What Size Battery? 32 or 40?

Waffles on 07/06/2025 - 16:29

I've been quoted $16,400 for a 32kwh SigenStore with 15kw Inverter & DC Coupled (to remove current 15kw fronious - I also have about 16kwh of Solar already)

I was then quoted $18,900 for a 40kwh (5x8) Battery and a 20kw Inverter. I asked for the larger sized inverter as I understand the batteries are 0.5C so could output up to 20kw. This is important as I have a 3 phase EV charger that goes for 11kwh and an AC that seems to go up to 6-8kw. I run them at night for cheap power and together they would exceed 15kwh.The batteries might be a bit big but it's a 5 person household so I think we could mostly self consume in winter and output in the summer.

In theory this leaves 1 slot on the Sig which I can then use for an extra battery or a DC EV charger if they stop being stupidly expensive or V2G becomes worthwhile.

It seems worth the extra money to get more storage and more inverter power (though im unlikely to get more solar anytime soon). Is there any reason I shouldn't go for the bigger size?

Electrical & Electronics Solar Battery

Comments

  • GordonD on 07/06/2025 - 18:17
    +3

    Is this a genuine question or just an opportunity to boast?

    • Waffles on 07/06/2025 - 20:18
      +2

      I have solar, an EV and am contemplating a battery, I'm lucky but it can't be that unusual…

      • 2esc on 08/06/2025 - 10:35

        Certainly not common most people just dont have that sort of money.

        • Waffles on 08/06/2025 - 19:49

          I didn't say common, but I see a lot of EVs and I assume I'm not the only one looking at the battery rebate… In any case, I wasnt trying to boast - sorry. In OzBargain fashion I was trying to get the most bang for my buck!

  • Malik Nasser on 07/06/2025 - 18:26
    +1

    Is there any reason I shouldn't go for the bigger size?

    It cost more…?

    I also have about 16kwh of Solar already

    Wow, you must have those next gen solar panels that also store energy…

    I have a 3 phase EV charger that goes for 11kwh

    You really need to get a grip on the difference between power (kw) and energy (kwh)

    • Waffles on 07/06/2025 - 20:17

      Yeah I never understood it sorry.

      • gyrex on 08/06/2025 - 09:24
        +1

        Power (instantaneous potential: solar panels, generators etc.) is measured in kW. Energy (power stored or used over time: battery capacity or how much power was used over time) is measured in kWh.

        • Waffles on 08/06/2025 - 19:47

          I knew that one was kinda throughput and one was kinda capacity but thanks - this helps.

    • Waffles on 07/06/2025 - 20:18

      Sorry I guess he point is if I'm going to spend more on a system, should I save the bit of cash and let is be a bit smaller or just go for it?

  • Pigey on 07/06/2025 - 18:44
    +1

    Do you really use 40kWh of power overnight throughout the year?

    If you do, that suggests you are in a poorly insulated house and you might be better off spending that money on insulation or sealing leaks.

    • Waffles on 07/06/2025 - 20:16

      If I'm charging the car. So not every night but some days we go through a lot

    • LFO on 08/06/2025 - 13:58

      that suggests you are in a poorly insulated house

      Depends on the size of a house. And location too.
      A 9 (nine) bedrooms house fantastically and thoroughly insulated will probably need that and more …

  • MS Paint on 07/06/2025 - 20:37

    What is the break even period?

  • aragornelessar on 07/06/2025 - 21:45

    Can I ask if there was any particular reason you're removing your Fronius? I have a Fronius Symo 3 phase and was going to look at adding a Sigenergy battery but not keen on removing the Symo cause it's only 17 months old

    • Waffles on 07/06/2025 - 23:00
      +1

      DC Coupling seemed a better idea and the fronius becomes another potential point of failure? Maybe I can flip it for a few hundred to offset cost.

      • NigelPearson on 08/06/2025 - 19:37
        +2

        Another way to look at it is that seperate components are cheaper/easier to replace when something goes wrong in the future.

        If, for example, you split your solar between the Fronius and your potential new battery inverter, you can still power some things if your SigenStore system has a fault.

        I would normally suggest this reduces heat failures in the battery inverter (because it is doing less work), but the SigenStore brochure says the batteries have a "voltage range" so maybe they have a DC->DC converter inside each battery module???

    • noz on 08/06/2025 - 15:22
      +1

      Yes it feels wasteful to get rid of such a new inverter, but connecting the panel strings directly in to the Sig controller means less DC<->AC conversions and hence a bit more of that generated PV will go in to the battery. Also keep in mind that I believe if you stick with your Symo inverters, then I believe you will be bottlenecked by the Sig inverter capacity, whereas if your panels are connected directly to Sig, then you are only bottlenecked by your max battery charge rate.

  • Jaspa7 on 07/06/2025 - 23:50

    32 kWh seems appropriate, if a bit much.

    I'd advise that if you're not nearly completely depleting the battery and fully charging it again every day - it's going to waste.
    Even with an EV and a power-hungry HVAC system, our typical consumption is only around 25kWh/day, meaning we get max use out of our 14kWh batt almost every day (granted, we are only a 3 person household) - it's only really there as a slight buffer and the majority of the savings come from solar.

    ~~Noting you're not even coupling it with solar panels? This makes it seem economically un-advisable.
    Some simple back-of-the-envelope calcs:
    (This all depends on provider and state of course, I'll just use typical)
    Lets say
    Peak = 50c/kWh
    O/P = 30c/kWh
    Shoulder = 27c/kWh

    Assuming you're utilising the entire 30kWh every day, saving 23c/kWh (HIGHLY unlikely) - that's $7/day you can save max.

    That's about $2.5k per year - making the 'break even period' ~7 years.
    (granted, modern LiFePO batteries last a lot longer than that, but doubt you'll still be the owner after say 12 years)

    This doesn't even factor in charge / discharge efficiencies - the inverter alone is probably only 90% efficient.
    AC-DC from grid to batt, maybe 85%.
    Once these stack up, full system is maybe 75, 80% best-case efficient.
    Then there's self-discharge/leakage.. maybe 1% per day.
    and finally, you're most likely only going to utilise it to 70%.

    True savings is probably only around $1500 - $1600/year, a 'break even period' of about 11 years.~~

    EDIT: disregard, I missed the 16kW of solar.

    Changes the equations entirely, but point still holds - look at your typical daily consumption and try to size it for only 80% of that.

    • Waffles on 08/06/2025 - 07:47

      Thanks, I figure I could always chuck on another battery module later if something changes but I won't get the battery rebate then.

      I was thinking the extra battery could be utilised by amber or something though I did see someone say that this battery rebate will make amber less profitable as the point is to reduce the spikes.

  • HelpMeiCantSee on 08/06/2025 - 10:28

    Always but always go for the biggest that you can reasonably afford.

    I have had a pw2 (13.5kWh) for about 3 years maybe 4. Havent had a bill since. The big issue I have had is output capacity (5kW for pw2). I have mitigated it via automations, but it has required I be aware of what’s being used when. I also have 15kW of solar (currently winter will MAX out at 9kW peak production at best, usually winter days max around 7kW peak.

    I have since added 15kWh more battery capacity with a 4.8kW output, bumping me to 30 kWh/9.8kW. This now allows me not to freak out if the AC turns on while I’m cooking (oven consumes 3.6kW, air fryer 2.4kW for example).

    The week of rain we had, I managed 3 days limping on battery being super conservative, then had to draw a total of about 20kWh from grid- easily 2 months worth of grid use for me, if not more.

    I am now adding a further 15kWh battery capacity with approx 9.5kW output, which will give me a grand total of 43.5kWh storage and about 19.5kW output- very very similar to what you have proposed.

    Adding my possible (instantaneous loads) like oven + air fryer+AC+pool heater+pool pump+water heater gives me 3.6+2.4+6+6+1+1=20kW. Unlikely to all be on at the same time sure, but it can pretty much cover my max possible use case. As far as the storage goes, 40kWh can be used up very quickly depending on how it’s used and what for.

    For me, the above battery combination of 43.5kWh would have let me get through pretty much the whole rainy week without needing grid power. It also gives me a big chunk to leverage on VPP or with amber or similar.

    Substitute my pool for your EV, and it would be pretty similar (I suspect you would use more power than my mostly automated house too) Go for the bigger option.

    For the value/payback period brigade, the sums are not as simple as they seem. My calculated PW2 payback period was 7-9 years, which actually came out at <3 years in reality.

    • Waffles on 08/06/2025 - 19:45

      I have a pool pump but not a pool heater or electric hot water. im mindful that if I wanted either than my usage will definitely go up too.

      Thanks :)

      • HelpMeiCantSee on 09/06/2025 - 13:11

        I don’t have an ev, so my pool usage would pretty much translate to your ev usage.

  • noz on 08/06/2025 - 15:29
    +2

    I'm not sure if such a large battery is appropriate in your case. If it's about charging the EV and running the AC at night, then you'd be much better off doing so on say, the AGL 8c/kWh EV plan (which will cover any loads, not just EV) rather than via a battery. Powershop have a similar plan also. To me, charging an EV from a home battery seems a bit nonsensical, but I can understand it to some degree in specific cases.

    • Waffles on 08/06/2025 - 19:44
      +1

      I was thinking more that if/when I ever had both running, I might want to be able to use (at least some) of the battery rather than pulling from the grid, but you're right since even a 20-80% charge on the EV is more than the battery. I guess i was thinking more that to cover my possible max load, future growth and given the (relatively) small different price I couldn't see a downside.

      But (what if) I could get a 12 kw battery in 2 years for half the price, what if having them all degrade at about the same time is bad for some other reason? more just trying to identify potential gaps.

  • HelpMeiCantSee on 09/06/2025 - 13:15
    +1

    Let’s also not forget that the purpose of a battery is not really a simple equation. Stuff like backup, preventing grid draw at any time, contingency etc are all “non financial” reasons. Plus, so many of the calculations assume full daily solar input and don’t take into account poor weather.

    As I have said many times, go as big as practical. my 2c

    • Waffles on 09/06/2025 - 16:11
      +1

      Thanks!

  • KBZ on 10/06/2025 - 17:51

    Can I ask where you're getting your quote from or is it like a range of quotes from solarquotes?
    You're quotes seem cheaper per kwh than what I got.
    24 kwh SigenStore, 20kw inverter for about ~14K. Feel like I'm getting overcharged a bit. Would love larger battery set up like yourself but using that battery payback calculator at the prices I'm quoted its like a 10yr or payback seems hardly worth it….

  • rong on 24/07/2025 - 19:08

    Forget about dc charging, it looks beautiful only in face value. Think about how fast your cycle count is gonna roof.

    No one can beat 50kwh NeoVolt for $6700 installed(https://www.ozbargain.com.au/node/915675), price is single phase. Need to add a bit more for 3 phase.

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