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Battery Impact - Melbourne

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I thought I would share some experience on battery performance in what is probably a typical installation in suburban Melbourne.  We have a 5.4kW PV system installed in 2012 with Tesla Powerwall (13.2kW storage) from 2018. 

My view is PV systems well and truly pay for themselves, but batteries not so - so maybe they have to go in the hobby or feel good camp.  We are on a two tier tariff and our numbers show pretty consistent $150 a quarter saving across the year on top of the PV system - so roughly a 20 year payback ($600 a year on $12k for battery), which is likely well beyond the expected cycle life of the battery -- but who really knows.  And to realise that level of saving we had to apply some smarts that don't appear very common amongst most of the battery installers we talked to.  Fortunately the fact our house is on two phases confused many installers at the time and that led us to a couple of the smarter ones.

But what interests me most now, and I wish I had known at the time, was the impact a battery has on our 'footprint' on the grid if managed well.  Below are some pairs of graphs across the extremes of Melbourne.  The first in each set is the basic solar generation versus house usage (instantaneous and cumulative) and the second is the battery view (so instantaneous Grid Use and Generation along with Battery Charge and flow).

Let's start with a good summer day.  On the first graph, strong solar generation between 8:00am and 6:00pm covering all the load (the heavy day loads are mainly swimming pool pumps) and a close to 15kW surplus of generation over need.  On the second graph the battery started the day (midnight) around 60% charged (the red line. scale to the right), powered the house until 8:00am when the solar kicked in to carry house load and recharge the battery.  Around 1pm the battery is full and excess is exported until 6pm when the battery takes over again and is back around 60% by midnight.  So nothing drawn from grid, and export through the peak afternoon period when power is scarcer.  The clear saving for us is not buying electricity at all, plus the feed in tariff.


At the other end you have a bad winter day.  From the standard graph our generation is well under house load for the day, even with pool pumps turned well down (and gas heating).  The battery pattern here is quite different - it charges off-peak to 100% overnight, carries the morning load. marshals what solar it can during the middle of the day and powers the house through to off-peak time again when it starts charging for the next day.  That is what locks in the winter gains.  The smarts to do this should be included with all battery systems - it just requires a look at the predicted cloud cover or solar radiation for the next day to make a charging decision.  There are some that do this, but in our experience beyond the skills of most we spoke to.  And yes, some argue we are killing the battery with extra cycling - we will find out sometime.


There are many variations between the two extremes that basically target a mix what morning charge should we target, and should swimming pool pumps run (along with judgement on washing machine, dishwasher etc.)

Anyway, some food for thought.  It fundamentally changes our footprint on the grid in a way that I sense is quite beneficial to an aging set of poles and wires - not driving large swings in export / import incessantly during the day as sun / clouds move in and out, using base load night time power, and not contributing to the peak in 'distributed generation'  through the mornings in summer.




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