Tesla Powerwall 2

Powerwall 2 at a glance

Specs

Datasheets

• Powerwall 2 brochure
• Powerwall 2 datasheet
• Backup Gateway datasheet

Functionality

The system offers a range of functionality:

• storage of excess solar output on site;
• off-peak charge and peak discharge (buy cheap, use peak) - perfect for time-of-use tariffs such as Economy 7 or smart tariffs from various suppliers;
• backup in a power cut.

Note if you have previously installed a Powerwall 2 unit without backup (likely pre-2019), you would need to upgrade the Gateway to enable backup functionality - see installation section below.

Installation

Powerwall 2 consists of the main unit (see picture in our downloadable guide) which houses the battery and inverter/charger, as well as a smaller Energy Gateway which measures the energy flows through the house and directs the battery to charge or discharge accordingly. It also 'islands' the house in a power cut, enabling the solar system to keep working and the battery to charge and discharge.

The gateway is a stylish frosted glass-fronted ‘mini’ version of Powerwall 2, measuring 380mm wide x 584mm tall x 127mm deep, and weighing 9.8kg. It is IP55 rated, so suitable for outdoors, and can be padlocked to stop any unwanted access.

The gateway is installed near the electricity meter. The main battery unit should be relatively close to the gateway, otherwise the installation cost increases. The ideal scenario is an integrated garage with the meter and fuseboard inside. If there is no garage, the main unit usually goes on the outside wall as close as possible to the gateway and electricity meter. The installation typically takes up to a day for two installers.

A typical schematic is as follows:

Battery chemistry

The Powerwall battery cells are made from lithium manganese cobalt (LMNC). In this respect, it differs from many of the other home storage technologies on the market (and all of the other batteries which we offer), all of which use lithium ferro phosphate (LFP). In our view, lithium ferro phosphate offers a superior chemistry to lithium manganese cobalt.

LFP offers many more lifecycles (one lifecycle being a round-trip in and out of a kWh) – i.e. many more storage slots. Typically 8,000 – 10,000 lifecycles for lithium ferro phosphate, compared to 4,500 for lithium manganese cobalt. This means that each 'storage slot' costs a lot less for LFP than it does for LMNC.

It is surprising that Tesla has gone against the market norm in their choice of chemistry.

Timed charge and discharge

Across the nation, electricity demand reaches a high in the late weekday afternoons (4-7pm), and then falls to a low overnight. However, to date most domestic properties have paid a single rate electricity tariff which doesn’t reflect the peaks and troughs of demand. Some domestic properties are on an Economy 7 (or Economy 10) tariff with a peak rate and an off-peak rate.

More suppliers are starting to trial or offer time-of-use (ToU) tariffs, with time banded tariffs reflecting demand and supply across the nation and the true underlying cost of electricity. Octopus has even launched a Tesla Energy Plan with 10-12p/kWh import and export, and low standing charge.

With the advent of smart meters, able to record half hourly electricity use, the market will gradually witness the introduction of many more such time-of-use tariffs. Clearly there are large savings to be made by buying electricity at night and using it in the day. Time-based controls are now in place for Powerwall 2.

The system user can choose between two different control modes in the Tesla app:

• Self-powered. This mode is purely for solar self-consumption, with no charging from the grid. It will prioritise charging from the solar at all times and discharge whenever it can to maximise self-usage.
• Time-based control. This 'load shifting' mode incorporates charging from the grid at off-peak times where possible to maximise savings, as well as charging from solar during the day. You set the peak and off-peak time slots of your electricity tariff. Powerwall learns your usage patterns to forecast energy demand, so this mode can take about a week to become available.

You don’t even need a solar system to benefit from time-of-use charge and discharge. That said, the best savings will be realised through a combination of solar self-consumption and ToU charging and discharging.

Backup

Powerwall 2 enables you to continue providing power to your home or business during power cuts. It also allows your solar PV system to keep running in a power cut (as long as the system is subject to size limits below).

Powerwall can be installed to back up the whole house, or it can be installed such that it backs up only certain loads. Even with ‘whole house’ backup, certain large loads (such as a car charger) can technically be excluded via load shedding, though our preference is to differentiate backed up and non-backed up loads when wiring up.

The system will switch automatically although the switch will not be to UPS (uninterrupted power supply) standard. Tesla claims that in general you won’t notice the switch (except via a notification on the Tesla app), but don’t rely on the seamlessness of the system if you have critical loads requiring seamless backup e.g. life support equipment.

You will need to specify a reserve percentage (e.g. 30%) and the system will always keep that percentage of the capacity in reserve for a power cut. This setting can be changed.

If the grid goes down and the loads in the house exceed 5kW, (or if they exceed 3.68kW for a system set on the lower 3.68kW power output at commissioning), the battery will shut down. The system will spend the next hour sending you reminders to turn off high power loads so that the total load falls below 5kW and the battery can restart. After this the battery will need a manual reset to reboot (via the gateway box).

Solar system will continue to function in a power cut

Not only will a backup enabled system keep the lights on in a power cut, it will also keep the solar system functioning. The system will continue to function whilst there is either a load or sufficient spare battery capacity to soak up the solar electricity it produces.

If there is insufficient load or battery capacity to soak up the solar, the system will automatically power down the solar. However it should be noted that for this to work effectively don’t connect more than 7.6kWp of solar to a system with a single Powerwall set at 5kW output. If Powerwall is set at 3.68kW (due to DNO constraints), don’t connect more than 5kWp.

If you want more solar, you need more Powerwalls, or you need to connect the solar 'upstream' of the gateway so that the solar disconnects in a power cut…

In practical terms...

The sizing of Powerwall 2 means that with full backup power (5kW), with some judicious juggling, your life should continue undisturbed through the average power cut. That said, you should probably wait until the power comes on before using an electric dryer, and if you want to ensure you can continue to cook up a storm in the kitchen, it’s probably best to install two Powerwalls. 

If you reserve 30% of your battery (4kWh), then you should be able to watch TV, use your laptop and keep the lights and freezer working for almost four hours, whilst enjoying a few cups of coffee in the process.

Typical loads are as follows:

Kitchen

• fridge / freezer: 500W;
• kettle: 3,000W;
• oven: 3,650W;
• induction hob: 1,500W;

Office and entertainment

• laptop: 50W;
• TV: 80W;

Lighting

• average lighting load 500W (2,500W all LED lights blazing for a typical 4 – 5 bed house);

Laundry / dishwasher

• dishwasher: 1,250W;
• washing machine: 500W;
• electric clothes dryer: 4,000W.

Note that if you try and run more than the 5kW load limit, Powerwall 2 will stretch to 7kW for a few seconds, but after that you will see some brownout behaviour, as indeed you would if you tried to import more than your 100A supply from the grid.

3-Phase connections

Most households run on a single phase connection (230V). If you have a 3-phase connection (400V), Powerwall 2 can still be used but with the following limitations:

• There is no 3-phase version so instead single units have to be put on each phase to power 3-phase loads. The maximum number of units that can be installed per site for 3-phase supply is 9 - 3 per phase. Alternatively, you can put up to 10 units on one phase to power single phase loads.
• Even with Powerwall 2 installed on each phase, the backup functionality will only allow loads to work on a single phase when the grid goes down.
  
  In our view, if you have a three phase grid connection, it would make more sense to install a Victron based system. This will allow three-phase solar to work in a power cut, and will also allow you to back up each phase in a power cut. If you have three phase loads, these can be backed up as well.

Powerwall pros and cons

Pros

• Beautiful design, wall hung and weatherproof. Space is a consideration in residential properties, as indeed is aesthetic appearance. The design is modern and minimalist, designed to be seen, not hidden away out of sight. And the waterproof case means that it can be installed outdoors. The ability to install outside is one of the major plus points of the Powerwall.
• Whole house back-up (for single phase (230V) grid connections). The system design allows for the whole house to be backed up in the event of a power cut. Whilst some loads can be excluded from back-up mode, this is not a necessity, making installation simple. With other systems (apart from Victron), it may be necessary to divide loads into ‘emergency’ and ‘non-emergency’; whilst this promotes good discipline and avoids battery drain in the event of a power cut, it does increase installation complexity and cost.
• 10 year warranty. The warranty and support is a strong point of the Powerwall. Tesla's warranty guarantees unlimited cycles for self-consumption, time-based control (grid charging) and backup. The warranty guarantees the battery will retain 80% of its initial capacity after 10 years. That means that after 10 years, Powerwall 2 should still be able to provide 10.8kWh of capacity. An important thing to note is that there is a UK freephone number to claim (not China, Korea, Germany or a sales rep’s mobile like some batteries). Tesla monitors all batteries, and only permits installation by a certified installer. If you prefer not to have your battery monitored by Tesla, this is possible, but it does limit the warranty. 
  • High capacity (kWh) and power output (kW). Powerwall 2 has a total usable capacity of 13.5 kWh and a maximum power rating of 5kW. This is well sized for most domestic properties. Tesla also ‘gets’ that most people wanting backup would like their life to continue relatively undisturbed (see our section on backup to understand the different load types). The system is also geared to ride the rollercoaster of the British weather system.
  • Increases power output of the average home by 20%. Most homes have a single phase 100A connection, limiting power consumption in the home to 100A. At 230 V, that’s around 23 kW. With a 5 kW continuous output from the battery, a full battery has the potential to ramp simultaneous power consumption by 20% for 2.5 hours. This may come in handy if yours is one of those busy households with a two hour window in which to wash the dishes, dry the clothes, cook dinner and charge the car… And if you’re running a home with two electric vehicles fighting to be charged, some sort of turbo-boost on the incoming grid power is going to be an absolute must. This facility is also offered by Victron.
  • Export Limitation Certification. Most battery systems require prior approval from the electricity grid (DNO) before they are installed, including all systems that provide grid backup such as Powerwall 2. Tesla is one of the only manufacturers to have certification in place which guarantees that the battery will not export any power to the grid, however, which makes grid connection permission more likely in areas where the local network may be constrained or when coupling with a large solar system.
    
    Cons
    
    There is no one size fits all system, and the Powerwall does have drawbacks. For this reason, we do offer a range of other batteries, summarised here, and on this PDF. The main drawbacks of the Powerwall are as follows:
  • 3-Phase connections. Tesla wins hands down against the competition unless you have a 3-phase (400V) connection, in which case the system has a couple of limitations. You won’t be able to power 3-phase loads with the system, and with a backup system, only one phase will be operational. A 3 phase solar inverter cannot continue to function in a power cut. For 3-phase setups employing ‘net metering’, however, the units can utilise a mode of operation called ‘phase compensation’. By monitoring all 3 phases of solar generation and grid supply, a single battery unit (or multiple units) can balance the total power across all 3 phases (L1 + L2 + L3) to zero. In our view, you would be better off installing a Victron system if you have a three phase grid connection.
  • Chemistry: Powerwall uses Lithium Managanese Cobalt (LMNC) rather than Lithium Ferro Phosphate (LFP), which is the favoured chemistry or most of the home storage market. As noted above, this is a big drawback, because overall lifecycles are much lower (4,500 lifecycles typical for LMNC, versus 8,000-10,000 for LFP).
  • Doesn’t allow DC coupling of solar. As above, the Powerwall does not allow the ‘DC-coupling’ of solar PV panels. Other systems (such as the MyEnergi ‘libbi’ or Victron systems, allow solar to be connected directly to the battery inverter-charger, reducing ‘AC-DC’ conversion losses, and allowing additional solar to be connected without the additional expense of another inverter. This is a benefit, particularly when you note that the VAT rate for retro-fitted battery storage is currently 20%, but if you add in some panels, then the VAT rate is 0%.
  • Non UPS backup. The backup doesn’t meet ‘UPS’ standards. So if you have loads needing a UPS (e.g. medical loads), don’t rely on Powerwall 2. For most of us, the non UPS backup is a non issue: you just may have to reset the clock on the oven when a power cut happens.
  • Not ideal for off-grid properties. Unlike the Victron systems, Powerwall 2 is not compatible with generators. It is therefore unlikely to be a suitable choice for a fully off-grid property, where some sort of generator is likely to be required unless the renewable energy system and battery are to be completely over-sized.

    Tesla battery solar compatibility

    Powerwall 2 is AC-coupled and therefore compatible with any solar system when connected to the grid. It can be configured to allow any single phase solar system up to 7.6 kW AC rating to continue to function in a power cut (subject to install restrictions).

    Tesla Powerwall cost

    Tesla Powerwall costs around £7350 excl VAT, with prices varying depending on installation requirements. If installed as part of a domestic solar system (or an 'extension system') the applicable VAT rate will be 0%. Otherwise the VAT rate will be 20%. 

Spirit is a Tesla Powerwall certified installer.

For more information, download our guide "Is Tesla Powerwall 2 Right for Me?"

Is Tesla Powerwall 2 right for me?

Or if you are having your Powerwall installed, check out our Tesla FAQs.

Battery storage for solar panels: summary page

Interested about finding out about other battery systems?

Read our summary page on battery storage for solar panels.