The Powerwall was installed at the end of February 2022 and it pretty much works as expected. When we have a full year of figures will post about the difference it has made to our electricity bill. Until then here’s a few things we’ve learnt since it was installed.
Summer operation – We turned the Powerwall into ‘Self Powered’ mode on 1st June – from then until the beginning of October we used less than 20 kWh of day rate electricity – that’s around £6 in total or 5p per day – much less than the standing charge! We used a bit more overnight charging the EV from time to time – but our total electricity bill including standing charge for June, July, August and September was around £30!
Not for control freaks – The Powerwall app on your phone lets you control the percentage of the battery reserved for grid outages and select between two operating modes. That’s it. Self-Powered charges the battery from excess solar (during the day) and discharges when solar can’t meet the house demand (day or night). Works great. Time-Based Control charges the battery overnight from cheap rate electricity and discharges during the day as required and it still charges from solar during the day. You enter the details of your electricity tariff – hours and costs – and the system works out when and how much to charge the battery overnight based on your expected use. The algorithm is in charge. It generally does a decent job. A recent update let you enter unit prices for your electricity tariff – hoping this will let the algorithm do an even better job in future. There’s no way to control the battery charging and discharging manually.
Storm Watch is a cool feature – If a storm likely to cause power outages is forecast for your area Storm Watch will fully charge the Powerwall and keep it fully charged until the storm passes. If there is a power outage you have a fully charged battery to carry you through. If there isn’t then you’ve paid a bit more for electricity while the battery has been on standby – we regard it as insurance. It works – it’s triggered only once and we did have a brief power outage. You can disable Storm Watch if required.
It integrates with Home Assistant – There is an integration available for Home Assistant which can access useful data from the Powerwall so it can be used to automate things. We use it to control EV charging from solar and when the heat pump heats hot water. Can’t underestimate the importance of this point – the electric future needs devices from different manufacturers to be able to work together. Shame that Tesla regard the local network interface only for installer use and recommend using the app to monitor battery status. The app does not integrate with anything. The local interface and the app don’t even report the same battery charge level – when the app says 20%, the local interface says 24% (it includes the additional capacity that Tesla reserve for battery management). Ho hum.
Get a wired ethernet connection – the Powerwall is most definitely a connected device, it has wifi, ethernet and its own sim for connectivity. It’s a condition of the guarantee that an internet connection is maintained. Our system was installed using the wifi connection but it kept disconnecting – easy enough to reconnect in the app but useless for automation with Home Assistant. We added a wired ethernet connection and network connectivity has been rock solid ever since.
You want G99 not G98 – G98 and G99 are regulations that govern the connection of micro-generation systems to the grid. The Powerwall has a 5kW output which requires a G99 connection. The District Network Operator (DNO) has to approve the connection which can take some time. The battery can be installed with the output limited to 3.67 kW in compliance with the G98 regulations. This lets the installers get the Powerwall installed without having to wait for the DNO OK, then when they get approval for the 5kW output it can be turned on by enabling G99 operation. You can check which mode your Powerwall is operating in via the local web interface (not the app) – connection instructions here: https://www.tesla.com/en_gb/support/energy/powerwall/own/monitoring-from-home-network Your installer should handle all this – ours needed quite a bit of prompting and still not sorted.
Solar diverter may not work with Powerwall – when our solar panels were installed the system included a solar diverter to automatically ‘divert’ excess solar power into the immersion heater. They are a really good idea (if you don’t have a battery) and we’d become used to using it. Once the Powerwall was installed a couple of years later the diverter no longer worked properly – seemed to come as a surprise to our installers. Thinking about it the battery and diverter are going to struggle to play nice. The diverter works by monitoring the export from the house and then adjusting the power going to the immersion heater until the export is zero. The battery is also monitoring the house load and supplying power to avoid importing from the grid. So no matter what the solar is doing the battery will supply the power needed by the diverter. The diverter will keep increasing power as no matter what it does there will be no export. Which is exactly what happened when we turned the diverter on – in a short time the battery is discharging 2.8kW to the immersion heater and excess solar is being exported. In reality it’s turned out not to be an issue. Watch out for a heat pump and hot water post that will explain why. We have read that some diverters and battery systems have threshold adjustments to try and address this issue but have no experience of how they work in practice and suspect thresholds may introduce more problems than they solve. Always happy to hear your experience. Finally – sorry this has turned out to be quite a complex issue – our installers offered to fit a current measuring clamp so that the Powerwall would ignore power going to the immersion heater. This potentially fixes the diverter issue but means that the immersion can never be supplied from the battery – which is definitely a bad idea as far as we are concerned. We said no thanks and the diverter is turned off.