Tag Archives: powerwall

Tesla Powerwall Payback Math for South Africa

The Tesla PowerWall is finally becoming available in South Africa, so I thought I’d do some “30% for math” calculations to work out payback periods.

This post is mostly inspired by the comments here – http://mybroadband.co.za/news/energy/154888-here-are-the-tesla-powerwall-systems-you-can-buy-in-south-africa.html

Thanks to MacAfrican for the criticism in my comments, otherwise I’d be too lazy to do this post 🙂

Onto the math –

Predicted cost at the moment for a daily use PowerWall is $4000.
Rand is currently hovering at R16 (lets hope Zuma doesn’t open his mouth in the near future, as that historically has lead to large drops in Rand values).

Assuming R16 x 4000$, we have a cost of R64,000 for the battery itself.
Obviously thats a large sum of money.

Does it make sense / cents to buy one?

Lets have a look. First up we need to try to work out total lifetime costs.

The warranty for the Tesla on the NaturalSolar.com.au site indicates this

The Tesla Powerwall includes a 10 year limited warranty.
The limited warranty covers defects in parts and workmanship, as well as at least 60% energy retention after 10 years, provided it is registered and used as intended.
The Tesla Powerwall is designed for daily use applications like self-consumption of solar and load shifting. Assuming full daily cycles, Tesla Powerwall is designed to provide energy for 3650 full equivalent cycles which is equivalent to 10 years of use.

Thats interesting, as it now gives us an indication of cycle usage.
From that, I can infer that each year we’ll see a drop of around 5% in capacity.

So year 2, we’ll see 95% of original capacity, year 10 down to 60% of original capacity, and at say Year 15, around 30% of original capacity. At year 15, I’d probably want to replace the unit, or have it as a secondary storage device..

With that in mind, we can do some math!

I’ve made a basic spreadsheet using those figures and worked out payback periods for the units.

I can’t predict Eskom pricing, so I’ve gone with current CoCT pricing per KW, and worked with annual % increase’s.
Total lifetime I’ve kept to 15 years, although you could probably scrape another year or two out of the units. I expect battery replacements to at least have halved in current Rand / Dollar terms in 10 years though, so replacement should be cheaper assuming Zuma doesn’t do any more Nene’s..

Below is what it looks like for a 5% annual increase

Screen Shot 2016-02-15 at 11.50.25 AM

You’ll see that it currently doesn’t make sense to use a PowerWall at a yearly 5% increase, even at a 15 year time frame. It comes close, but no cigar..

What happens at 10%?

10% increase

At a 10% annual increase (which might be closer to what Eskom pricing will eventually be than at 5%), we see breakeven in the 12th year of ownership. By 15 years we’re safely into profit.

Lets look at a best case – well, “worst case” scenario with a yearly 15% increase:

15% increase

15% annual increase see’s break even at Year 10.

Its unlikely that we’ll see continued 15% increases though, I guesstimate using thumbsuck that we’ll see continual annual increases of 8%, which leaves us breaking even at around Year 13.

Feel free to play around with the values, I’ve uploaded the Numbers file here (as I’m a larney Mac user), or as an Excel sheet here.

In other news, am fully expecting Rich from HomeBug to critique this, hehe 😉

Some points to note:

The Rand Dollar rate is going to be the main cost influence on whether the PowerWall makes sense. If the rand drops further (and the indications are that it will), then it doesn’t make sense at R20/ dollar. If by some miracle the rand recovers to say R14 or R12 to the dollar, buying a PowerWall is a no-brainer.

NERSA approved increases may or may not beat my guesstimates. Historically we’re much more expensive per KW than 10 years ago by a large factor, so its likely that a moderate value of 10% increase per annum is going to correlate with actual figures. This will also increase once Eskom/ Muni’s introduce further daily connection fee’s and other non tariff increases on top of per KW pricing.
(Actual historical figures can be found here – http://www.eskom.co.za/CustomerCare/TariffsAndCharges/Pages/Tariff_History.aspx )

I don’t calculate round trip values for Electricity in /out of the PowerWall. Tesla documentation indicates that this is 92%, so final KW generation figures probably should be discounted by 8% for further accuracy.

I also assume you’ll be generating electricity to go into the unit from a solar install. Costs for that are not included, as we are looking purely at the viability of the PowerWall. While I can do full system calculations, its already clear that Solar generation is already cheaper than Eskom in South Africa, and has been for a few years no. Rehashing that again is of no interest to me.

Tesla’s Powerwall

As there is a lot of interest on Tesla’s Powerwall, I thought I’d collate what I could find out about it.

The official press release details are here – http://www.teslamotors.com/presskit/teslaenergy and the end user details here http://www.teslamotors.com/powerwall


From those pages we can glean this:

Mounting: Wall Mounted Indoor/Outdoor
Inverter: Pairs with growing list of inverters
Energy: 7 kWh or 10 kWh
Continuous Power: 2 kW
Peak Power: 3.3 kW
Voltage: 350 – 450 volts
Current: 5.8 amp nominal, 8.6 amp peak output
Round Trip Efficiency: >92%
Operating Temperature Range: -20C (-4F) to 43C (110F)
Warranty: 10 years
Weight: 100kg
Dimensions: H: 1300mm W: 860mm D:180mm

There are 2 different Powerwall units available:

10KWhr for weekly cycle use @ 3500$ to installers.
7KWhr for daily cycle use @ 3000$ to installers.

Up to 9 units can be chained together to provide more power.

Simple math for the given values shows that the continuous / peak sort of make sense –
350V @ 5.8A = 2030W +-
350V @ 8.6A = 3010W +-

The voltage is curious though – at that sort of voltage, it means you will be connecting to a hybrid grid tie inverter rather than a battery inverter. Most commercial battery inverters run at 24/36/48 (or other higher multiples of 12), so that eliminates using those (if the voltages are correct).

The press pages do mention that the Powerwall is supported by the Fronius Symo Hybrid inverter.
The datasheet for those is here – http://www.fronius.com/cps/rde/xbcr/SID-3DAF0C16-CD8F5048/fronius_international/SE_DS_Fronius_Energy_package_EN_386411_snapshot.pdf

If we take a look at that, we can see that they only seem to offer a 3 phase solution currently, and that it talks to batteries using Modbus RTU, and at various voltages, depending on capacity.

Fronius’s 10.5KWh unit offers 8.4KWh capacity (80% DoD), and connects at between 280-400V @ 16A, so would be the closest offering they have to the Tesla Powerwall. Its also about twice the price!

There is other data out there though. This page purports to show details of TESLA’s 10KWhr unit.
(pictured below)

Screen Shot 2015-05-04 at 5.36.30 AM

That unit (if its real) looks a lot more compatible with battery chargers as it runs at a more common 48v.

What batteries are they using?
Tesla are using NCA batteries (LiNiCoAl)- Lithium Nickel Cobalt Aluminum in the Powerwall 10KW (weekly cycle unit)

Tesla are using NMC (LiNiMnCo) in the 7KW unit.

Elon Musk – quoted from the recent press conference:

The 10kWh device is designed as back-up, suitable for 60-70 cycles per year. Its chemistry is similar to the Tesla Model S electric vehicle, and is nickel-cobalt-aluminium cathode.

The 7kWh system is designed for daily cycling – when homes and businesses will store solar electricity produced during the day. Its daily cycling control constituent is nickel-manganese-cobalt, and Musk expects it to daily cycle for “something on the order” of 15 years.

“Actually the warranty period would be a little bit less than that,” Musk said.

“But we expect it to be something that’s in the kind of 5,000 cycle range capability, whereas the high-energy pack is more like around the maybe depending upon on how it’s used anywhere from 1,000 cycles to 1,500 cycles. And they have comparable calendar lives, and for the high energy one, it’s important to appreciate that this actually has a lot of interest from utilities “

What actual capacity is the 7KWhr unit? (daily cycle)

 Option 1

The 7KW unit is really a 10KW unit, and they’re running it at 70% of actual capacity.


Lifetime for NCA is in the 10 year range (3000 cycles +-) @ 70% DoD (depth of discharge) before it drops below 80% of original capacity.

Lithium doesn’t like being 100% charged, and it doesn’t like being 100% discharged.
Data sheets indicate that Lithium prefers 15-85% for longer lifetimes, so thats the “sweet spot”.

 Option 2
It might be that the 7KW unit is only 7KW storage though, which means actual capacity is really 70% of that or just under 5KW (4.9KWhr usable).

I personally think that its going to be option 2, sadly.

Why the cooling?
Lithium and other battery lifetime is extended dramatically if you keep it cool.
I’ll assume for lifetime purposes they’ll try to keep the batteries down to 25C

Why is it a game changer?
Pricing is about half of current retail battery units.

…but Lead Acid is cheaper!
Lead Acid isn’t cheaper.

Lead Acid provides about 30% usable capacity for a battery.
Lithium provides about 70% usable capacity for a battery.

So, if you need 10KW of usable storage, you need
30KW of Lead Acid
14KW of Lithium

Lead Acid will take up far more space.
Have less lifetime.

Whats the difference between the 10KW unit and the 7KW unit?
Other than the sizing / cycle usage, we don’t know (yet).

10KW unit is rated for weekly cycle, so would probably be for UPS style usage – eg to assist with grid outages like Eskom’s regular ones. Given Eskom’s reliability though, the 7KW unit may be more applicable here!

7KW unit is rated for daily cycle, so would probably lend itself to off grid, or time shifting, or if power outages get to be regular day to day outages here; ideal for that.
South Africa doesn’t currently have time of day rates for residential end users, although thats coming at some point, so time shifting isn’t useful here (yet).

Some thoughts on this –
If the 10KW unit is rated for weekly discharge @ 10 years (3650 / 7), thats gives you roughly 500 cycles.
500 cycles for NCA batteries @ 100% DoD is the rough lifetime, so it looks likely that 10KW is actually 10KW.

If the 7KW unit is rated for daily discharge @ 10 years, thats 3650 odd cycles. 70% DoD for NCA batteries gives about 3500 cycles before it hits 80% of original total capacity, so would indicate that the 7KW unit probably is going to be 4.9KW usable, as that also matches with the lifetime / usage.

Their pricing points for the 2 units sort of confirms it – 10KW @ 3500$, 7KW @ 3000$

So, my guess is that the 7KW is going to turn out to be 4.9KW usable out of 7KW actual storage.

Some real life testing for the NCA battery tech Tesla uses here – http://ma.ecsdl.org/content/MA2011-02/17/1282.full.pdf+html

What do I need to use a Powerwall?
At a best guess, it looks like its intended to be used with a hybrid inverter.
A hybrid inverter is a grid tied inverter with battery charging capability that can create a “mini house grid” if Eskom goes off.

So far Tesla says it works with the Fronius Symo Hybrid inverter.

Assuming that there really are a number of different units, it looks like they may offer a 48V version for those with existing MPPT chargers (as per the graphic up above somewhere).

Fronius’s Symo Hybrid inverter uses Modbus RTU (over RS485 – serial) to talk to its batteries, so I would expect the Powerwall to support that (amongst other things)

The (alleged) data sheet says IEC 61850 ( http://en.wikipedia.org/wiki/IEC_61850 )

Tesla has also been advertising for staff who have experience with RS485, Modbus, IEC 61850 and DNP3

IEC 61850 is the european favoured smart grid standard, DNP3 is the american standard.

Support for those is more likely to be in place for Tesla’s larger units eg the 100MW and up options.

Whats the price likely to be in South Africa?
If one counts in duties, clearance, shipping it will probably be in the 5000$ range or R60,000 for the 7KW.

I’ll try to keep this post updated when more info comes out.
Feel free to ask questions in the comments.