Posted by So as part of my aim to reduce by carbon emissions, the next logical move to me has seemed to be to buy a Tesla Powerwall, though to put that in context I already have an electic car and electric bike, I have maxed out our roof with solar (only 4.8 kw unfortunately due to funny roof shape) and have also installed a Sanden Hot Water Cylinder which is set to run outside the morning peak. We also have our washing machine and dishwasher set up to run during the middle of the day to maximise their use of solar.
With all of that, I still had a power bill in 2024 of just over $2,300 so a home battery seemed to be a good way of both reducing my carbon emmissions and my power bill, though to be honest I still expected to lose some money overall (which I was cool with so long as it reduced my carbon emissions).
So, about a year ago, I diligently started entering my daily energy usage from the Aurora+ App into a spreashseet so I could do some modelling on the potential savings. When I finally got around to trying to do some analysis on this, I quickly realised it didn’t really give me the data I needed as it didn’t break the consumption into morning and afternoon peaks and the assumptions I needed to then use quickly got quite complex.
So, aftethen reading that Solar Quotes had developed a Battery Calculator to let you see how much you’ll save, I tried to get the required NEM12 data download from Aurora.
Apparently Aurora doesn’t provide NEM12 files and instead they sent me this impossibly big csv file, which I swear they have deliberately engineered to be unusable. I spent nearly a full day trying to just convert it into excel, but because they use the american date format and time format in a single cell, not even ChatGPT could provide me with a solution (that worked) to convert that into a date format that I could use (no matter what I did it would import the date and time field as a text, and then the palaver I had to go through to then convert that into a date was doing my head in).
So congratulations Aurora on finding yet another way of making sure people can’t use their own data to try and improve their lives.
Anyway, after doing all this for a year, I figured … maybe I don’t need to do the modelling down to such a detailed basis. Maybe I can just make some broad assumptions around savings based on my monthly bill and get a ball park figure … so that’s what I did.
I won’t go through it blow by blow, but in simple terms I took my monthly bill and did the following:
- If my monthly peak usage was less than my excess solar generation, then I assumed that all of that peak usage would be offset by the excess solar generation going into the battery, and so I would save the peak usage consumption (less the solar rebate) so about 26.9 cents per KwH in 2024;
- If my monthly peak usage was more than my excess solar generation (which it was for the five winter months from May to September 2024) I still assumed that all excess solar generation was put into the battery and first used to offset my peak usage as per the step above, but then I assumed the system would also charge the battery from the mains during the off-peak period and use that charge to offset any remaining peak usage giving me a saving of around 19.2 cents per kWh. I did do a quick check of average daily excess peak consumption and except for June/July where the daily storage would have needed to be around 21 kWh per day (noting this is across two peak periods, so was possible) this seemed a reasonable assumption.
- Finally for those months where the excess solar generation exceeded peak usage (which occured Jan to April and October to December), I assumed that any residual excess solar generation (after offsetting peak usage) was able to be captured by the battery to offset my off-peak consumption (noting this only saved around 7.75 cents per kWh in 2024 due to the small difference between the off peak rate and the solar rebate). A quick check done by dividing the excess solar production by the number of days in that bill period showed that for six of the seven months my excess production was between 9 and 13 kWh per day (the other month was only 6 kWh per day), so while a bit optimistic, this seemed reasonable for this simple analysis given a Tesla Powerall 2 has a useful energy storage of 13.5 kWh.
So, noting that this probably modelled a best case situation and it would be wise to probably discount this by between 10% and 30% to allow for daily variations where I couldn’t optimise things so nearly, this simple modelling found I would have saved $738 (32% of my bill) in 2024 if I had a powerwall, with:
- 38% of those savings coming from peak consumption from stored consumption from the solar panels
- 47% of those savings come from peak consumption from off-peak storage (most of these in winter); and
- 15% off those saving from off peak savings coming from stored consumption from the solar panels.
I found it interesting that nearly half the savings didn’t require solar. However, even in this optimistic scenario, if I achieved all these savings and my Powerwall lasted 15 years, then I’d still only be saving around $11,000 (without discounting) and the powerwall would have cost me around $16,500 to install. And yes, I know energy prices will go up over those years, so the savings will be more, but I’m trying to get a ballpark here, not an exact prediction and who knows if there will be a real increase in prices and what that will be.
On a more realistic scenario, if I apply a 20% discount to that $738 annual saving to reflect less than optimal optimisation of solar and off peak usage by the the battery software due to basic uncertainty and to allow for daily variances where solar is lost during extremely sunny days in summer and I can’t offset all of the peak consumption on some really cold days in winter, and if I go with the more commonly used 10 year life of a battery, then the savings are only $5,900 over the ten years (and that’s not allowing for discounting). So the payback is pretty terrible.
But to be honest … with those sort of figures, I’d still be happy to pay for a battery if it really did something significant in reducing my carbon emissions, which neatly takes me to my next question …
Will a battery offset carbon emissions in Tasmania?
So, the current government line is that Tasmania’s energy production is 100% renewable, though as has been pointed out by people far more qualified than me this is not technically true – while we produce more renewable energy than we consume, we export some of that to the mainland during peak times and then import enegy from the mainland during off-peak times – and we can’r guarantee all of that imported energy is renewably produced, plus in 2024 we did also run the bell bay gas plant during winter due to low water levels in our dams.
So, in theory at least if we are importing energy and using gas energy produced in Tasmania right now, then potentially storing our own solar from our rooftops could be offsetting non-renewable energy sources, except … the amount of gas energy produced was pretty small in the scheme of things (and occured during winter when I wouldn’t have any excess solar to offset it anyway) and if you look at the OpenNEM site for Tasmanian consumption, you will quickly see that (simplifying a little) most of our imports of electricity are occuring during the day where it looks like Hydro use the excess solar rooftop energy and imports to reduce Hydro usage so it can be used at night.
My simple logic therefore says that if I start capturing my excess solar energy production in a battery, all I am likely going to be doing is increasing imports from the mainland (which I’d see as a negative, although some of it is probably solar rooftop from the mainland) or I’m increasing the use of Hydro production (which given our dam storage levels are ok, but not great (46% in December 2024) – also doesn’t seem like a great win to me).
Conclusion
So … I may have completely stuffed up my analysis or assumptions in the above, but it seems to me I’d be outlaying quite a lot of money for very little to no carbon benefit by buying a Tesla Powerwall or any other home battery in 2024 in Tasmania as it appears my solar is being used in Tasmania during the day to offset gas and imports from the mainland.
I did think about the power blackout benefits of having a home battery, and while I don’t dismis them, I do have a massive battery sitting in my driveway (with vehicle to load capacity from my BYD Atto 3), so in an emergency (and assuming I’m home) it would be the work of five minutes to run an extension cord from my car up the stairs into the kitchen to keep the fridge and a few other key appliances running (and even if the powerout was for a very extended period, assuming that one of the fast chargers around Hobart is working, I could go off and recharge the car if needed).
Plus I also already have an EcoFlow 0.5 kWh camping battery which is fully charged (plus several smaller power banks) which combined could be used to recharge phones and run the modem or computer through most black outs.
So, as much as I want to buy me a Tesla Powerwall, unfortunately I don’t think the case for it stacks up … so now I need something else to do … maybe window tinting to reduce heat transfer? Other ideas?