Following on from the first generation Powerwall, the most popular and cost-effective home storage product on the market is Tesla Powerwall 2.
|Power output:||3.68kW or 5kW (subject to grid permission)|
|Round-trip efficiency %||> 90%|
|Mounting:||Wall- or floor-mounted can be stacked|
|Location:||Indoor or outdoor installation|
|Dimensions (main unit):||H 1150mm W 755mm D 155mm|
|Energy density (including inverter / charger):||100 kWh / m3|
The system offers a range of functionality:
Note that back-up is a relatively new feature (end of Q1 2019), thus if you have previously installed a Powerwall 2 unit without back-up, you would need to upgrade the 'Gateway' to enable back-up functionality - see Installation section below.
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 580mm 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:
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).
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.
As an example, here is Bulb’s smart tariff for South East England:
Time of day
Cost per kWh
(week day 4-7pm)
(week day 7am-4pm and 7pm-11pm, weekend 7am-11pm)
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 (up to 34p per kWh under with Bulb) 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:
|Mode||Backup Reserve||Solar Self Usage %||Savings||Who should do this?|
|Self-powered||Configurable (0-100%)||Great!||OK||"I want to minimise my carbon footprint"|
|Advanced - Time-based control (Balanced)||Configurable (0-100%)||Good||Good||"I want to minimise my carbon footprint and I have a ToU tariff"|
|Advanced - Time-based control (Cost-Saving)||Configurable (0-100%)||OK||Great!||"I have a ToU tariff and want to maximise my savings"|
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.
As of the end of Q1 2019, Powerwall 2 now 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 (subject to size limits below).
Powerwall 2 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’ back-up, certain large loads (such as a car charger) can be excluded.
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 notification on the Tesla app), but don’t rely on the seamlessness of the system if you have critical loads requiring seamless back-up e.g. life support equipment.
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.
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 re-start. After this the battery will need a manual reset to re-boot (via the gateway box).
Not only will a back-up 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 the 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…..
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 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:
Office and entertainment
Laundry / Dishwasher
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.
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:
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.
The cost of installing a Powerwall 2 varies, but as a guide, allow up to £9100 incl VAT @ 20% for standard installations. If installed as part of a solar system the applicable VAT rate is 5%. The Tesla battery cost is generally lower per kWh than any other system, giving a lifetime cost per kWh competitive with the current grid cost of electricity (around 15-20p per kWh).
Spirit is an Accedited Powerwall Installer.
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