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Tesla Powerwall 2 review - AC battery storage system

Jason Svarc

UPDATE: March 2017 - The Powerwall 2 will only be available as an AC battery. (The DC version is no longer available)

The highly anticipated Tesla Powerwall 2 battery system from Tesla Energy is finally here, but with limited units available until production increases. The first Powerwall with 6.4kWh storage capacity was released less than two years ago and in that time the engineers at Tesla managed to develop the second generation Powerwall with double the storage capacity at close to half the price.

The new Powerwall 2 (PW2) with 13.5kWh storage capacity is AU$8800 (US$5500); with the first Powerwall costing a little over AU$7200 this is roughly a 40% price reduction. Although the new distribution model also made it much more affordable with less middleman involved in the sales and distribution.

Tesla/Panasonic battery

The remarkable cost reduction has been achieved due to the enormous Tesla battery manufacturing facility known as the Gigafactory in Nevada. Both Powerwall’s use Panasonic battery cells which are very similar to the cells used in the Tesla electric vehicles, although the new generation, larger 2170 lithium battery cells are used in the Powerwall 2. The lithium NMC (Nickel-Manganese-Cobalt) battery cells used in the Powerwall 2 are manufactured in collaboration with Panasonic within the Tesla Gigafactory.

 Powerwall 2 battery Competitors - See how the Powerwall 2 compares to other battery technologies including the new range of home lithium batteries by LG chem

The new PW2 has been completely redesigned to be more compact, easier to handle and install. Gone are the sleek curves, giving way to a more practical square shape with venting either side for cooling and a bright green LED strip. Interestingly it also has a simple on/off switch on one side. Still weighing in at 120kg it is definitely not light, although thankfully the PW2 can now be ground mounted as well as wall mounted. The IP rating (weather rating) remains high at IP56 although I would still recommend installing in a reasonably sheltered location such as carport or garage and out of direct sunlight if possible.

Battery Inverter included

The AC Powerwall 2 includes an integrated AC-DC inverter which gives it a price advantage over the competition. The inverter power is reasonably high with a continuous output of 5kW and peak of 7kW, compared to 3.3kW (DC output) for the previous model. The higher power output is much more suited to the typical loads of an average home which can have several high powered appliances running simultaneously. Although the 7kW peak is not as high as similar sized dedicated battery inverters, plus it can maintain 7kW for only 10 seconds which means it is only suited for brief power surges.

 

The Powerwall is still the only battery system on the market to incorporate liquid cooling and has an operating temperature range of -20 to 50 degC. Although it is likely to de-rate or reduce its output once above 40degC.

For larger capacity systems the PW2 will have the ability to be setup in split or 3 phase installations with up to 9 battery modules able to be linked together in series (although this capability is not yet enabled).

See the full AC Powerwall 2 specifications / datasheet here

 

Separate Solar Inverter Required

Since the Powerwall 2 is an AC battery (AC in / AC out) in order to function together with solar the Powerwall 2 requires a solar inverter as well (Refer diagram below), although the solar inverter can be any model since it operates independent of the Powerwall. AC battery systems are becoming more popular due to the simple installation although they are not as efficient as more common DC systems when used with solar. This is due to losses in the conversion from DC to AC power - first the DC solar energy is converted to AC by the solar inverter, then back to DC to be stored in the battery and finally back to AC when it is needed by the appliances (with typically 3-5% losses in each conversion).

Many hybrid solar systems available have incorporated both a solar inverter and battery inverter in one complete unit, this makes installation much easier and requires less space to install. Redback Technologies, SonnonBatterie Eco and Alpha ESS just to name a few, some of these can also provide back-up and pass-through power as standard. I would expect the next generation Powerwall to move in the same direction and offer a complete solar/battery/back-up solution.

Backup Power

Unfortunately the PW2 cannot function as a backup power supply or UPS without additional components. To enable backup power an optional Backup Gateway will need to be installed at additional cost. The rather large Gateway box, which is not as aesthetically pleasing, communicates with the Powerwall over wifi and put simply it's job is to isolate the house from the grid if it detects any issues with the electricity supply. For those who were hoping to purchase a Powewall and go off-grid, it can operate in off-grid (back-up) mode but it is not designed to function as an dedicated off-grid power system. I would recommend reading this first before considering disconnecting from the electricity grid.

Typical system layout with the Powerwall and solar inverter feeding power the home.

Typical system layout with the Powerwall and solar inverter feeding power the home.

DC VERSION - No longer available - Since the DC PW2 like the PW1 operates at the higher 350-450V DC range the number of compatible solar inverters is very limited which is one of the disadvantages of ‘high voltage’ batteries. Although the 2 main inverter options are Fronius (3 phase) and SolarEdge (single phase) which are both highly regarded in the industry and are 2 of the leading solar inverter manufacturers. Another well regarded inverter manufacturer SMA is also compatible but only using the 'Sunny boy storage' inverter (limited to 2.5kW output) which was ok for the PW1 but a little undersized for the PW2.

Controversy

Tesla is a disruptive and innovative company which has had several last minute changes to its products before release. This may be due to a number of reasons including raising public interest through its clever PR strategies, but there is also likely to be unforeseen technical and engineering difficulties which need to be ironed out before release. This has caused some issues, especially in the case of the Powerwall 2 as some solar installers have installed DC Powerwall compatible systems only to be told the DC coupled version will no longer be released.

Tesla Energy’s aim is to focus on safety, quality and customer service. A business model that has enabled Tesla to become one of the largest companies in the world without actively promoting their products other than through product launches. The customers and press simply promote it for them. Quite clever really.

We review the best solar panels from leading manufacturer's LG, Sunpower, REC, Trina and more.

We review the best solar panels from leading manufacturer's LG, Sunpower, REC, Trina and more.

Warranty

The warranty for PW2 is 10 years with unlimited cycles (self-use and back-up use only). This is much better than the original Powerwall 1 warranty but is still a little ambiguous, for example if it is charged using off-peak energy during winter (a feature which is not yet enabled possibly for this reason) then the number of annual cycles may be much greater and it will reduce capacity at a higher rate. Although since all batteries reduce capacity over time, the 70% retained capacity after 10 years is reasonable. See the full Warranty details here

First section of the Warranty - Powerwall 2 (DC version)

First section of the Warranty - Powerwall 2 (DC version)

Wrap up

Powerwall 2 monitoring app

Powerwall 2 monitoring app

Tesla learnt a lot from the first generation Powerwall and took on board many of the issues highlighted by installers, in particular the limited compatibility, since it was a high voltage (400V) battery it required a specialised hybrid solar inverter to function. Tesla overcame this by developing the PW2 as an AC battery which operates independently of the solar inverter and thus is compatible with any solar system, new or existing. However the built-in inverter does not have the ability to isolate or island itself from the grid and thus requires additional equipment to function as a back-up power supply, also unfortunately it is not designed for completely off-grid installations.

The Powerwall 2 is one of the best value battery systems on the market, with a large 13.5kWh storage capacity it is well sized for most large modern homes. Also it is the only battery to incorporate liquid cooling which may mean it outlasts other batteries especially in hotter climates. With a 10 year warranty and building on the experience from the Tesla vehicle batteries it is a well engineered system and has so far proven to be a high performing battery. Although with the current supply issues you will have to wait at least until early 2018 to get one. Luckily there are many other 'hybrid systems' available which include batteries and are very good value for money, plus most include a solar inverter and back-up function in one complete unit.

See how the Powerwall 2 compares to other battery technologies in the complete solar battery comparison here.

 

Commercial size Tesla Powerpack

Large Tesla 100kWh Powerpack module

Large Tesla 100kWh Powerpack module

Tesla Energy also has large scale Powerpack modules available for utility scale and commercial energy storage. The 100kWh Powerpack (ref image) is simply made up of multiple smaller battery units and available in a modular system which can be easily linked together to create several MWh of energy storage. With each Powerpack costing around AU$40K this is a breakthrough price for large scale storage.

 

Recycling

Recycling the complex Lithium based batteries used in the Powerwall poses several challenges compared to common lead-acid batteries which are easily recycled, although much research and development is underway. A recent breakthrough by American Manganese Inc or “AMI” has shown very good results in extracting the raw materials from Lithium NMC batteries for re-use. The Lithium NMC batteries used in the Powerwall are difficult to recycle but Tesla claim it will be feasible once the volume increases; since most Tesla lithium batteries are still in use and are expected to last 10+  years there are simply not enough used batteries available yet to refine the recycling process.

 

How does the Powerwall 2 compare with the competition?

See the complete battery storage comparison and review here - Both the Powerwall 1 & 2 compared to leading lithium and sodium based batteries LG Chem and Aquion Energy, plus proven lead-acid and advanced lead-carbon batteries in the solar hybrid battery challenge.