Predbat

There are a couple of limitations with the ‘native’ behaviour of the Tesla PowerWall 3 inverter-and-battery system in my specific circumstances (dynamic electricity pricing and grid export limitations):

  • The ‘smart charging’ behaviour of the Intelligent Octopus Go EV charging tariff means that sometimes it schedules an EV charge slot outside the standard daily off-peak tariff time slot (23:30 – 05:30).
    • When this happens, the import tariff applied to the monthly electricity bill is actually the off-peak tariff – despite it falling in a ‘peak’ time slot.
    • The PowerWall doesn’t know this, assumes any grid import will be at the ‘peak’ rate, and tries to charge the EV from the PowerWall.
    • The answer is to spot when Octopus have scheduled a peak-time charging slot and put the PowerWall into ‘Standby’ mode for that half-hour (or potentially charge the PowerWall battery from the grid).
  • On sunny days in Summer (when there’s no heating required so the electrical load is much lower than in Winter) it’s not sufficient to simply export to the grid when the solar PV is generating – the export limit imposed by the grid operator (5.5kW) means that the PowerWall battery receives a significant charge too, leaving it ‘too charged’ by the next morning and resulting in curtailment of solar generation when consecutive days are quite sunny.
    • The answer here is to:
      • Predict how sunny it is going to be tomorrow – and how much electricity will be consumed on-site before and during tomorrow
      • Force-export to the grid during the evening, to reduce the SoC of the PowerWall battery so it has enough ‘headroom’ to accept all of tomorrows’s excess solar generation (and then export that tomorrow evening).

The Predbat add-on for Home Assistant does a lot of the work around Home Energy Management.

The instructions for installing Predbat are fairly good, but a few surprises included:

  • By default, the File Editor App limits access to the contents of the config directory – which stops it seeing the configuration files for add-ons like Predbat. The File Editor configuration needs to be adjusted, with “Enforce Basepath” set to False.

CC BY-SA 4.0 Predbat by Marsh Flatts Farm Self Build Diary is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.