Powerwall 2 - How large of the PV system do I need to charge it? Are they in stock now?

[who177e]

Hi All

Looking to install a PW2 with a new PV system.
Are current usage is only about 12 - 18kWh a day but it will double when we get an EV. Most of our usage is at night. I am wondering what size PV system do I need to charge 1 PW2 during the day so that I can use that power at night to charge the EV?

Also, my Tesla sales rep told me that they have Powerwalls back in stock and there are no more delays. I'm not sure If I believe him. Can anyone confirm if this is true? I am trying to get the PV system with a PW installed by before the end of the year.

Thanks.

 

[bradbissell]

I've been waiting since March '18 for an install, with no communication from Tesla Energy since I signed the final paperwork. I was told by the sales team (and written on my sales agreement/permit) that my install would be in May or June, but obviously that didn't happen. I wouldn't necessarily believe that Powerwall 2's are in stock at this moment. Sales reps will say a lot of things to make a sale.

 

[ewoodrick]

The amount needed to charge the PW2 depends on the amount that you discharge it. The amount that you discharge it depends on how much the house uses during the evening. The amount needed to charge the car depends on the amount of miles that you drive daily.

Depending in the car, it will be about 250W/mile. 100 miles * 0.25kWh = 25 kWh. You probably want to double the number to be safe.

 

[who177e]

I've been waiting since March '18 for an install, with no communication from Tesla Energy since I signed the final paperwork. I was told by the sales team (and written on my sales agreement/permit) that my install would be in May or June, but obviously that didn't happen. I wouldn't necessarily believe that Powerwall 2's are in stock at this moment. Sales reps will say a lot of things to make a sale.

Wow that is very unfortunate.
I'm hoping that because I am in Northern California that this will not be the case but communication with Tesla Energy so far has not been the best....

 

[who177e]

The amount needed to charge the PW2 depends on the amount that you discharge it. The amount that you discharge it depends on how much the house uses during the evening. The amount needed to charge the car depends on the amount of miles that you drive daily.

Depending in the car, it will be about 250W/mile. 100 miles * 0.25kWh = 25 kWh. You probably want to double the number to be safe.

I would assume that the battery will be fully discharged at night with charging the EV. My commute is 75 miles per work day and I am planning on getting a model 3.

 

[miimura]

I would assume that the battery will be fully discharged at night with charging the EV. My commute is 75 miles per work day and I am planning on getting a model 3.

Why do you want to discharge one battery into the other?

You should get an estimate for a solar system sized to offset your utility bill. Not your total kWh usage, but your energy charges. Powerwalls will allow you to ride through an outage and avoid paying the high Peak prices for your evening electric usage. EV charging should always be from the grid overnight. This is the benefit of Net Metering.

 

[who177e]

Why do you want to discharge one battery into the other?
@miimura
You should get an estimate for a solar system sized to offset your utility bill. Not your total kWh usage, but your energy charges. Powerwalls will allow you to ride through an outage and avoid paying the high Peak prices for your evening electric usage. EV charging should always be from the grid overnight. This is the benefit of Net Metering.

So my initial thought was to make sure our PV size was enough to cover our projected monthly usage. I think once we get the EV it will be about 900kWh a month. We are currently using about 450kWh a month now with an EV ($40 per month).
The plan was also to get a PW2 so that we can store the excess energy that the PV produced during the day, so that we can use that stored energy at night when we get home and also charge the EV with it. So by morning the PW would be close to empty but then when the sun comes out it gets charged up full again. I wanted to make sure that our PV system was large enough, based on our day time usage, to feed the house and also completely charge up the PW before we get home.

We do have PG&E net metering but don't have a clear understanding about what rates we will be actually getting a credit on and how that relates to when we use the energy.

 

[MorrisonHiker]

So my initial thought was to make sure our PV size was enough to cover our projected monthly usage. I think once we get the EV it will be about 900kWh a month. We are currently using about 450kWh a month now with an EV ($40 per month).
The plan was also to get a PW2 so that we can store the excess energy that the PV produced during the day, so that we can use that stored energy at night when we get home and also charge the EV with it. So by morning the PW would be close to empty but then when the sun comes out it gets charged up full again. I wanted to make sure that our PV system was large enough, based on our day time usage, to feed the house and also completely charge up the PW before we get home.

We do have PG&E net metering but don't have a clear understanding about what rates we will be actually getting a credit on and how that relates to when we use the energy.

Depending on your utility, you might not be able to install that much solar. Our utility limited us to 120% of our monthly usage over the past year. They also had an allowance of 300 kWh per month for a new car. We were able to use that allowance as well and put in enough solar for as estimated 141% of last year's usage. Even with that much solar, we are finding it wasn't enough because I switched jobs and no longer have the ability to charge at work every week.

We do have three Powerwalls and they do a great job powering the house. We can charge one or two cars but don't have enough capacity to charge the other cars. We were exclusively on self-production for a couple weeks until we got TBC enabled. We tried TBC but wish it was more customizable since it doesn't use the Powerwalls at all during off-peak periods. Also, it tries to send as much back to the grid as possible instead of recharging the Powerwalls. I know this probably makes sense if the utility pays you a lot for production but ours pays a measly 1.5 cents per kWh. I was able to tweak things slightly by extending the part-peak and peak settings in the app but it's still not perfect. We definitely like that we can be close to 100% self-powered on sunny days.

BTW, you will most likely need at least two Powerwalls to effectively power the house and charge the car. If you only have one Powerwall, it can't provide whole-house backup functionality and you'd only be able to charge the car at low amps.

 

[Ampster]

Why do you want to discharge one battery into the other?

......... EV charging should always be from the grid overnight. This is the benefit of Net Metering.

I know you have done the math and I assume the system losses from going from one battery to another is the reason to not charge the car from the Powerwall.
I have not figured out what the Non Bypassable Charges are going to be when I get my PTO. I also have a Powerwall on order and I know the strategy will be to eliminate any consumption during Peak hours.

 

[miimura]

I know you have done the math and I assume the system losses from going from one battery to another is the reason to not charge the car from the Powerwall.
I have not figured out what the Non Bypassable Charges are going to be when I get my PTO. I also have a Powerwall on order and I know the strategy will be to eliminate any consumption during Peak hours.

Yes, losses are one reason, but in Northern California, with an EV rate plan, it is in your economic best interest to let all your solar go to the grid during the Peak period while the Powerwall is powering your household loads. The rate differential from Peak to Off-Peak, especially in Summer is too big to ignore. PG&E's EV Summer Peak is now $0.48/kWh while the Off-Peak is $0.13/kWh. Non-Bypassable charges are less than $0.05/kWh. I still haven't come up with a NBC calculation that matches PG&E's billing, even after going over all the related tariffs multiple times. Since I don't pay them, I don't spend any more time on it. Anyway, the point is that even if you pay NBCs for EV charging, you still get to charge at least 3kWh Off-Peak for each kWh you push to the grid during Summer Peak. The ratio in the Winter is 1.9X after NBCs.

Regarding solar sizing, make sure you do a full year analysis because solar production and usage patterns vary A LOT throughout the year. My daily kWh production in the dead of Winter, even on a completely sunny day is 1/5th the Summer max.

 

[Ampster]

Regarding solar sizing, make sure you do a full year analysis because solar production and usage patterns vary A LOT throughout the y

Great tip, but my situation is unique because my system was installed in December 2017 shortly after I bought the home. I sized it based on historical consumption in an all electric home in Southern Caliornia plus a factor for AC needs in summer. I assumed EV charging would be the same.
Because of the fires in Sonoma County the vendor is having a hard time coordinating PG&E and County building officials to meet for the electrical panel upgrade that was part of the contract. I haven't paid for the system but someone must have flipped a breaker and my generation for 10 months is better than PV Watts predicted. Because I have no PTO there is no net metering so I have been trying to use the power generated during the day by charging my cars, running my water heater cloths dryer and ocassionally cooling the house during late afternoons.

 

[NuShrike]

Regarding solar sizing, make sure you do a full year analysis because solar production and usage patterns vary A LOT throughout the year. My daily kWh production in the dead of Winter, even on a completely sunny day is 1/5th the Summer max.

Technically, you could get by with only 6 months as long as it's basically solstice to solstice.

 

[miimura]

Technically, you could get by with only 6 months as long as it's basically solstice to solstice.

For solar that's true. However, things like heating and cooling load may not be symmetric about the solstice. My heating is natural gas, but the air handler fan does take a significant amount of electricity.

 

[Shygar]

I have a 7.5 kW system, one powerwall 2, a Model 3 and a Pacifica Hybrid (and AC in the house). As @miimura said, your most cost effective use of the powerwall is to let the house draw power from it while you dump as much solar as you can during your peak period. I have a south facing roof and I almost think I would have been better off putting my panels on a west facing side due to this (my other house has a 6.3 kW system west facing and produces almost as much value as my 7.5 kW south facing system).

Over all I still produce more than I use, even with a daily roundtrip Model 3 usage of about 45 miles and about 5 miles in the Pacifica. If you have AC in your house and like to keep your house cool in the summer, I'd say you need somewhere between a 6.5 and 7.5 kW system. Although if you live in Union City, that should be cooler than here in Pleasant Hill.

As @miimura alluded to, basically if you dump 1kW of power to the grid between 2pm and 9pm (PG&E EV rate, peak time), that's worth 4kW of power when you charge at night, roughly. So if you dump at least 8kW of power during that peak time, and charge your Model 3 at night, then that should cover your commute on a sunny day during the summer.

 

[MorrisonHiker]

I have a 7.5 kW system, one powerwall 2, a Model 3 and a Pacifica Hybrid (and AC in the house). As @miimura said, your most cost effective use of the powerwall is to let the house draw power from it while you dump as much solar as you can during your peak period. I have a south facing roof and I almost think I would have been better off putting my panels on a west facing side due to this (my other house has a 6.3 kW system west facing and produces almost as much value as my 7.5 kW south facing system).

Over all I still produce more than I use, even with a daily roundtrip Model 3 usage of about 45 miles and about 5 miles in the Pacifica. If you have AC in your house and like to keep your house cool in the summer, I'd say you need somewhere between a 6.5 and 7.5 kW system. Although if you live in Union City, that should be cooler than here in Pleasant Hill.

As @miimura alluded to, basically if you dump 1kW of power to the grid between 2pm and 9pm (PG&E EV rate, peak time), that's worth 4kW of power when you charge at night, roughly. So if you dump at least 8kW of power during that peak time, and charge your Model 3 at night, then that should cover your commute on a sunny day during the summer.

I have a 16.5 kW system and 3 Powerwalls. Our peak period is only 4 hours and our entire peak period usage is probably only 4 to 6 kWh. The Powerwalls can power the house during peak but then they only run for another 3 hours until off-peak begins at 9 pm. The next day, they only run 5 hours before peak starts up again (from 9 am until 2 pm) so they only use another 5 to 8 kWh.

My "problem" is that by default, over a 24 hour period, the TBC mode only uses about 12 to 18 kWh of my 40 kWh capacity. I've found I can force the Powerwalls to power the house for a longer period if I set fake part-peak times in the app...but it seems like they should just allow the option to use the Powerwalls during off-peak if there's still something left in the Powerwalls above the reserve limit.

Does anyone else have to set fake park-peak times to extend the hours the Powerwalls will power the house?

 

[Shygar]

You might not even need TBC. Try just self powered and see if that mostly works.

 

[MorrisonHiker]

You might not even need TBC. Try just self powered and see if that mostly works.

That's what I've been wondering. Some have been saying to use TBC since it doesn't make sense to be self-powered. I think that the benefits of TBC vary depending on how much the utility company pays you, how large the solar system is, how many Powerwalls one has and how much the daily consumption is.

Self-powered usually covers our consumption for over 23 hours a day, depending on how many cars need to charge. With our system, we can often cover 100% of our consumption if only two cars need a regular daily commute charge. I changed the third car to charge during the day on days that I work from home.

With TBC, it's similar to self-powered if I fake the off-peak time to only be from 3 am to 5 am instead of 9 pm to 9 am. During the day, it will use grid power to power the house in part-peak and charge the Powerwalls. Then in peak, it powers the house from the Powerwalls and sends the solar to the grid. TBC does use a lot more grid power whereas self-powered will use the Powerwalls more.

I guess it's six of one and half a dozen of another.

 

[who177e]

After speaking with a few other Solar companies about our situation and what we are trying to accomplish, having the PV system cover all of the costs of electricity including charging a model 3 for 75 miles a day.

Almost all of the people I've spoken with, including the Tesla Energy advisor, says that financially speaking, I don't need a PW and that the PG&E net metering would take care of "banking" of excess electricity we are producing. I'm not sure if they are just saying that to get my PV business because they can't get their hands on PWs right now or what. My Tesla Advisor still says that he has them in stock and that we can have the PV system with PW installed by year end.. (still not sure if I believe him based on what people are saying on this forum).

But when I asked him straight up if he thinks I need one or if its a good direction to go, he says no. The says that he doesn't think that the technology is far enough along for it to make sense yet. He thinks that once the gigafactory is up that the PW production and technology will get better.

Me and the wife keep going back and forth on what to do here. PW or no PW.
Solar Companies are saying it doesn't make sense but I don't have the math to back up what they are saying about Net metering. Not sure if they are just saying that so that my install would be more "simple".

Currently we can fit 18 panels nicely on our roof (x 315w black panels) (5.67w PV system) but not sure where to go with PW or no PW.
Trying to cover about 10,000 kWh per year (regular usage and the model 3 charging).

I think that there is definitely a cool factor of having a system with the PW but not sure if makes financial sense but I don't have the numbers to back it up either way.

 

[reddy]

Your rep told you WHAT ??

I've had a reservation for 13 months and counting. I called Tesla last week, and they are hoping that 'I will hear some news' sometime early in 2019.

Definitely NOT in stock.

 

[who177e]

Your rep told you WHAT ??

I've had a reservation for 13 months and counting. I called Tesla last week, and they are hoping that 'I will hear some news' sometime early in 2019.

Definitely NOT in stock.

Yea.......kinda what I thought.

I was giving him the benefit of the doubt and maybe they give priority to solar installs..????