Solar and battery system - 1 year later


New member
Background info: Couple with two toddlers. South-East QLD location, house built September 2021, HSTP system, gas cooktop with electric oven, gas instantaneous hot water system and five split system A/C units (4 units in the bedrooms + 1 large unit for the living/lounge/kitchen area). 3-phase connected to the house.

Installed a solar and battery package in November 2021. System comprises of 21kW of panels, 5kW Fronius standard inverter & 10kW Fronius GEN24 hybrid inverter and 13.8kWh of battery storage (BYD HVM unit). Total cost was about $29k out of pocket after STCs (291 STCs @ $36 each), no other loans/rebates were available at the time.

I specifically wanted this setup as I wanted to maximise my export limit (15kW on 3-phase) whilst also having the ability to charge the batteries and power the house from solar during blackouts. I live in an area that can become isolated during floods and if the power goes out then, the power won’t be coming back until the flood waters go down, which can sometimes be over a week. Off-grid capacity is limited to 10kW, but have rarely seen usage go above 4kW which is generally when the oven starts up.

Most nights see the battery capacity drop to around 20% and during 2022 there were only a handful of dark days when the battery did not fully recharge during the day.

Now for the data and bills, from 9 November 2021 until 9 November 2022:

Solar generation: 29,682 kWh

Energy exported to grid: 24,586 kWh

Energy imported from grid: 127.31 kWh

Total cost savings is calculated at ~$4,100 ($2,950 from feed-in credits @ 12c/kWh)

2022 total generation and consumption graphs from Fronius solarweb portal shown for general info, as well as yesterday’s daily graph which was a pretty cloudy day (used the oven for dinner).

As for gas, we’re on 2x 45kg exchange cylinders and only needed 1x 45kg delivered in 2022. We’ll be needing another one soon, but our gas cost since moving in has only been $381 (3 x 45kg cylinders @ $105 each, $33/cylinder rental with first year free)
Total cost was about $29k

Total cost savings is calculated at ~$4,100

So at this rate, about a 7 year RoI. Assuming that all your other costs (like 5 x split systems) would have been paid for anyway.

That being said, bringing your batteries down to 20% SoC each night will chew through the cycle count pretty quickly - so you might be up for replacing those at about the 10 year mark - which would be ~3650 cycles.

Depending on the tech, you'd get between 3000-6000 cycles to stay above 70-80% remaining capacity - but it drops off pretty quickly after that in most life expectancies.

Likely, the 3 years additional savings might pay for the new batteries - which at least means you'd break even....
@kelbel81 Yeah, roughly 7 year ROI without factoring for anything else. Installer's quote showed a payback time of 6 years 3 months, which I thought was a bit ambitious.
If after 10 years the batteries need replacing, so be it. The primary reason for them was to be able to ride out a week long blackout without loosing power to the fridge/freezer, and being able to do it in air conditioned comfort is a bonus.
The battery
warranty does state 60% capacity after 10 years, or a minimum energy throughput of 42.69MWh from a 13.8kWh pack, which roughly equates to 11.7kWh out of the battery every night for 10 years.
And the final battery test centre report from March 2022 included a BYD HVM which showed no apparent capacity fade after 1180 cycles.
@djwhalen This is the reason why I’m not switching yet. Too many years for ROI and edging close to warranty running out.

I’ll wait till LFP-batteries become more readily available for solar storage.

Good numbers either way.
@djwhalen You’re not factoring in you’ll probably have an electric car by then that you can essentially charge for free saving you $50 a week on fuel (that’s what I use anyway) or $2500 a year
@christianityistrue .... you don't lengthen or shorten an ROI it either goes up or down. The payback period will shorten or lengthen and in that you're also incorrect. Any additional personal electricity usage increases the ROI and decreases the payback period as you're taking advantage of the ~10c price difference between what you can sell the power for and what you can buy it for....
@jink926 Any extra usage of electricity is negative gearing towards ROI. In order to break even it takes time and it’s based on rate; I.e rate of supply vs usage. Supply should be generally stable (dependant on weather condition) but usage can vary greatly and any extra such as charging EV will increase the rate of usage, thus effectively lengthening projected pay pack date (i.e. ROI).

Like you said, ROI is a %, a unit-less figure which in this case is essentially ‘usage vs supply’.
@christianityistrue That was one giant word vomit and completely incorrect. Just stop digging. Increased personal usage of the electricity you are generating results in an INCREASED ROI due to the due to the delta between your feed in tariff and general electricity purchase rate.
@thinker108 But you’re spending more now because you now have an EV to charge; i.e. it’s reducing the feed in profit because batteries have to be charged longer or instead of feeding in you are using the electricity to charge EV.
@kelbel81 6000 cycles would be some pretty high end batteries. BYD uses LFP cells which typically are rated at 2, maybe 3000 cycles before 80% capacity. Lithium titanium oxide would easily deliver 6000 cycles if not many more but would cost many times as much and take up a lot more space as well.
@kelbel81 The 12c feed in tariff won't be around for long. Current market price for electricity during peak solar is usually ~ 3 to 4 c.

The power companies are being forced by regulation to buy power at above market rates. So they are passing this cost onto other customers so its not achieving emissions reduction just transferring money from people with panels to customers without.

Similar threads