The Tesla Powerwall 2 lithium battery system from Tesla Energy has made a big impact in the solar world and pushed home energy storage into the mainstream. The first generation Powerwall released around 4 years ago had a much smaller 6.4kWh capacity and was a high voltage DC coupled battery, while the second generation Powerwall 2 has over double the storage capacity at almost the same price, including an inbuilt inverter/charger.
The Powerwall 2 (PW2) with 13.5kWh storage capacity had a price increase in late 2018 bringing it up to $10,000 (US$7400). This is possibly due to the original price being at break even point to get sales traction and fill the capacity of the Tesla gigafactory. With the first Powerwall selling for $7200 for 6.4kWh of storage capacity, the Powerwall 2 still offers a 30% price reduction per kWh and is competitive when compared to other leading battery systems.
Powerwall 2 main features and specifications
13.5kWh usable capacity and 100% depth of discharge
AC battery with integrated inverter
5kW continuous power output with 7kW peak output rating
Liquid cooling system
10 year warranty to 70% retained capacity*
Listed price AU$10,000 plus $1700 for the backup gateway 2 **
* Based on information available at time of writing - subject to change ** Plus installation costs. Does not include solar inverter, panels or BOS.
Tesla Lithium NMC battery cells
The lithium cells used in the Powerwall 2 are produced in the enormous Tesla manufacturing facility known as the Gigafactory in Nevada. The Powerwall 2 uses lithium NMC (Nickel-Manganese-Cobalt) battery cells developed in collaboration with Panasonic which are similar to the cells used in the Tesla electric vehicles, although the new larger 2170 lithium battery cells are used in the Powerwall 2.
Compared to the first Powerwall the Powerwall 2 was designed to be more compact, easier to handle and simpler to install. Gone are the sleek curves of the first Powerwall, giving way to a more practical rectangular 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 it is still recommend installing in a sheltered location such as carport or garage and out of direct sunlight if possible. Heat and batteries just don't go well together.
Battery Inverter included
The Powerwall 2 is an AC battery which means it has an integrated AC-DC inverter/charger. This has both advantages and disadvantages explained in more detail later, but one main advantage is the ability to be easily retro-fitted to homes with an existing solar installation.
The Inverter power is reasonably high with a continuous output of 5kW and peak output of 7kW, compared to 3.3kW (DC output) for the old 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. The 7kW peak rating is rather high but it can only maintain 7kW for 10 seconds which means it is only suited for brief power surges and not designed for modern off-grid home installations. See more about off-grid solar power systems here.
See the full AC Powerwall 2 specifications / datasheet here
The Powerwall 2 is still the only battery system on the market to incorporate liquid cooling which enables it to operate in a wide range of temperatures from -20 to +50 degC. Although like most lithium battery systems it is likely to derate or reduce its output at temperatures above 40 degC. Power derating is common for most lithium based batteries but the Tesla's liquid cooling should enable the PW2 to release more heat under high loads and rapid charging which should result in longer life span and better performance at higher temperatures.
Multiple Powerwalls for greater storage needs
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, however the additional Gateway unit is required in these more complex installations. Maximum capacity per phase is 3 powerwalls
Additional Solar Inverter Required
The Powerwall 2 is an AC battery (AC in / AC out) so in order to function together with a solar array the Powerwall 2 requires a separate solar inverter (Refer to diagram below). An advantage of this is the solar inverter can be 'almost' any model since it operates independent of the Powerwall.
AC battery systems are also able to be easily retrofitted to homes with an existing solar installation, but they are slightly less efficient compared to the common DC battery systems. 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). This adds up to approximately 89% round trip efficiency compared to around 95% efficiency with a DC coupled battery.
Tesla Powerwall 3?
There is some speculation about the next Powerwall from Tesla but as yet there has not been any official announcements made. Considering that many alternative hybrid solar battery systems have both a solar inverter and battery inverter in one complete unit I would suggest Tesla might move in this direction and offer an all-in-one Powerwall. All-in-one systems make installation much easier and requires less space to install. For example Redback Technologies, Solax and Alpha ESS supply all-in-one systems with back-up power and additional pass-through power as standard.
I would expect the next generation Powerwall 3 to move in the same direction and offer a complete solar/battery/back-up solution. Based on the previous development timing a new generation Powerwall 3 would likely be announced mid to late 2019.
The Powerwall 2 cannot function as a backup power supply or UPS without the additional Backup Gateway. The new Backup Gateway 2 (described below) costs around $1700 extra and functions as the household energy management system. In the event of a blackout it also isolates the backup circuits from the electricity grid and enables the system to function in ‘off-grid’ mode. The gateway also provides advanced monitoring and time-based controls.
For those who were hoping to purchase a Powewall and go off-grid, it is only designed to operate in off-grid (back-up) mode for short durations (until the battery is depleted) and 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.
The new gateway 2
Tesla announced a new backup gateway 2 in November 2018 which among the many new features is also much more aesthetically pleasing than the old grey backup gateway box. The new gateway’s improvements include higher speed 4G communication, three phase operation with greater control, more accurate CT monitoring and in a package that is about 30% smaller and lighter than the old gateway. See full backup gateway 2 specifications here.
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 not 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.
The warranty for the PW2 is 10 years with 70% minimum retained capacity (solar self-use and back-up use only). This is much better than the original Powerwall 1 warranty but if charged using solar and also low tariff (off-peak) energy, a feature which has just been enabled as of May 2018, then the number of 'daily' cycles could be greater than 1 and this will likely reduce capacity at a higher rate. This is where the warranty details comes into effect in regards to 'Operation Limitation' (see image below).
Keep in mind all batteries reduce capacity over time as they are charged and discharged. This is part of the normal aging (degradation) of a lithium (NMC) battery and the min 70% retained capacity after 10 years is reasonable. Other lithium battery chemistries have a lower degradation rate but usually a higher upfront cost so it's a matter of balancing the cost/benefit. See the full Warranty details here
The Tesla Powerwall 2 is a huge leap forward from the first generation Powerwall and has overcome many of the issues highlighted by customers and installers, in particular the limited solar inverter compatibility. Tesla overcame this by developing the Powerwall 2 as an AC battery which operates independently of the solar inverter and is thus compatible with most new or existing solar systems. However unlike many hybrid inverters the built-in inverter does not have the ability to isolate or island itself from the grid during a blackout and requires the additional 'gateway' to function as a back-up power supply. Also it is not designed for off-grid installations.
In our complete solar battery review the Powerwall 2 came out as one of the better 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 experience from the durable Tesla electric vehicle batteries it is a well engineered system and has so far proven to be a very high performing battery. Although with the current supply issues you might have to wait 3-6 months to get one.
Alternative battery options
There are many other 'hybrid systems' available which include batteries and are also very good value for money, plus many include a solar inverter and back-up function in one complete hybrid unit. For off-grid installations it is recommended to use on of the well known, dedicated DC or AC coupled off-grid inverter systems which incorporate a number of features designed specifically for off-grid installations.
Commercial size Tesla Powerpack
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 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.
Tesla Powerwall 2 Vs LG Chem RESU
See the complete battery storage comparison and review here - The Powerwall 2 compared to leading lithium based batteries LG Chem and SimpliPhi, plus proven lead-acid and advanced lead-carbon batteries.