Powerwall - Numbers don't add up (for me at least)

[dckiwi]

I want to want the new Powerwall. But none of the use cases add up for me (all estimates based on buying two Powerwalls for $13,000):

1) Time of use load shifting

I have an EV pricing plan from Dominion Virginia. Off-peak power is ~4c per kWh, on-peak power is ~8c per kWh.
Assuming I can charge my Powerwalls entirely during off-peak periods and use this power during on-peak periods, I would save $421/year.

This means it would take 30 years for the Powerwalls to pay for themselves based on time of use shifting.

2) Backup

Based on my energy usage, I estimate that two Powerwalls would keep my house up and running for ~17 hours during the winter and ~12 hours during the summer. This is a really nice benefit, as we do get relatively frequent power outages. But is it worth $13,000?

3) Solar self-consumption

I don't have solar yet, but I'm very interested in the new solar roof (or even traditional panels). I recently received a proposal for a 6kWh system that should provide about 50% of my power usage during the winter (when we use 1,100 kWh/month) and about 30% of my power usage during the summer (when we use 2,000 kWh/month).

How will the Powerwall help me in this situation? It's likely I would consume all the solar power generated during the day during the day. I assume that using the Powerwall for solar self-consumption only makes sense if you have surplus power at the end of daylight hours, in which case you can store this power for night-time use.


I'd love feedback from others about how they think the Powerwall will benefit them in these various areas.

 

[dckiwi]

PS I think I also need to figure out why my energy use is so high (we have a 4,000 square foot house). We actually peaked at 3,000 kWh this past August! The biggest culprits were our pool pump and keeping the HVAC thermostat way too low. I'm planning to get an energy efficiency assessment.

 

[ecarfan]

Off-peak power is ~4c per kWh, on-peak power is ~8c per kWh

That is amazingly low. I don't think you can make an economic case for a Powerwall. But you can certainly make a "health of the planet" case.

 

[dckiwi]

That is amazingly low. I don't think you can make an economic case for a Powerwall. But you can certainly make a "health of the planet" case.

I can't even make that argument (unless I get solar). Time shifting usage of my power from Dominion Virginia (where the majority of their power still comes from coal) doesn't help the environment.

 

[JCLeonard]

PS I think I also need to figure out why my energy use is so high (we have a 4,000 square foot house). We actually peaked at 3,000 kWh this past August! The biggest culprits were our pool pump and keeping the HVAC thermostat way too low. I'm planning to get an energy efficiency assessment.

If you don't have one already you might consider getting a smart thermostat (Nest, Ecobee, etc.). Tracking and adjusting our HVAC usage has been drastically simplified and the data we get (from Ecobee 3) is super awesome for a data nerd like me. Up here in MN we are paying $0.10 - $0.12 /kWh with no real useful time of use/off-peak options.

 

[dckiwi]

If you don't have one already you might consider getting a smart thermostat (Nest, Ecobee, etc.). Tracking and adjusting our HVAC usage has been drastically simplified and the data we get (from Ecobee 3) is super awesome for a data nerd like me. Up here in MN we are paying $0.10 - $0.12 /kWh with no real useful time of use/off-peak options.

Thanks, good point. Unfortunately I have a Carrier Infinity system (requires you to use their thermostat). But it does allow you to program by time of day, so I could certainly get smarter about HVAC usage.

 

[ecarfan]

I can't even make that argument (unless I get solar).

Agreed. So get solar.

 

[JCLeonard]

Thanks, good point. Unfortunately I have a Carrier Infinity system (requires you to use their thermostat). But it does allow you to program by time of day, so I could certainly get smarter about HVAC usage.

Yeah even basic time adjustments can make a difference.
The Cor series might be an option? Thermostats | Carrier - Home Comfort
You may have already done this, but the compatibility checker on the Ecobee site is pretty good, and their support team is pretty awesome:
Thermostat compatibility checker | Smart WiFi Thermostats by ecobee

 

[LargeHamCollider]

That is some cheap power, pretty hard for solar/battery to compete with that right now. Number 2 would still be attractive to me personally though.

 

[cantdecide]

That is some cheap power, pretty hard for solar/battery to compete with that right now. Number 2 would still be attractive to me personally though.

If you have solar thent he backup option becomes more attractive... as you can then use the solar power during an outage.

 

[strider]

Yeah, here in NE Oklahoma we pay $0.08/kWh peak (14:00-19:00 M-F, June-October). Off peak is $0.02/kWh. I'm planning to do solar just to feel better about myself but the payback will be very long. A friend had a system professionally installed and he's looking at a 19-year payback. I'm hoping to do all the labor myself and am still looking at a 9-10 year payback.

But to answer your question, batteries combined w/ solar are a very good backup solution. You would easily spend over $10K on a nat gas generator and if the outage is bad enough the pumps that move the nat gas will stop working and you'll be out of gas. With solar and batteries you could go on for a very long time. Yes, you'd have to cut back on your usage (unless you build a crazy large system) but you would do that w/ a nat gas generator too (identifying critical loads and such). The idea is that you won't run your entire house on batteries (A/C cranked, pool pumps, etc).

Idk if I'll get a powerwall but I do plan to add some batteries at some point as we do get ice storms here and the power lines are all on poles.

 

[TacC]

TOU load shifting is more relevant in places like CA and HI where prices regularly exceed $0.30. Self-consumption is more attractive in these same areas, too.

For folks living with exceptionally high energy prices, cutting your consumption by 50% can reduce your bill by 60-75% just by avoiding peak rates and higher monthly tiers,

I don't think there's a great business case yet for folks living in VA, PA, etc. The bargain basement prices I've seen quoted here give me a little heartburn. And then I realize I wear shorts in January and I'm happy again.

 

[Dborn]

The reason powerwall were pushed hard in Australia is our power prices. My averaged cost is 19.5c per kWh! Peak is around 48c!! A bit different to the single digit prices you have in the USA.

 

[lolachampcar]

My very first observation was why so low on the PV? I'm about 4K sq-ft and did 10 KW-Dc which still does not cover my whole bill. I'm looking for a way to do 3KW more......

Hard to make a case for the PW as long as my utility continues to be my battery for free (net-metering).

 

[lklundin]

Agreed. So get solar.

Really?

The user asks for advice, informing us that with the Powerwall he can reduce his marginal electricity cost to 4c$/kWh.

Solar is not currently at a price-point where it can compete with that.

But an additional argument for the Powerwall and for solar is the reduction in price risk it gives. Who knows how the utility companies will react when their business model becomes obsolete?

Btw, why is the residential electricity consumption so high in the US?

My annual electricity consumption is 3800 kWh, my 4-person house is about 300 m^2 (but not using electricity for heating nor for transportation).

Maybe there is some money and sustainability to be had in reducing consumption.

Or is the extra electricity consumption maybe explained by the charging of BEV's?

 

[winfield100]

I want to want the new Powerwall. But none of the use cases add up for me (all estimates based on buying two Powerwalls for $13,000):

I'd love feedback from others about how they think the Powerwall will benefit them in these various areas.

Since you (and I) are in the DC area, consider the edge cases over the past 10-15 years
1) Snowpocolypse we lost electricity for 3-4 days (blizzard)
2) Snowmagedden same deal, blizzard, no elec 3-4 days (DC shut down for a week, again)
3) Derecho a few summers ago, (Inland hurricane) lost elect day or so
4) swarm of 300 tornadoes a few years ago from Alabama to Dumfries, Va
5) few summer tornadoes that hit elec a couple of summers
(one missed Rockville Tesla SC by appx 1/2 mile)
6) Ice storm a few winters ago lost elec again
This winters forecast of Polar Vortex (you can use outside for refrigerator though)
Hurricanes Sandy, Matthew, Agnes, Hazel (im that old, 1954, saw the eye as it passed over our house south of Manassas

 

[nativewolf]

Good question Lklundin. The average consumption is really high because

• He's living in a slightly larger home, say 350sq meters.
• Reston VA is blessed with 4 real seasons, so summer is very hot & humid-makes cooling challenging where you average highs from late may to mid sept of 30C+ and july and august can be 35C for several weeks. The winter is really not much warmer than in Munich, in fact maybe just a degree or two different and there can be weeks where it rarely gets above 5C and nights go to about -5C to -8C . Now what gets really bad is that nighttime summer temperatures stay over 20. Very difficult to be comfortable (for many Americans) without running an AC almost all day, only for a few hours 4-6am maybe does outside temperature not trigger the AC. Then we get to the humidity thing...makes the AC system work harder,
• then he we have some bad HVAC legacy players here that are not very efficient. Carrier and others are getting challenged by mini splits with much higher efficiency.
• In his case he's also got some luxury items like a pool, with an automatic heater and cleaning system (the only way to have a pool in my opinion but I took my pool out).
• Power prices are very low. He (and I live close by) is just 2 hours from the Appalachian coal belt that has some of the highest energy dense coal in the world, they strip mine nowadays and it is cheap cheap cheap coal. So cheap the coal trains rumble down to Baltimore and Newport with trains of coal every day, even as coal prices fall the coal trains move sending the coal to China or elsewhere in the US, etc. This cheap power encourages misuse.

What is the implication:
Because power costs are so low (and we don't have bankrupt power utility challenges like Germany unless PE groups got involved) green energy has to be very disruptive to succeed. Not a bad thing, just means when change happens it is ..significantly more likely to be impactful. Like Tesla competing in the US car market, everyone sells in the US car market and there is a price/quality point competition that crazy even with high regulation and oversight and litigation. For Tesla to have succeeded in the market means they will succeed, period. However, continued advances in wind turbines mean that the great plains wind belt is just an untapped pool. Oklahoma and the panhandle of Texas (maybe throw in the Dakotas) could literally power the US in the evening if you could move the electricity from coast to coast (not trivial). In the US our grid is our issue, lots of power companies have grid sharing challenges and there are even 3 or 4 separate grids (TX is 1 in and of itself and is flirting with brownouts without major changes).

Collapsing the battery and power electronics into one system is very very disruptive, this may kill the enphase smc outback etc type folks because I don't think there is any way to bundle up 14kw of LI battery and the power electronics and look at $6k. WK057 would be pleased I think. It is the collapse of the system that is impressive to me, if I am building a new home I can almost just say don't hook up to the grid. The logical implication would be that with the PW2 and PP2 you could stress the utilities into bankruptcy and that is certainly impactful. The original PW...not so impressive.

On the other hand the powerpacks could be a huge seller in the TX market because it would provide storage solutions for the wind farms, compared to Central/South TX Reston VA has a mild summer climate. It could completely shift the green power discussion in that grid and stave off bankruptcy for the utilities that were raided by PE firms in 2007.

Anyway, an outcome in the US is likely to be more disruptive and messy as it is easier to innovate faster than social norms can progress.

 

[aesculus]

The reason powerwall were pushed hard in Australia is our power prices. My averaged cost is 19.5c per kWh! Peak is around 48c!! A bit different to the single digit prices you have in the USA.

Not all of the USA is created equal. By the Tesla factory, in much of NorCal, we also enjoy peak rates of 41c, partial peak of 22c (7AM - 9PM) and off peak (11 PM to 7AM) of 10c. There are even some cases where people who use a lot of power can push 50c kWh too.

But I hear you and you do get the award for the highest costs so far.

None the less I don't think a PW is in the cards for me,at least right now. Suffering and time shifting, and a bit of solar with NEM works more economically today. But as soon as California starts having measurable solar production (ie outside of the 10-15%) we have today, then NEM value is going to start to diminish more and more thus we will have to look at eating our own cooking. The rate plans are already shifting to take value away from solar production periods so over gen value is softening.

 

[dckiwi]

My very first observation was why so low on the PV? I'm about 4K sq-ft and did 10 KW-Dc which still does not cover my whole bill. I'm looking for a way to do 3KW more......

They said they could only fit a 6kWh system on my roof. I have a 4k square foot house but the roof has a ton of angles. Needless to say I'm eager to hear more about the new Tesla solar roof products.....

 

[dckiwi]

Since you (and I) are in the DC area, consider the edge cases over the past 10-15 years
1) Snowpocolypse we lost electricity for 3-4 days (blizzard)
2) Snowmagedden same deal, blizzard, no elec 3-4 days (DC shut down for a week, again)
3) Derecho a few summers ago, (Inland hurricane) lost elect day or so
4) swarm of 300 tornadoes a few years ago from Alabama to Dumfries, Va
5) few summer tornadoes that hit elec a couple of summers
(one missed Rockville Tesla SC by appx 1/2 mile)
6) Ice storm a few winters ago lost elec again
This winters forecast of Polar Vortex (you can use outside for refrigerator though)
Hurricanes Sandy, Matthew, Agnes, Hazel (im that old, 1954, saw the eye as it passed over our house south of Manassas

Wow, you're bringing back some bad memories! The worst was actually number 3 (the derecho). We lost power for 3 days in early July, with temps in the high 90s. Maybe a couple of Powerwalls would be worth it. That'd only power my home for ~12 hours, but that's with us using everything as we normally do. I'm pretty sure we could scale down fast if needed, maybe getting to 24 hours or so of power.

Good question Lklundin. The average consumption is really high because

• He's living in a slightly larger home, say 350sq meters.
• Reston VA is blessed with 4 real seasons, so summer is very hot & humid-makes cooling challenging where you average highs from late may to mid sept of 30C+ and july and august can be 35C for several weeks. The winter is really not much warmer than in Munich, in fact maybe just a degree or two different and there can be weeks where it rarely gets above 5C and nights go to about -5C to -8C . Now what gets really bad is that nighttime summer temperatures stay over 20. Very difficult to be comfortable (for many Americans) without running an AC almost all day, only for a few hours 4-6am maybe does outside temperature not trigger the AC. Then we get to the humidity thing...makes the AC system work harder,
• then he we have some bad HVAC legacy players here that are not very efficient. Carrier and others are getting challenged by mini splits with much higher efficiency.
• In his case he's also got some luxury items like a pool, with an automatic heater and cleaning system (the only way to have a pool in my opinion but I took my pool out).
• Power prices are very low. He (and I live close by) is just 2 hours from the Appalachian coal belt that has some of the highest energy dense coal in the world, they strip mine nowadays and it is cheap cheap cheap coal. So cheap the coal trains rumble down to Baltimore and Newport with trains of coal every day, even as coal prices fall the coal trains move sending the coal to China or elsewhere in the US, etc. This cheap power encourages misuse.

Seriously, you're right on the money with every one of these points. I gotta say that the pool was the biggest eye opener - the pool pump can run for up to 17 hours a day (depending on what you're doing), and adds about 700kWh/month during the summer months.

Our HVAC systems are brand new Carrier units (two 3-ton systems), supposedly high efficiency but who knows.


Anyway, thanks everyone for the feedback. I'm just trying to think through the main benefits of the Powerwall. As you've confirmed, it's entirely dependent on your situation. In my situation, the biggest use case would be coverage during power outages. But for others I see why time of use shifting and solar storage are important too.