Welcome to Tesla Motors Club
Discuss Tesla's Model S, Model 3, Model X, Model Y, Cybertruck, Roadster and More.
Register

2 Powerwalls + "Free Nights" Plan...can they pay for themselves?

This site may earn commission on affiliate links.
Hi folks,

I am moving to a new home in a couple weeks, and am trying to validate whether the plan below is financially viable.

There are several energy plans in my area for "Free Nights and Weekends" where you only pay the TDU charges. I don't get enough regular sun to use solar - but would it make sense for me to get Powerwalls anyway?

I live in South Texas, HVAC use is HEAVY. My power bill in the summer months is about $300, and about $180 in the winter. That breaks down to

Energy: $0.071400 PER KWH
TDU: $0.044700 PER KWH
plus $8 in tax/fees.

If the PowerWalls run my house during the day, I can remove the majority of the "Energy" line, which would save me between $100-200 per month depending on the time of year.

My thought is -- what if I have the Powerwalls and my car charge at night during the "free" period, and then use that energy during the day, so I'm not consuming during peak hours. Rinse and repeat.

I have a brand new 2800 sq ft home with 2 AC units, and I'm thinking 2 PowerWalls would be plenty to keep me running for a full day. I don't know much about the loan length/terms for PowerWalls, but could 2 Powerwalls pay for themselves in this way within the warranty period?
 
That's about $3,000 per year in energy. So you will need to do the math on how long you want to wait to recapture the cost.

The other question is can the two PWs handle the load? You need to figure out how many kWhs you will be using them each day (cannot exceed 23 kWh total for the two during your off grid time) and also you cannot exceed 10 kW at any given time.
 
If I understand this correctly, you currently pay 11.6 cents/kWh, so $300/month in summer means about 2600kWh/month or 86kWh/day. On average during 24 hours, that's 3.6kW power draw or 150 Amps at 240 VAC. Assuming you want the Powerwalls to supply power for 14 hours (charging 10 hours at night), you'd need 4 PW, and consume down to 8% capacity each day. This assumes that the power consumption is level throughout the 24 hours. That deep of a discharge will, I believe, shorten the life of the PWs. The Tesla PW app sets a default of 19% of capacity as the lower end of discharge unless the grid is unavailable.

At night, you'd need to supply both the house power load and additionally the PW charging load. Again ignoring the possibility of uneven power loads between day and night, and realizing that the PWs need to be fully charged in 10 hours, I think the night time current draw would be 150A (house load)) + 14/10*150A (PW load) or about 360 Amps at night. That's a lot of current: Do you have a 400+ Amp service?

I don't see how this gives a good return on investment. Looks like around $35K to save $30K. And I have to wonder if these plans have a minimum percentage of use during the daytime hours.

If it is a brand new home, how do you know the projected power consumption?
 
If I understand this correctly, you currently pay 11.6 cents/kWh, so $300/month in summer means about 2600kWh/month or 86kWh/day. On average during 24 hours, that's 3.6kW power draw or 150 Amps at 240 VAC. Assuming you want the Powerwalls to supply power for 14 hours (charging 10 hours at night), you'd need 4 PW, and consume down to 8% capacity each day. This assumes that the power consumption is level throughout the 24 hours. That deep of a discharge will, I believe, shorten the life of the PWs. The Tesla PW app sets a default of 19% of capacity as the lower end of discharge unless the grid is unavailable.

At night, you'd need to supply both the house power load and additionally the PW charging load. Again ignoring the possibility of uneven power loads between day and night, and realizing that the PWs need to be fully charged in 10 hours, I think the night time current draw would be 150A (house load)) + 14/10*150A (PW load) or about 360 Amps at night. That's a lot of current: Do you have a 400+ Amp service?

I don't see how this gives a good return on investment. Looks like around $35K to save $30K. And I have to wonder if these plans have a minimum percentage of use during the daytime hours.

If it is a brand new home, how do you know the projected power consumption?

I think you might be off by an order of magnitude on the amperage, 3.6kW is not 150A at 240V, it should be more like 15A.

I about 3.5kW/hour does look correct. Power wise one Powerwall can supply it. Energy wise the OP will need about 4 of them if the load is evenly distributed to cover all.

One can calculate an effective kWh cost for a Powerwall. If that is below the daytime rate of about $.116 then the OP will have a winning plan. They don't have replace all their usage.
 
Thank yall for the insight. I'll try to put together a spreadsheet to see where the numbers land. The house has standard 200A service, I've never heard of a non-commercial site with more than that. It sounds like based on arnolddeleon's correction, I'm well within the power output/input requirements for the PowerWall.

My concern would be in needing four powerwalls....power is fairly cheap here in TX, and four powerwalls is about $40k - so that might be a hard number to offset within its lifespan.

I'm basing power consumption on my current home which is 14 years old (same size), so the consumption will be slightly lower in the new home with the more efficient materials.
 
Thank yall for the insight. I'll try to put together a spreadsheet to see where the numbers land. The house has standard 200A service, I've never heard of a non-commercial site with more than that. It sounds like based on arnolddeleon's correction, I'm well within the power output/input requirements for the PowerWall.

My concern would be in needing four powerwalls....power is fairly cheap here in TX, and four powerwalls is about $40k - so that might be a hard number to offset within its lifespan.

I'm basing power consumption on my current home which is 14 years old (same size), so the consumption will be slightly lower in the new home with the more efficient materials.
Would love to see our numbers. With power that cheap, no idea why you would want to put that kind of money in PW's. I would max the heck out of your solar, and that may get you your zero energy bill. But, maybe your numbers might prove me wrong.
 
Would love to see our numbers. With power that cheap, no idea why you would want to put that kind of money in PW's. I would max the heck out of your solar, and that may get you your zero energy bill. But, maybe your numbers might prove me wrong.

see bolded below

Hi folks,



There are several energy plans in my area for "Free Nights and Weekends" where you only pay the TDU charges. I don't get enough regular sun to use solar - but would it make sense for me to get Powerwalls anyway?

Im not sure how the OP came to that determination, but a recommendation to "max out your solar" when the OP states " I dont get enough sun for solar" doesnt make sense.
 
see bolded below



Im not sure how the OP came to that determination, but a recommendation to "max out your solar" when the OP states " I dont get enough sun for solar" doesnt make sense.
So does this imply he will have zero solar to charge the batteries? At those energy rates, all I asked was to see his numbers. I just have no idea how he could justify a 40K cost, but again, maybe his number at those low energy rate will prove me wrong.
 
So does this imply he will have zero solar to charge the batteries? At those energy rates, all I asked was to see his numbers. I just have no idea how he could justify a 40K cost, but again, maybe his number at those low energy rate will prove me wrong.

I have no idea what his numbers say, and personally dont think powerwalls make sense in the OPs situation, but his post was clear in his intent "I dont get enough sun to use solar, but would it make sense to get powerwalls anyway" was the direct statement, so recommending "max our your solar" didnt make sense to me given that statement.
 
I have no idea what his numbers say, and personally dont think powerwalls make sense in the OPs situation, but his post was clear in his intent "I dont get enough sun to use solar, but would it make sense to get powerwalls anyway" was the direct statement, so recommending "max our your solar" didnt make sense to me given that statement.
Yep, in your first response, yep, I missed that, which is why I did not ask it again. What he does not say directly is whether he was planning to get solar, even though yep, he stated he gets no sun. But yep, I am assuming for whatever reasons he has no sun, he will have zero solar. I am not jumping to conclusions on PW ROI for him. Since he said he was going to do numbers, just interested in what he comes up with. If I end up doing the battery stuff I am looking at, I will never return my money. But as I said to the wife last night, but at least I will be having some fun.
 
If you fully utilise one powerwall daily and get say 11kwh out of it you save $0.78, if you go with another provider that waives the TDU during free nights you increase your savings to about $1.28 per day. 5 powerwalls look to be around 44k with taxes from tesla and would at best save you $6.4 per day so you might breakeven in 18.8 years not counting degradation, loss of rate plan or failure before then.
 
Tax write off? you'd have to check with an accountant but last I heard if you install power walls with solar then they also get the 27% tax write-off. it may be that adding even a small amount of solar would make tremendous sense because of the tax write-off. 27% is quite a hit!
This is discussed in more detail elsewhere, but to clarify a couple things, first, the credit for 2020-2022 is 26%. But, more importantly, the requirement for claiming the credit for batteries is that they be charged exclusively from solar. So adding a small amount of solar makes no difference to claiming the credit if you still plan to charge the PWs from the grid. What it does do is make it much more difficult to get Tesla to enable grid-charging since their default position in the US seems to be that they only enable grid charging if the PW is not connected to solar.
 
  • Informative
Reactions: henderrj