Tesla power wall - worth it or not?

[pavelcherepan]

Many of you have probably seen the news about the new Tesla Power Wall, which is basically a lithium-ion battery that's meant to be recharged with solar power (or other renewable source) and keeps you from using any electricity from the grid at all. For those who haven't seen the news, you can watch the presentation by Elon Musk, who is admittedly a great entrepreneur, but in my opinion might need to work a bit on his presentation skills.

 

What do you reckon, is it worth getting one? I've been thinking of one and it seems barely worth it because:

 

- average daily electricity consumption for our house is ~8 kWh (averaged over the last 4 months)

- solar panels on the roof with ~3 kW combined power

 

If I get a Power Wall for 10kWh, there's plenty of sunlight and day is long enough here in Victoria even in winter to charge it completely and then feed back to the grid for the remaining day time. Power Wall has been introduced with a price tag of $3500 for 10 kWh system, so in Australia it will probably cost closer to AU$5500, knowing that everything here is ridiculously overpriced. Even so, I'd be saving about $70 a month on electricity bills and getting some $20 more from the power company for feeding into the grid. So at $90/month it would take about 5 years to break even, while Tesla has a 10-year warranty on batteries. So far so good.

 

But if I include $3500 solar panels in the calculation too, now it will take over 8 years to break even, and so, assuming that the battery will go 'boom' straight after the warranty runs out, it's barely worth it to get one. Of course, you'll be saving the planet and all that...

 

Maybe I'm missing something here?

 

[swansont]

The point is you will still ultimately save money, even if the break even point is ~8 years out. 2 years at $90/mo is >$2000. When you buy the replacement battery, odds are it will be even more cost-effective, since you will have had 10 years of battery development and more widespread adoption to drive costs down. Even if you plan to sell your house before 10 years are up, the presence of a solar power system should add value to it.

[pavelcherepan]

swansont, actually it's more than that. Power Wall also gives 350-450V DC current, so I'd need to buy an AC/DC inverter, plus whatever it'd cost to install it as I'm no electrician and eventually I'd be looking at some $11-12k for the total system cost, which will push braking-even point to 10 years away or more. I was really excited about the whole thing when I first heard the initial announcement, but now it seems like it's still awesome, but not financially awesome

[swansont]

swansont, actually it's more than that. Power Wall also gives 350-450V DC current, so I'd need to buy an AC/DC inverter, plus whatever it'd cost to install it as I'm no electrician and eventually I'd be looking at some $11-12k for the total system cost, which will push braking-even point to 10 years away or more. I was really excited about the whole thing when I first heard the initial announcement, but now it seems like it's still awesome, but not financially awesome

 

You'd need an inverter for the solar panels. Do you need a second one?

[pavelcherepan]

You'd need an inverter for the solar panels. Do you need a second one?

 

Good point. Didn't think about that.

[Danijel Gorupec]

I don't get it.... If you can feed to grid and get paid, why do you need the Power Wall? (Is it because grid pays more during night? Hmm...)

[pavelcherepan]

I don't get it.... If you can feed to grid and get paid, why do you need the Power Wall? (Is it because grid pays more during night? Hmm...)

 

Most of the power usage is during the evening/night hours obviously and the rate at which power companies buy electricity and what they sell it for are two totally different things. Different by an order of magnitude.

[Danijel Gorupec]

So, when you say you can get $20 for feeding into the grid, you are only talking about eventual excess energy you gathered?

[swansont]

I don't get it.... If you can feed to grid and get paid, why do you need the Power Wall? (Is it because grid pays more during night? Hmm...)

 

Power generation and power use don't match up. You need a battery if you want to run anything electrical at night. Also the utilities (and the machinery) are probably happier if they can adjust the drawdown of your excess power, rather than being fed in real time.

[Danijel Gorupec]

Thanks for clarification.... I was confused because here we can sell solar-generated power to grid at higher price than we buy from grid. No market for Power Wall in Croatia.

 

Still I would be interested to see the pavelcherepan calculation if no Power Wall is used (while I do understand that some 'battery balast' is probably beneficial to your home equipment). Just sell/buy the power difference from grid in real time.

[Enthalpy]

[...] assuming that the battery will go 'boom' straight after the warranty runs out [...]

 

The 10-year warranty by Tesla Motors can be prolonged an other 10 years, so expect the battery to last >20 years.

http://www.teslamotors.com/powerwall

Battery life is one thing I was uncomfortable with, and their warranty is the proper answer to give me confidence.

You'd need an inverter for the solar panels. Do you need a second one?

 

Tesla's Powerwall accepts and restores DC only, at 350-450V (supposedly varying with the progress of charge), so the user needs a DC-DC conversion from the solar panels and a DC-AC to the home network.

 

Differences between the networks among the countries (and even within countries like Japan) are an excellent reason for Tesla not to include the inverters in the offer; differences among the energy sources (solar DC, wind 3-phase and so on) an other reason. But sure, the costs add up.

[...] here we can sell solar-generated power to grid at higher price than we buy from grid. No market for Power Wall in Croatia. [...]

 

Quite the opposite! Install a 4cm² solar cell in your basement to be an electricity producer, plus three dozens Powerwalls in your house, buy cheap electricity from the company, resell them expensive green electricity.

 

Or put a other way: electricity suppliers will have to change their policy, and governments too where they decide it.

 

It would make technical sense that electricity companies pay individuals to store electricity according to the needs of the supplier. Storing near customers reduces the transport losses, and individuals can invest the capital needed for electricity storage on a large scale.

 

Though, I doubt that electricity companies stuck to fossil energy sources (including uranium) want to promote electricity storage, as this favours renewable sources.

[StringJunky]

 

The 10-year warranty by Tesla Motors can be prolonged an other 10 years, so expect the battery to last >20 years.

http://www.teslamotors.com/powerwall

Battery life is one thing I was uncomfortable with, and their warranty is the proper answer to give me confidence.

Battery life is one thing, battery capacity over time is another. It's a great idea but I'm not sure lithium-ion is the optimum medium for it, as it's not that robust in maintaining charge with lots of charge/discharge cycles and it's relatively temperamental about catching fire. There are new technologies hot on its heels though, so I think there will be a better alternative in the next five years; batteries are a very hot research project going by the number of articles I see every week about it on science news sites. My personal feeling is let the wealthier people who can afford to upgrade as it happens provide the testing ground. If it's a major investment for a person it's possibly a bit too early at this stage in the game to be jumping in. Just my 2p.

[swansont]

Battery life is one thing, battery capacity over time is another. It's a great idea but I'm not sure lithium-ion is the optimum medium for it, as it's not that robust in maintaining charge with lots of charge/discharge cycles and it's relatively temperamental about catching fire. There are new technologies hot on its heels though, so I think there will be a better alternative in the next five years; batteries are a very hot research project going by the number of articles I see every week about it on science news sites. My personal feeling is let the wealthier people who can afford to upgrade as it happens provide the testing ground. If it's a major investment for a person it's possibly a bit too early at this stage in the game to be jumping in. Just my 2p.

 

 

While both have had issues, there is a difference between Lithium and Lithium ion batteries. AFAICT it's the lithium batteries that have been most problematic

[StringJunky]

 

 

While both have had issues, there is a difference between Lithium and Lithium ion batteries. AFAICT it's the lithium batteries that have been most problematic

I didn't know this ...will check it out.

[iNow]

Costs will come down. Efficiency will go up. Rebates will help. These can be infinitely stacked.

 

I'm thinking this plus solar plus geothermal for heating/cooling when building new house in a few years. That with low-E glass and closed cell spray foam insulation inside 2x6 construction walls and full exterior wrap plus basement seal... I'm all in.

[Enthalpy]

Battery life is one thing, battery capacity over time is another. It's a great idea but I'm not sure lithium-ion is the optimum medium for it, as it's not that robust in maintaining charge with lots of charge/discharge cycles and it's relatively temperamental about catching fire. There are new technologies hot on its heels though, so I think there will be a better alternative in the next five years [...]

 

Bettry life is universally defined by a loss of capacity, often -20%, precisely because batteries can't be used down to zero capacity. Here one cycle per day is a benign environment, where 20 years mean only 7,000 cycles, a difficult but realistic value for lithium-ion.

 

Yes, much research is being done. but lithium-ion dominates for years, so it's difficult to outperform. Whether one chemistry will equal or exceed lithium-ion is impossible to predict, especially because research papers and newspaper reports are so unshamingly bloated in their claims: "we could absorb 20% more ions in graphite during two cycles" becomes "we can build a car with 3,000km range".

 

Fire is an issue, and I'm pleased that the Powerwall can operate outside.

 

To me, the best bet is with today's technology since it's usable and makes some economical sense, not to lose five full years of activity.

[Sensei]

Power Wall has been introduced with a price tag of $3500 for 10 kWh system,

 

This price seems ridiculous, to me at least.

 

The first accumulator from net 12 V 120 Ah = 1.44 kWh, 2 years warranty, 86 usd.

To have 10 kWh there is needed 7 pieces, 86 usd * 7 = ~603 usd.

 

6 times less..

[Enthalpy]

Yes, I had previously evaluated storage in lead batteries and they were indeed cheaper. But lithium brings other important advantages:

• The (claimed) 92% round-trip efficiency isn't attainable by lead. 70% disqualifies a battery for mass storage.
• The Powerwall sized for an individual house fits at a wall, lead by no way.

As for risks and cleanliness, I'd say: yuk both - but both are recycled, and we know to live with worse gasoline, propane and the like.

[iNow]

Sensei - You don't seem to be accounting for differences in the life-cycle of lead-acid and the replacement frequency upon failure (especially in warmer areas). Surely if you're replacing one at multiple times the rate of the other that must be factored into your calculations.

 

A higher front-end cost on a higher quality product is often cheaper than a lower-cost on a lower quality product that needs repeated service and replacement.

 

http://www.altenergymag.com/content.php?post_type=1884

Lithium-ion has significantly higher cycle life than lead acid in deep discharge applications. The disparity is further increased as ambient temperatures increase. <snip> the AGM pack must be limited to a 30% depth of discharge to get comparable life to a lithium-ion that is at 75% depth of discharge. This means that the AGM battery must be 2.5 times larger in capacity than the lithium-ion to get comparable life.

 

In hot climates where the average temperature is 92°F, the disparity between lithium-ion and lead acid is further exacerbated. The cycle life for lead acid (flooded and VRLA) drops to 50% of its moderate climate rating while lithium-ion will remain stable until temperatures routinely exceed 120°F.

 

[StringJunky]

It becomes apparent when you compare directly. The Powerbrick is the Li-ion.

 

Click to enlarge.

 

http://www.powertechsystems.eu/en/technics/lithium-ion-vs-lead-acid-battery/

 

Peukert's Losses

Edited May 12, 2015 by StringJunky