 PWs without even charging 1 car, let alone 2.

We will pay $0.80+ this summer and already pay more than Hawaii so I always sorta laugh when I see Hawaiians complain about power rates now.

For EV charging, I think it's better to get a very large system so you can charge only on solar and never touch your batteries (save that to avoid peak rates) after they are fully charged. This won't work of course if someone simply drives too much.