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POWERWALL 2 BY TESLA an article by Solar Quotes

Powerwall 2 Full Spec Released: Low cost but ‘unlimited cycles’ warranty has limits.

SOLAR QUOTES ARTICLE BY FINN PEACOCK

What is a tesla powerwall battery?

Yesterday Tesla kindly sent me the Powerwall 2’s full data sheet and warranty.

We already knew the headline specs:

Storage: 13.5kWh

Power: 5kW steady, 7kW Peak

How much is a tesla powerwall?  Price: AUD$8,000 + GST and Installation

But there were lots of questions over the exact configuration and warranty details that could make or break Tesla’s shiny new battery.
Most of these questions have now been answered.

Here’s what the new specs reveal:

Hells Bells It’s True! AC / DC versions

The specs reveal the rumours of 2 versions of the Powerwall 2 are true. An AC coupledand a DC coupled version will be released, both for $8,000 + GST + installation. This is a very smart move.  The AC version has an internal battery-inverter to change your home’s 240V AC  to the 50V DC the battery operates on and vice-versa. This means it should be a piece of cake to retrofit to any existing grid connected solar system.

The AC version also allows you to use any brand of solar inverter you like – including micro inverters.

Tesla are claiming the AC powerwall can also be configured for backup operation with a flick of the switch, but I’m skeptical. Here’s the data sheet for the AC version:

Download (PDF)

If you want backup, then you are probably better off with the High Voltage DC version, for reasons I’ll go in to  tomorrow. In the meantime here’s the data sheet for the DC version:

Download (PDF)


No Solar Inverter Included

I was really hoping that Tesla would have gone Rock or Bust and included a solar inverter as well as a battery inverter (AKA a hybrid inverter), as that would have made full solar and battery installs much simpler and cheaper. But I guess it is a Long Way to the Top and I’m predicting the Powerwall 3.0 will come with an integrated hybrid inverter that you simply plug the panel array into.
 

System Efficiency

The AC Powerwall 2 has a roundtrip efficiency of 89%. The DC version is 91.8%.

This means that the most efficient configuration for overall system efficiency is to DC couple the DC Powerwall using a hybrid inverter:

[dc coupled - dc powerwall]
dc-coupled-dc-pw2-effy
If DC coupled with a hybrid inverter with an average efficiency of 96%, the overall efficiency from solar generated electricity to supplying battery power to the house would be 88.1% for the DC version.

If you use the AC version – you are looking at a total system efficiency of around 85.4%.

[ac coupled ac powerwall 2]
ac-coupled-ac-pw2-effy2
And if for some crazy reason you decide to AC couple a DC powerwall1 then you only get 84.6% system efficiency.

[ac coupled dc powerwall]
ac-coupled-dc-powerwall-effy
The AC version claims it can supply 13.2kWh of AC power.  This means its built-in battery inverter would operate at an average efficiency of around 97.8% which is exceptional compared to a top-end battery inverter like the Sunny Boy Storage which is 96.5% efficient.
 

Depth of Discharge

Tesla says the batteries are 100% depth of discharge.  I don’t actually believe this because completely discharging lithium-ion batteries is bad for them.  I think the batteries are larger than 13.2kWh and when they reach what Tesla calls a 100% discharge they’re actually not completely flat.

But I do approve of Tesla describing them this way, as it keeps the usable capacity and nominal capacity the same – eliminating confusion for consumers.
 

Adjustable Power Factor

The AC version will have adjustable ‘Power Factor’ which means that it should comply with the latest Australian inverter standards (many older model inverters don’t meet these latest standards and so can’t be installed any more).
 

The Powerwall 2.0 Warranty

This was the document I really wanted to see. The warranty for the DC version is here:

Download (PDF)

(I have not seen the AC version’s warranty)

I was not a fan of the Powerwall 1’s warranty. But I’m happy to say that the Powerwall 2 warranty is much, much better. While there are still a few clauses that should not be in there – as they are clearly overridden by Australian Consumer Law – the warranty is generally clear, short, in Plain English and, apart from one point, unambiguous.

I like to rank batteries by a factor called ‘cost per warranted kWh’. To calculate this, I look at how many kWhs of storage are actually warranted, then I divide the retail cost of the battery by that number. This gives me a cost to store each kWh of energy in the battery, as warranted by the manufacturer.

Now you’d be bloody unlucky if your Powerwall failed the day after the warranty expired – so your actual cost per kWh will be less. But as a relative comparison of the cost of storage – that the manufacturers will warrant – it is a really useful number.

The really exciting thing about the Powerwall warranty details are that they show a cost per warranted kWh that is 70% of its nearest competitor according to our freshly updated battery comparison table.
cost-per-kwh

 

Calculating cost per warranted kWh


The Powerwall 2 has two different warranty categories depending on how you use it.

pw2-2warranty-conditions

Category #1: Tesla warrant the Powerwall 2 DC, for 10 years and unlimited cycles when used for “solar self consumption / backup”.

They include a footnote that defines this as:

 “Storing solar energy generated by an onsite array, and using that stored solar energy (i) for daily self-consumption and/or (ii) for use as backup power.”

But this definition does not include:

Charging the battery with off-peak electricity as is often done with battery systems that are on-grid and is something that services such as Reposit do.
Charging the batteries with a generator which is often done with systems that are used off-grid.

I am surprised that Tesla won’t allow these two common activities without the warranty falling into the second, worse category:

Category #2:  For all uses that do not fall into category #1 Tesla warrant the Powerwall 2 DC, for 10 years or until it provides 37,800 kilowatt-hours of stored electricity (approx 3,200 cycles2 )

Obviously category #1’s unlimited cycles are better, but in practice the two categories are much the same.

The first category with unlimited cycles is impressive. In theory you could get an average of 2.6 cycles a day or potentially more, if you had a huge solar array and did nothing but charge and discharge it at full power all day every day. But only a complete loonie would do that.

Realistically, even large users of electricity with 10 kilowatts of solar panels are likely to average below 0.85 cycles a day when limited to only charging off solar electricity as per category #1.

And because the Powerwall 2’s capacity is so large and most people have smaller solar systems, the actual average will be considerably less.  So realistically, Tesla ‘unlimited cycles’ warranty will only have to warrant up to around 3,100 cycles over 10 years, which is a little less than the approximate total number of cycles the second warranty category gives.
 

It Is An 8.8 Year Warranty When Cycled Once A Day

Because many people are going to want to charge their Powerwall 2 with off peak grid electricity, or use a service like Reposit, or charge it with a generator if off-grid, most people are likely to fall into the second warranty category and have a hard limit of 37,800kWh for their warranty. So what does this hard limit mean in practice?

Generally, households are getting 0.8 to 1.2 cycles per day from their batteries in Australia3.  At 1 cycle per day, taking account of battery degradation4, the warranty will last for just under 9 years:
pw2-warranty-graph
When fully cycled, once per day, and used for solar shifting, peak tariff arbitrage and/or Reposit, the warranty is just short of 9 years.

Because the Powerwall 2’s storage is so large, most households won’t use it at this capacity and so its warranty will last for 10 years.  But note that if you are using it at less than full capacity then the cost per stored kilowatt-hour increases.

Here’s a graph that shows how cycling your Powerwall 2 more or less than 1 cycle per day affects its warranty for ‘other uses’:
cycles-vs-warranty2
 

Warranted Degradation / Energy Retention

All lithium-ion batteries degrade over time. Tesla warrant that the Powerwall 2 will still have at least 70% of its initial capacity after 10 years. That compares to 60% for the LG Chem RESUs and 80% for the Enphase AC batteries.

The only ambiguity I can find in the warranty is that they don’t specify the rate of decline. If the battery drops to 70% on the second day of operation you would not be a happy camper. It would be nice to see Tesla warrant a linear degradation down to 70% at 10 years. Like this:[powerwall 2 warranty graph]
Assumed warranty capacity
I’ve assumed this linear decline in energy capacity for my calculations.

 
I’d be pretty happy if my home battery still had almost 10kWh of storage left in 10 years time.
 

Warranted Cost Per kWh

At a retail cost of AUD$8,800 inc GST and 37,800 warranted kilowatt-hours of storage, that gives a cost per warranted kWh of 23.3 cents. This is 70% of the cost per kWh of the little known DCS PV5.0, and 60% of the cost per kWh of Tesla’s main rival, the LG Chem Resu 10.

If you want to assume your Powerwall 2 will provide 50% more kilowatt-hours of storage than it is warranted for before it dies, then you are looking a cost per kWh of 15.5 cents.  That’s pretty compelling for large users of electricity with large solar systems.
 

Conclusion

For early adopters looking to buy a large battery storage system, then the Powerwall 2 looks to be unbeatable right now.

I expect the competition, especially Enphase and LG to reduce their prices in response to Tesla’s challenge very soon5 which should make for an exciting 20176. Watch this space.
 

Footnotes

  1. The only reason I can think of that you might do this is for a retrofit to an existing modbus inverter with backup functionality  [↩]
  2. I have assumed the battery pack will have 75% of its original capacity by the end of its warranty and the degradation in its capacity will be linear.  This gives an average of 11.81 kilowatt-hours per cycle and 37,800 kilowatt-hours divided by that amount gives 3,200 cycles.  [↩]
  3. according to Reposit who have Terabytes of real battery usage data.  [↩]
  4. Again, I have assumed that by the end of its warranty the Powerwall 2 will have 75% of its original capacity left at the end of its warranty and its decline in capacity will be linear.  [↩]
  5. It certainly looks like LG have got the margins to reduce their prices substantially  [↩]

http://www.solarquotes.com.au/blog/author/admin/

About Finn Peacock

I'm a Chartered Electrical Engineer, Solar and Energy Efficiency nut, dad, and founder of SolarQuotes.com.au. My last "real job" was working for the CSIRO in their renewable energy division.
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