The Tesla Powerwall is pretty much a fit-and-forget piece of kit – beyond an isolator it has no physical controls and it just sits more or less silently on the wall, pulsing with green neon light when it’s doing something useful like charging or discharging. But the battery links to a phone app. In addition to allowing setting of off-peak charging hours, this allows monitoring of the system as a whole, given everything’s connected to the battery. So, it shows battery status but also domestic use, PV output, and mains use.

Over the past year Tesla have updated the app a few times and it now gives some very useful options. All historic information is available – pretty much on a day-by-day basis and it can be downloaded for analysis. Simply interrogating the app gives some interesting stats though. Taking our own installation, we’ve been here a little over eighteen months now. So – with the exception of a few weeks last summer when the system was down (due to a blown fuse, as it transpired. We Don’t Get Fooled Again) we have a full year’s figures (2019, above).

As is clear, the situation varies wildly between the seasons. In winter we’re using a lot more space heating (although, reassuringly, very much in line with PHPP predictions) and getting less sun. In summer there’s a three-month period where we’re producing more than we’re using (although performance for this period would have in reality been better as this was when the system was offline for a while).

Overall, the figures are as above. PHPP predicted annual space heating of 6731kWh so that home usage figure – including cooking / lighting / office / electric bike etc – looks about right. Almost exactly a third of this came from the sun (wooohooo!) and slightly more came via the Powerwall. The app seems to be wrongly reporting the annual split between peak / off-peak, but checking this on a month-by-month basis gives around a one-quarter peak / three-quarters off-peak balance.

At some point in the future – when we’ve even more data – I’ll update this!

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