Tesla Powerwall: Crunching The Numbers For Australia

Tesla's Powerwall is a great concept — with the potential to reduce electricity costs, tie in with solar and create a smarter, distributed power grid. But how long will it take to actually save you money?

In case you were living under a rock, Powerwall is a 7kWh daily use battery that can be used to power your home and offset some electricity costs. There is also 10 kWh backup model that is aimed at businesses instead of a backup generator.

The Powerwall is likely to be used in a few different ways. Firstly, it could be charged using cheap off-peak electricity, then run your house during the morning, day and night. You could also charge the Powerwall with solar during the day, then use that power to run your home in the evening, night and morning. You could go off grid completely, charging the Powerwall with solar and cutting ties with your electricity company.

Buying a Powerwall and inverter, as well as having it installed is estimated at $7340 US by SolarCity, or about $9300 AU at current exchange rates. It's likely that it will be slightly more expensive here, due to shipping costs as well as increases install costs for our smaller user base. A cost of $10,000 AU installed is a good starting point.

The daily use Powerwall is rated at 7kWh. The round trip battery efficiency is 92%, and a good inverter can be 95% efficient. A good (if slightly optimistic) starting point is to figure we need to use about 7.5 kWh to charge the Powerwall, and will get about 6.5 kWh back out.

Electricity prices vary a lot in Australia, so we went straight to our own power bill. This is for a suburban house in Sydney Australia, with AGL, so your numbers may vary.

We pay $0.241 per kWh at peak, and $0.0676 for off-peak power on controlled load 1. This currently runs a hot water tank, but could be also used to charge a battery, provided it’s a hard-wired connection. Controlled Load 1 is the cheapest, and is usually turned on for at least 6 hours a night - plenty of time to charge the Powerwall.

At the off-peak rate, it would cost us $0.51 to charge the Powerwall each night. If we use the entire capacity the next day (easy enough for a typical house), we save $1.57 of electricity charges.

So after charging costs, we can save a maximum of $1.06 a day. This gives us a payback time of 25 years. Considering the warranty is only for 10 years, this isn't a great figure.

In comparison, what if we pulled the $10,000 to buy the Powerwall from our home offset account? Assuming 5% interest, it would cost an extra $500 a year in interest, which is more than the the $387 we would save a year in electricity.

So what if we add solar into the mix? The problem is the very best we could do is save the $0.51 a day charging cost, which amounts to $186 a year. Prices for small stand alone solar installations are hard to find, but we should be able to get a 1.5 kW system (enough to just charge the Powerwall in a day) for around $5000 installed. Assuming few cloudy days, the payback time would be around 26 years.

So what if we go off grid entirely?

According to our power bill, we use 816 kWh in 95 days, or 8.6 kWh a day. This is actually crazily low, and AGL actually states that the average 1 person household uses 1216 kWh in 95 days or 12.8 kWh a day. Chalk that up to efficient LED lighting, off-peak hot water and no air-conditioning.

At 8.6 kWh a day, we need at least two Powerwalls to cope with cloudy days. Even so, we would need to be extra frugal through long periods of cloudy weather.

Fully installed, we are unlikely to end up with much change out of $30,000.

On the plus side, we can now save our entire power bill, which includes the supply charge. In this case, $2.61 a day. Unfortunately, this actually means our payback time is around 31 years, even worse than being tied to the grid (at current electricity costs).

Powerwall is a great concept, but it's not exactly new - off-peak and solar battery banks have been around a long time. The main benefit is that it's a lot smaller and easier to install. It's also great to get people thinking about ways to save energy and be more efficient.

One major benefit is that Powerwall keeps you going during a blackout. In our experience, blackouts are very short in duration and few and far between. Not to mention, with a USB Power bank that can output 12v for our modem, plus laptops and tablets, the essentials are covered.

As prices drop with increased production, Powerwall will become more economically viable, but for now it's only going to appeal to early adopters, or those in niche situations.

It's also worth noting that electricity companies are likely to respond to technologies like Powerwall, which could include raising off-peak electricity costs. It’s hard to predict, but this could make the payback time significantly longer.

Of course, these are not hard and fast numbers. Electricity costs can vary a lot, so take a look at your own bill to see how much you can save. Check out the Powerwall for yourself over at Tesla.

Do the numbers stack up for your home? Tell us in the comments.

Lifehacker's Loaded column looks at better ways to manage (and stop worrying about) your money.


Comments

    "This is actually crazily low, and AGL actually states that the average 1 person household uses 1216 kWh a day"

    I don't think this is right

      You are correct - that should have been 95 days, with 12.8 kWh per day.

      @25 cents kw hr that is $400 a day . Gina Rhinehardt's mansion perhaps !

    What about if you already have a 2.5Kw solar system installed and paid for?

    Last edited 25/05/15 1:12 pm

      I would also be interested to see how the numbers play out for this particular usage scenario. Someone with a solar system in place is probably the ideal candidate for this product - they won't require an inverter, so take 2 - 3 K from the Powerwall installation charge, and then subtract the 5K for the 1.5kW system also. Should make a significant difference to the payback time while providing power for emergencies and reducing, but not eliminating, grid dependance.

        If you are paying $5000 for a 1.5kw system these days you are in trouble.

      That will depend on how much you are getting paid for the solar you are generating.

      If you get paid more than the off-peak power that you can buy, charging from your solar will actually be worse than charging off-peak.

      If you get paid less than the off-peak, then the savings will be the difference between the off-peak power cost and the feed in payment.

      The AGL website suggest that I could get 8 cents per kWh is I had solar. So I would be better off charging the Powerwall off-peak, and just selling all my solar.

      If the solar feed in price was 6 cents per kWh (as Origin energy gives) I would save around 20 cents a year in electricity costs by charging from solar.

        Not all regions in AU are able to sell power back to the grid. But isn't the goal of the Powerwall to charge by solar not recharge it off-peak from the grid?

        I understand the effectiveness of your solar comes down completely to your panels and not all are equal with effectiveness ranging from 10% to 45% with the current technologies. But a Powerwall if connected to a 30%+ set of panels surely offers a much better return?

          Powerwall is about being able to "time-shift" your power to save money. So solar and off-peak are both part of that.

          Solar is the ideal way to do it, especially if you could have a smart, redundant grid that shares power between homes without needing large scale fossil fuel power generation.

          But in this case, we were just specifically looking at the costs involved, so off-peak is a very viable way to charge.

            Power costs from the grid will likely increase significantly. A base minimum of 3% increase per annum seems reasonable. Such figures would reduce pay back time by several years (and as flagged initially payback should be sooner).

      I'm not sure what it's like in other states, but here, when you get a solar system you get hit with the 'single-charge' rates for your energy. So, every 1kW I take from the grid is 25c. Every 1kw I take from the Solar Array is 0c. Every 1kw I give back to the grid I get 4c. So, essentially if I store 1kw from the solar and use it at night, that's a 21c saving per kWh compared with selling it to the grid and buying it back later. Compare these numbers to the numbers above (7c and 24.5c respectively = 17.5c saving) and you're going to come out with a scenario where it's about 20% better to be doing this with Solar in perfect conditions.

      However, solar conditions are far from perfect.

      You have a limit of 7kw stored energy, and not every day is sunny. We are heavy energy users with a 5kWh system. The solar array generates 8kw to 30kwh per day depending on the season and day, and averages about 16kwh per day year round. We use about 30kw per day, and (due to the difference between when the energy is generated vs used) we contribute 5 to 10kw to the grid. So, the 5kw system should fill the 7.5kw battery on a sunny day, but you're not going to get the same benefit in winter.

      On a 2.5kw system, you have less chance of filling the battery, so it's going to be worse. However, this obviously depends on how much energy you use during the day.

      Which is a very roundabout way of saying, the calculations above probably hold about the same, (long term) for solar.

      As someone said above - for $10,000 it's not worth it. But try the numbers again on a $3,000 investment and it starts to look attractive.

      you still need to count those costs.
      you couldnt use this battery without a preinstalled system, and you didnt pay nothing for it

        and you didnt pay nothing for itThe point is,that it IS paid for and no longer a cost burden!

          You still need to factor it in though.

          If the solar system has been paid for, but has not yet paid for it's cost in savings, then you still need to include that in your calculations. You also need to factor in the changed ROI by storing solar vs selling it back into the grid.

          If the solar system has already paid for itself in savings, then you only need to factor in the change in savings from storing solar power vs selling it to the grid.

    It’s also worth noting that electricity companies are likely to respond to technologies like Powerwall, which could include raising off-peak electricity costs. It’s hard to predict, but this could make the payback time significantly longer.

    The thing that is bullshit about this and solar panels, because the electricity company are losing money from people with powerful solar panel setups, they drive the price of electricity up for people who cannot afford to install solar panels. I have seen my power bills rise because i can't afford solar panels.

      This is true. What's going on is that electricity distributors are having to make changes to the network that handle the feed-in from home solar, but that home solar doesn't help alleviate the evening peak usage time. That evening peak is a problem - it keeps growing and we have to keep increasing our generation capacity for it, even though it represents only 10-15% of a day (in terms of hours).

      Interestingly, if we had widespread Powerwall uptake, we'd see the evening peak usage from the grid decreased as people use their stored energy for their evening activities. There wouldn't be so much feed-in from home PV arrays (everyone's charging their batteries), and the infrastructural improvements that have been made in the last few years to handle it would still be good for a fair while into the future.

      But yeah, for the poor schmoes who missed the boat, I'd love to see an enterprising electricity generator come out with some sort of "Rent/Buy a Solar Panel" scheme. So if you're renting your premises and can't put an array up, you get into a financial arrangement one of the generators to help fund more large-scale solar plants, and get the benefit of some of the profit. Dunno if it would work; I'm sure smarter people than me have crunched the numbers.

        Funny you should say that. They did, and they started a company called SolarCity.

          Thanks for pointing this out.

          I'm poking about on solarcity.org, and trying to fathom where they're doing a scheme like this, or where as I private investor I can throw money at them. I see they're listed on the Nasdaq, so that's one way, but I was after something more direct.

      I'm actually in a similar situation.

      I live townhouse estate is unable to have solar panels (size of room, body corp issues, ajoining roof is a rental property) so I am in the same boat, pretty much getting charged extra because I can't get something.

      The other probl em is there are less people using the grid. But there are still poles etc everywhere. So same infrastructure but less people paying.

      I don't think you understand how prices are set in a capitalist system.
      The price charged is the highest price the seller can charge and still get a sale.
      The cost to make the product has no impact when deciding what price.

      The mechanism means that competition from private solar farms can only serve to decrease the market price.

        That's a slightly inaccurate thing to say. The cost to make the product is the starting point for deciding the price, because clearly no business could possibly survive in an ongoing manner if they charge less than the cost of producing their product.

        The ability for the electricity firms to make super-profits comes, in this particular market, from the inability of consumers to easily compare pricing between electricity companies, and the inability for consumers to easily change companies. But the electricity market, because electricity really is homogenous and electricity from AGL is no better or worse than electricity from Origin, could actually be really competitive.

          It's not really inaccurate at all. He is talking about setting prices, not base price constraints (below which you would close up shop).

          Electricity companies don't make 'super profits' (which aren't a real thing anyways) because of information asymmetry or switching barriers. They make supra market profits (which are a thing) because they are a regulated service and the Government allows them to do so.

        The price is set by the market rate and I was also talking about distribution which is another thing all together.

          "The price is set by the market rate"

          This doesn't appear to be a coherent thought, can you clarify it?

            Ok. The price is set by the NEM The national electricity market. Is that better.

              You realize we're discussing the consumer price, right?
              The NEM wholesale price changes every 5 minutes.
              Clearly the consumer price does not do so, and is thus not contingent on the NEM price.

    AGL actually states that the average 1 person household uses 1216 kWh a day

    Well, my 2 person household usage come in at about 1/100th of that average, so I think we're doing ok...

    I think the figure is wrong above... on the AGL site it states

    The approximate AGL Green Energy premium per quarter is $49.01, based on the national average household electricity consumption of 891 kWh per quarter effective from 1 January 2011. Actual amount will vary each quarter dependent on electricity usage.

    So 1216kWh a day is MASSIVE.

    Good number crunching - it might not be economical today but I can see a time in the future when this becomes a much better option.

    Would also be keen to see what impact this will have as a renter - could you 'plug and play' the technology. i.e. take it with you when you leave? Especially for long term renters or business/commercial premises. Generate all the power overnight when the business is shut and expend during the day.

    Thanks for doing the work!

    The assumption in this article are completely off base!
    From Clean Technica
    "The assumption has been that the $3,000 price for a Powerwall is the wholesale price.
    I reached out to Treehouse this week, one of the initially listed Powerwall partners, to find out what kind of markup it would be putting on the Powerwall. I was informed that the Powerwall will actually retail at $3,000 on the site. Wow."

    The price in Australia with exchange rate and shipping will be $4000 or under! It is of "Plug and Play" design with it's own inbuilt charger. If you have a solar system or you are installing a system they already have an inverter - no need to purchase! Installation is straightforward and should not be very costly. The PV Solar system price in the above assumption is WAAAY TOOO HIGH as well!

    Re asses your figures on this basis and I think you will find Powerwall installation quite compelling!

    Last edited 25/05/15 3:56 pm

      The figures are based of those for installation in the US by SolarCity.

      It may well be cheaper (or more expensive) to have installed here and we will certainly revisit the numbers when we have an official Australian installed price.

      In terms of solar, it depends entirely on the price you can get paid for generating solar, compared to the price you can pay for off-peak power.

      In my situation, it would cost me more to charge a Powerwall from Solar, instead of just selling that solar and charging off-peak.

      Of course the numbers are different for every situation.

        You will find that regulations will not allow you to charge the battery on your low rate ( Hot Water) or even regular off peak power! If you combine a 3kw PV system with a Powerwall - $3.5k + $4K - so say $8k on the generous side. That gives you 20kwh/day - 13kwh (Solar) + 7kwh Battery. 24 cents x 20kwh x 365 days gives you $1752.00. Now management of your use can optimize the system - cool during summer days with excess power so evening it's not needed much. Do laundery during the day etc etc. Use the low priced controlled load to heat the water - not the battery. Now even if you take a factor of 80% for more cloudy days than usual it still means the pay back is still 6 years!! The average Australian household uses 22kwh per day. What kills your analysis is the inflated price you have used! So if as you say, the prices are not known - why write the article?? PV Solar prices are easy to find. You can purchase 3kw PV installed for less than $3500 from a multitude of sellers.

          Control Load 1 can be connected to hard-wired appliances and includes battery charging systems. How this may be changed in the future is not known, but it is currently possible.

          I didn't say the pricing is not known - the installed pricing is based on quotes given for installed pricing in the US. There are no Australian prices yet, installed or not, to base it on. The US prices are a good starting point though.

          To save $1752 a year, the 3 kW solar system needs to produce the full 20 kWh + efficiency losses. In reality, the solar system will only produce around 2/3 rd of that - about 14.5 kWh. Of the 7.5 kWh not used to charge the battery, only a small portion will be able to be be offset by running appliances. This is why it's typically fed into the grid instead.

          Each day such a system would be able to offset charging the Powerwall (saving approx 50 cents of off peak power), a few kW at most of daytime appliance use (saving approx 60 cents) and feed another 4 or so kWh back into the grid (generating 24 cents @ 8 cents a kWh).

          This saves $1.35 a day, which gives a 16 year payback on the $8k cost.

          Using these numbers, if just the $4k Powerwall was installed an used with off-peak power, then the payback is actually lower at 10 years.

          These numbers assume a much cheaper install cost than has been quoted for the US though. That may well be true though, which would change the ROI.

    The figure of 1216kWh is supposed to have a decimal place I believe, 12.16kWh. People need to also be reminded that the importance of this device (and other's like it) it not just about deriving an economic benefit. It also about supporting renewable energy and it's continued development. I believe that in the short term electricity companies will (stupidly) increase prices in order to offset the profit they are losing from people going solar (instead of actually investing in renewables themselves and driving a new business model). However, they will need to walk a very careful line, otherwise they will simply provide greater incentive for others to go solar and hasten their own demise.

      Yes, everyone seems to be about the cash back investment. It will be a hard sell to say "Hey do it for the future" but hopefully many will jump on board and not worry if in 3-4 yrs they will be making money off the system.

        I'm fine with the 'do it for the future' argument, it would be interesting, but it's not just a gimme, it's also an analysis which needs to be made.

        I'm no expert but I'd expect the parameters to be things like;
        What materials are used to make this?
        Can they be recycled?
        Are any of them extremely rare or toxic?

      If the horse and cart owners who changed to motor cars only looked at the cost savings there would be no motor industry so the cost only argument is for the short sighted. We must all see the need for using renewable energy over fossil fuels and hang the expense to some extent and the more we make that change the more man will find ways to make it cheaper and better in the future. To keep burning coal maybe the only option for some 3rd world countries but you and I must pay back our abuse of these fossil fuels for our children's sake.
      Sadly with corporate executives rewarded by quarterly profits I fear they will be as stupid as 'Ben of the Plains' says and that's fine because I also agree that will motivate investment in renewable energy. What is sad is to listen to Aussie politicians bow to their paymasters as, like Nelson they put their blind eye to the telescope and in doing so, to me they lose their last ounce of credibility.

    Interesting article, but needs to go a little deeper:
    - factor in the rising cost of electricity, which is way in excess of inflation or general interest rates. Or (whisper it) a carbon tax.
    - I believe that the 10 year guarantee is renewable. And just because the guarantee is 10 years, it doesn't mean the powerwall will suddenly stop working.

    What is more interesting to me is that the power companies may actually subsidise powerwall type gear in future. The grid is currently specified for peak use, which wastes billions in overcapacity under normal conditions. People with powerwalls are effectively removed from the peak. Power companies could cater for expansion of normal demand by distributing powerwalls, reducing the current overcapacity, and save investment in new overspecified infrastructure.

    Sure, right now powerwalls are borderline economic and for committed enviro types. But give it a few years, and I suspect they will be as commonplace as solar now.

      He doesn't even discount the future savings by inflation, which means his math is a confusing mess of present and future values. Add in heroic assumptions about 'offset accounts' and ugh.

    I suspect the target for devices such as this is - those with solar systems.
    With the government winding back rebates, those with relatively new systems suddenly find themselves getting a lot less back. Why go through all the hassle of chasing the electricity company for your refund? Why not just save the energy, for your own use?

    The next step after this is the concept of Uninterruptible Power Supplies [UPS]: solar [and mains] constantly "top up" batteries which run your house. When summer hits, and everyone turns on air conditioners, electric companies get brownouts, transformer failures and load shedding. Your battery backup should ensure your home is good for several hours worth of power outage.

    Charging the electric cars of the future (i.e. saving on petrol / diesel) should also be taken into consideration as these will become the norm no doubt.

    I'm actually super interested in this, but it doesn't work out for me either. See below.

    My situation:
    - 3.5 kW PV system installed on my roof (on the east facing side, which I think is odd)
    - In WA and only get 7.135 c per kWh back from Synergy
    - I pay is 22.36 c per kWh (no on peak/off peak discounts)
    - I'm averaging 9.2 kWh usage per day (after any deductions due to generating while using)
    - I’m generating 10.5 kWh per day (after any usage while generating)

    So, assuming an ideal world, by installing this system, I’ll use all 7 kWh per day of the storage, and top it up during the day, whilst still selling 3.5 kWh back to Synergy.

    Thus I have to pay for 2.2 kWh (49.192c) and will sell 3.5 kWh (24.9725).

    So over 67 days (my latest bill period) I would be charged $16.22, vs my current $138.41 for usage only (supply charge is separate) for a daily saving of $1.82 or a buyback period of 15.2 years on a $10,000 install.

    There is also the added benefit of having a house wide UPS system if the power goes down.

    It's very very close, and will depend a little bit on what sort of discounts are about. I think when it gets down to the 8-10 years mark i'll jump. I think we're due a price increase for electricity so this might change it up too.

    Last edited 25/05/15 2:55 pm

      East is not a good direction for PV should be N or NW - W as afternoon sun is stronger than morning. The PW will come into Australia for around $4000 or less and install should be low cost. Read Clean Technica article "Tesla Powerwall DOES = $3000"!

      Last edited 25/05/15 3:54 pm

        As I said in my post i was a bit weirded out by the east placement. I just bought the place, the PV was a bonus.

        At $10,000 it doesn't make sense, if its more like $5000 installed, it really does and i'll grab one. If it's less than that? that's just candy.

    I think a couple of tweaks might be needed to those numbers.

    We had a 3kw solar system fully installed for $3000 in january and in all the quotes I got (other than a couple of stupidly expensive ones) a 3kw system should nowadays come in between $2000 - $5000 installed.

    Secondly this first domestic powerwall release is really aimed at solar owners, while it doesn't have an AC inverter it does have a built in DC/DC inverter. So it's actually designed to fit between the panels and your existing solar inverter.

    It has a peak output of around 3.3kw so it's meant to fit perfectly into any existing solar system with a 3kW or greater AC inverter with minimal installation costs as no extra inverter wiring is needed (hence the comment in the launch video that it takes less than an hour to install).

    By the time it gets to Australia in a year or two it's quite conceivable that you could get a powerwall and a bundled 3kw solar system for under $10k

      "Stupidly expensive" is sometimes code for 'will actually last 10 years'.

      "It has a peak output of around 3.3kw so it's meant to fit perfectly into any existing solar system with a 3kW or greater AC inverter"

      The most important question is how much maximum power will be required by the household. A toaster (2.2kW), a water pump if on tank water (1.6kW) some lights on and TV will trip the overload already just making a cup of tea in the evening.

      This only works if the inverter is capable to boost it's production with power from the grid. So it would generate 3.3Kw from the batteries and take the rest from the grid.

    Still not the best battery technology for storage. Flow batteries will win this battle I believe...

      hmm.. I just checked wikipedia and it states that these flow batteries are more complex and have rather low energy densities compared to lithium iron. Not going to happen me thinks.

    Thanks for the maths.
    I would like point a bit more into this math. As all the maths goes to the point of how much cost to install in different scenarios and the ROI of it, how different would be considering the costs if how much from our taxes goes to build electric power plants X the ROI of that back to us!? Plus, I believe this maths will not match in money wise for those who doesnt care about it as soon as it is greener and efficient. My point is I dont believe this a product for those who wants to save money, but it is for those who wants to change the world in sustainability matter. Right now for me the biggest down side is how to recycle those batteries once it reach their lifecycle. Probably not worse than what we do on the environment to create electricity to cities. Again I am happy this cones to a topic of discussion, and I am sure there room for improvement.

    7.5kw to charge and 6.5kw back? Closer to 87% efficient.

    But pedantic me aside, there is more information in this article than at their website. And their estimates on power usage are a bit over the top and there's no decent technical information on their systems. I don't agree with their average power usages but that's just being picky.
    You did ask for our input.

      Yep, because there are efficiency losses in the Powerwall and the inverter.

        Was more just trying to point out they were quoted seperately not as a combined system loss. And as it's used as a Complete system Not Separate Components Total Loss Is What The End User Would Be Interested In. And battery efficiency isn't a set value it varies with current draw and battery voltage.
        Eg.
        http://www.vonwentzel.net/Battery/00.Glossary/Peukerts.jpg

        I know it is picky but if you are properly setting up an off grid dependant property these are things you need to know. And if a manufacturer won't tell you. It's usually because they don't want you to know.

        Last edited 26/05/15 4:38 pm

          What sort of battery is this chart for? This post is about lithium iron batteries.

    Makes sense for people with lots of solar already though.

    In 18 months the $0.60 FIT stops for me, so my 4Kw system will be making around 15-20kw a day that is going to be unused, as no-one is home during the day. I doubt the power co. will pay more than 0.06 for the excess production.
    ,
    Put 2x Powerwalls in between my panels and the existing 6kw inverter, change to net-meter, and cost should be around $10K (2x $3k US, no inverter needed, simple install), which should have a payback of 8-9 years. That's on today's prices - if the AUD improves, and the price will come down once the Gigafactory is online, it'll only get better.

      You likely won't be able to use your grid inverter for home consumption. It needs the grid signal or it will cut out and it's not designed to ramp up/down production depending on the load. It's just churning out as much as it can to the grid.

    The discussion only talks about people taking money our of their home loan account. What a bout a pensioner who has money in an income or asset assessable account or share portfolio which is only yielding 2%? The buy the system and make it part of their primary residence and it is no longer assessable. Their pension goes up and/or they get a health card. Their return on saved power bills (non taxable income) is vastly higher than anything else they currently have.

    Big point to remember is the connect to grid per day charge.
    Under a dollar per day at the moment but, "watch this space".
    Nearly $300 for just synchronising with your PV.

      Yes, we are still using the grid as our backup and the fee to be just connected sucks. When I come across a good generator like one of those nice honda ones I'll cut the grid for good.

    Apart from the math I believe there are plenty people that want their energy independence. And those that have been treated badly by their energy utility. A step towards independence might be worth more to people than any savings.

    I have a home built off-grid solar system with lithium iron phosphate batteries. The capacity of the battery bank is 24v @ 400 amp hours which translates to 9.6 kWh capacity. That battery bank has cost me about $5.4k delivered. On my roof is a 3600w solar array ($2.3k direct from china) which feeds into 2 Outback FM80 charge controllers (US $550 each delivered) which keep the batteries topped up. A 2nd hand 4kW Latronics Inverter ($2300) feeds the mains of our home. A battery monitoring system ensures the batteries are never over charged or over discharged. All up with racking and wiring we have spent about $12k. The batteries are only discharged once the sun goes down and are more often than not fully charged again well before lunchtime. The batteries accept a very high charge rate. I often see more than 90 amps on a sunny day. For overcast days, we are able to switch back to the grid with a transfer switch. We do this about twice per month.

      May I ask where you obtained the $5.4k battery?
      Being 9.6kWhr, that battery is considerably cheaper than Telsa's Powerwall.

      The Powerwall's output is 2kW continuous. 'Peak' output is for a short time. Tesla do not define that time. It would be easy to exceed 2kW with just one or two household items.

      Another point not mentioned in the article, is that Tesla's battery is 400V, so can't be self-installed, nor is it compatible with low-voltage inverters such as you have.

      Last edited 27/05/15 2:06 am

    Origin energy just launched a similar scheme to Solar City... https://products.originenergy.com.au/4688/Solar-as-a-Service, and AGL has payment options that are comparable http://aglsolar.com.au/payment-options/

    payback becomes MUCH more interesting when on Time of Use tariffs (increases differential between OP and Peak rates). Unsure about whether its possible to use your Controlled Load to charge the battery... it's triggered/controlled by the network which (i imagine) would limit your charging behavior. If you're not getting a state/distributor subsidised FiT for your excess solar, you should use excess solar to charge the battery.

    I have had my 5Kw grid connect system connected for 2 years now. I also have a separate system for storage which is dedicated to running our pool.
    Combined between grid connect and storage, i am saving up to $3000 per year. The whole grid system will pay for itself in 4 years. I would look at the Telsa or Samsung storage system dependent on cost.

    If it just came down to economics it makes better sense for the grid companies to buy the batteries rather than the end users. They have economies of scale.

    In the real world, everyone is trying to game the system. Bigger companies with more market power are going to be able to do this better; there's plenty of evidence for this. End users can try to game the timed pricing of the suppliers. In practice, suppliers would rather put in some batteries themselves to reduce their peal generation cost and their peak charges than hand control to consumers. However, there's always a mix of cost, usage, continuity and other considerations, so batteries will be work for some end users. Especially anyone with existing solar.

    The good news is that cheaper batteries make power cheaper and improve the economics for solar and other intermittent renewables. There is no fundamental reason why batteries can't do the kind of price performance drop that solar has done and is continuing to do. We certainly haven't seen the end of this. Welcome to the stored energy future.

    Who cares? Buy cheap now. In 5 years solar will cost half what it does now, at which time you just buy another cheap system.

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