Another point to consider Ben are the losses in the inverters. Unless you are charging the battery from the DC of the PV system, any usage of the battery to power the immersion will have been converted 3 times if the energy originated from the PV panels or twice if the power came from the grid.
Haven’t a clue how efficient the Tesla Powerwalls are or a typical solar inverter but you could ‘lose’ a lot in all the conversions back and forth between AC and DC.
Simon
EDIT - see here for some experience with a Powerwall and PV → Solar PV + Tesla Powerwall (or storage battery) - Photovoltaics (PV) - BuildHub.org.uk