Tesla Powerwall 2

Following on from the first generation Powerwall, the most popular and cost-effective home storage product on the market is Tesla Powerwall 2. 

Powerwall 2 at a glance

Functionality

The system offers a range of functionality:

• storage of excess solar output on site;
• 'off-peak' charge and 'peak' discharge (buy peak, use cheap) - perfect for time-of-use' tariffs such as Economy 7 or the pioneering Green Energy TIDE tariff;
• (Q4 2018) back-up in a power cut.

Note that back-up functionality is achieved using a different gateway from a system which doesn't provide back-up. Back-up can be installed retrospectively, but the original gateway will be redundant.

Installation

Powerwall 2 consists of the main unit (see picture on page 11 of our 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.

The gateway for a system without back-up is a small box (~250 mm x 250 mm) that 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.

With back-up, a larger gateway is required.  

A typical schematic is as follows:

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 (15-20p per kWh) and an ‘off-peak’ rate (7-9p per kWh). In January 2017, Green Energy UK introduced a ‘TIDE’ tariff with a banded tariff reflecting demand and supply across the nation and the true ‘underlying cost’ of electricity:

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’ (ToU) tariffs. Clearly there are large savings (up to 24p per kWh under ‘TIDE’) 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 three different modes in the Customise tab of the Tesla app. The different modes are summarised below:

• 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.
• Advanced - Time-based control (Balanced) Balanced mode is the same as self-powered, except you can tell Powerwall the peak and off-peak times for your tariff and the battery will DISCHARGE at peak times in preference to off-peak times to maximise savings. There is still no charging from the grid in this mode.
• Advanced - Time-based control (Cost Saving) Cost-saving 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 don’t even need a solar system to benefit from time of use charge and discharge. Although the best savings will be realised through a combination of solar self-consumption and ToU charging and discharging.

Back-up

From Q4 2018, Powerwall 2 will enable you to continue providing power to your home or business during power cuts. The system will switch automatically although the switch will not be to “UPS” (un-interrupted power supply) standard. Tesla claims that in general you won’t notice the switch (except via a text from Tesla), but don’t rely on the system if you have critical loads requiring seamless back-up.

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

Solar PV will still work in a power cut. When the batteries are full, the system will ensure that the solar inverters power down if there is no load to utilise the excess solar.

The sizing of the Powerwall 2 means that with full back-up power (5kW), with some judicious juggling, your life should continue undisturbed through the average powercut. 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 kithen, 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: 3000W;
• oven: 3650W;
• induction hob: 1500W;

Office and entertainment

• laptop: 50W;
• TV: 80W;

Lighting

• average lighting load 500W (2500W all lights blazing for a typical 4 – 5 bed house (LED));

Laundry / Dishwasher

• dishwasher: 1250W;
• washing machine: 500W;
• electric clothes dryer: 4000W.

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 ‘brown-out’ 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 back-up functionality will only allow loads to work on a single phase when the grid goes down.

We recommend Tesla Powerwall 2 for several reasons:

• Design and efficiency The system uses a highly efficient battery and inverter (>90% AC in to AC out) combined with a battery management system controlling the state of charge, voltage, temperature and other parameters.
• Quality control and safety The system has been developed in-house by Tesla following their substantial experience from automotive battery technology. Every system undergoes extensive testing and inspection in their “Gigafactory” in the U.S. and the in-built management system incorporates an inherently multi-layered safety system.
• 10 year warranty Unlike most battery manufacturers, Tesla prefers to talk in terms of years rather than cycles, cycles being a less easily understood concept than years.  Tesla's warranty guarantees unlimited cycles for self-consumption and time-based control (grid charging). The warranty guarantees the battery will retain 80% of its initial capacity after 10 years.  That means that after 10 years, the Powerwall 2 should still be able to provide 10.8kWh of capacity. An important thing to note is that there is a UK free phone number to claim (not China, Korea, Germany or a sales reps mobile like some batteries).
• Lowest lifetime cost per kWh Powerwall 2 offers great value for money, giving one of the lowest “lifetime cost per kWh stored” on the market, well below the current cost of grid electricity.
• 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.
• Flexibility Tesla has designed a system that will allow AC-coupling with any PV system, or even without a PV system if used only for off-peak charging and backup. Furthermore, up to nine Powerwall 2 units can be installed together to help power systems of varying size.
• 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 above the size of competitor systems (even though they cost more…). That’s because as well as being geared to storage of excess power, Powerwall 2 is geared to the future: time of use charging. Tesla also ‘gets’ that most people wanting back-up would like their life to continue relatively undisturbed (see our section on back-up to understand the different load types). The system is also geared to ride the rollercoaster of the British weather system.
• Reduced peak demand charges Powerwall 2’s large usable capacity can be combined with Economy 7 tariffs to help reduce your electricity bill. By charging Powerwall 2 overnight from the National Grid on the lower tariff, daytime peak demands can be met through battery discharge rather than more expensive Grid import.
• Grid back-up facilities From late 2018 Powerwall 2 will enable you to continue providing power to your home or business during power cuts. See above.
• 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.
• Export Limitation Certification Most battery systems require prior approval from the electricity grid (DNO) before they are installed. However, Tesla Powerwall 2 is one of the only systems to have a certification in place which allows it to be installed alongside existing or new solar PV systems up to 4 kWp without prior approval. This not only saves up to £250 of application cost, but can also avoid weeks of delay and potentially avoid a connection refusal (although this is rare unless the solar PV system is very large).
• Warranty Tesla doesn’t talk in terms of life cycles, a concept which is not easily quantified by most of us. Tesla talks in terms of years and minimum output. If you use your battery for solar storage only, Tesla guarantees that the storage capacity will still be at least 10.8kWh after 10 years. 

Tesla battery solar compatibility

The Powerwall 2 is ac coupled and is therefore compatible with any solar system when connected to the grid, and back-up is not required.  

Tesla battery cost

The cost of installng a Powerwall 2 varies, but as a guide, allow up to £7,600 incl VAT @ 20% for standard installations.  If installed as part of a solar system the applicable VAT rate is likely to be lower.  The Tesla battery cost is generally lower per kWh than any other system, giving a lifetime cost per kWh well under the grid cost of electricity of 15p per kWh.

Spirit is an Accedited Powerwall Installer.

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

Is the Tesla Powerwall 2 right for me?

Battery storage for solar panels: summary page

Interested about finding out about other battery systems?

You can read our summary page on battery storage for solar panels here.