Last week, I mentioned a bloke who was beaming after placing his order for a new 30 kWh home battery. The plan was simple: charge it with solar during the day, then use it to both run his house and charge his EV at night. He reckoned he was set for energy independence.
Here’s the problem: unless his installer has done some careful system design – and unless this budget battery actually has the control interface required – he’ll still be pulling power from the grid every time he plugs in his car.
What the Fine Print Reveals
The retailer’s marketing looked impressive, but the small print told a different story. Backup power during a blackout? That’ll cost extra.
That’s already a red flag. Most people assume a home battery always provides backup. It doesn’t. Always check the quote and get it in writing.
Then I dug into the datasheet. It was billed as a:
“30 kWh battery with 5 kWh hybrid inverter.”
There’s no such thing. They meant 5 kW, not kWh. If the people selling you the system can’t tell the difference between power and energy, that’s a worry.
Why 5 kW Isn’t Enough
A 5 kW inverter1 running a 30 kWh battery is like trying to drain your hot tub through a drinking straw.
To fully charge from solar in winter, it would need about six hours of perfect sun. To discharge, the limit is still 5 kW.
Now think about the EV. A standard single-phase home charger pulls 7 kW. That means as soon as the car plugs in, the battery can only cover 5 kW of that demand. The missing 2 kW? Straight from the grid.
Add in the rest of the household loads – cooking, lighting, heating, hot water – and the grid is doing even more work. The battery will run out of power way before it runs out of energy.
Can It Be Fixed?
In theory, yes. A good installer can set up a smart EV charger to monitor grid draw and throttle the charging speed so you always stay under the 5 kW battery limit. But that requires:
1. A battery, inverter and EV charger that support those smarts.
2. An installer who’s sat down with you and worked out your load profile.
I’ll be blunt: at under $10k for 30 kWh installed, I doubt either of those things have happened.
The Bigger Picture
The federal battery rebate has triggered a flood of deals on oversized battery systems.
Cheap, giant batteries are flooding the market. And they’re being sold like flat-pack furniture: one size fits all, just whack it in the garage and “energy independence” is yours.
But batteries aren’t flat-packs. They’re high-energy machines that only pay off optimally when the design matches the household. Without that, the dream of “no bills” becomes a reality of “mystery imports.”
So by all means, grab a 30 or 50 kWh battery while the rebate makes them tempting.
Just don’t expect a cheap, cookie-cutter install to make your bills disappear. Without proper design, all you’ve really bought is an oversized box that still needs the grid every night.
Phase Shift is a weekly opinion column by SolarQuotes founder Finn Peacock. Subscribe to SolarQuotes’ free newsletter to get it emailed to your inbox each week along with our other home electrification coverage.
Footnotes
- Even 10kW hybrid inverters often allow 10 kW of solar power but only 5 kW of battery power – always check the specs! ↩
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Honestly, should 5kw inverters even still be a thing? It would be a small household indeed that never used more than 5kw of power at once during busy periods these days.
People selling systems with a 5kw inverter are no better than snake oils salesman in my opinion.
Secondly, charging an EV and a big battery from your solar array is still a big ask if you are draining them both significantly daily – given the size restrictions on both inverters and arrays imposed by regulation. That means a very well designed system is required.
Unfortunately the biggest inverter you can connect to the grid in WA is 5kw.
Hi Durra,
That’s not entirely true these days. Fronius has set about educating the authorities so they’ll allow 10kW per phase now.
Very true Andrew,
I just had a new system fitted and because my preferred Option of a 8kw Inverter with multiple batteries 30kw, would not fit sideways along my internal garage space available and rules that apply
So the only option the installer gave me was a smaller 25kw vertical stacked, but with only a 5kw Inverter sadly 😥
That means you can only ever draw 5kw at an instant ever from my batteries 🔋
Which is easily exceeded if you switch your HWS to heat up and a moderate air conditioner load during the evening.
So say goodbye to any dreams of being VPP
Take care
Andrew, I’ll second your motion. It’s grid-mandated misdirection which has some outdated thinking stuck in the Tiny Inverter Trap. There’s proven to be no big battery or inverter limit, I find.
My on-grid brother’s solar set-up with 25 kWh of LTO batteries was installed 5 years ago, and there is zero issue, because it was designed for purpose. He has 8 kW of battery inverters + 12 kW of PV inverters, and a 10 kW export limit. There is ZERO relation between that limit and inverter size(s) – just export throttling instead.
My off-grid system’s inverters, with 46 kWh of LFP batteries, are pumping out only 10 kW at the moment, into the BEV, HWS, two aircons, and two fridges – 14 kW earlier today. With 24 kW of inverters, that’s a modest load. The DC coupled arrays are supplying 8 kW, the AC coupled delivering 3 kW, with capacity to spare.
Energy resilience requires load-proportionate generating capacity. How hard is that to understand, really? Half measures half do it.
Another great piece, I’m seeing so much of this in the community now, bigger is better status games. To link it back to another recent post from you about efficiency, we have a single Powerwall 2 at home, have had it about 3 years. When the rebate happened, I thought hmmm, do I need a second Powerwall 2? And then I zoomed out and looked ay my design not of the hardware, as you talk about here, but the design of my weekly energy use.
And I realised for example that our EV travels a few hundred kms per week, mostly on Thursdays and Fridays. And that it can do that without recharging, so that if I leave that until the weekend it can all be from our own solar. Previously I was charging overnight, every night, using cheaper overnight rates. But I realised I didn’t even need those, through consideration of the design of our own energy use, another dimension of design that should probably precede any solar install? Maximising self-use, as you say, but that needs design.
Thanks Finn. As usual, critical information for those of us who are seek a good quality, well designed and supported installation under the current rebate system. I have learnt so much here that, while at times overwhelming for my small, non-electron focussed brain can be a bit too much, but at least I now know the main traps to avoid and the right questions to ask.
I have recently requested you provide me quotes for expanding my system with addition solar and a battery and I will probably buy an EV with a larger capacity battery in the not too distant future. But my charging will be predominantly at home. Blackout coverage will be useful but not critical as the infrastructure locally is relatively new and seems reliable. It will probably outlast me!
I know what I need and want I now know what I don’t need and don’t want. Extremely useful. Some of my friends who have bought big systems at very reasonable prices may not necessarily have received what they hoped for.
Patrick, especially if retired, BEV range + judicious use can greatly boost self-consumption. I’ve delayed BEV charging through the last six consecutive dim overcast days here, running it down a bit with trips to town – to finally charge today in a little 3hr, 20 kWh, catch-up. Even without V2H, that effectively extends the house battery a bit, as it similarly performs consumption time-shifting.
Solar-surplus-only would have charged it over the 6 days, but I find it convenient to just whack in 7.2 kW when it’s sunny. That predicates a sufficient inverter or two – best to have with a BEV or other substantial loads, to avoid grid draw.
A grid isolator and islanding capability is easy, if manual switchover is sufficient for your needs. Automatic failover can be pretty simple too, given adequate inverter capacity.
Thanks Eric I am retired, I will be living in a well insulated house (when I finally finish it!) with a rented flat powered from the same system (so no control on usage there) in the temperate climate of the mid-north coast of NSW. I will be able to manage/time shift loads as necessary.
But I want minimal hassle or maintenance and time input to all this and I am not the sort of person who will constantly watch an app or spend time changing parameters. I don’t want to spend time monitoring and setting it up, I have much more important things to do, including travel! After a long career sorting out complex problems in government and business and having to learn heaps of new systems and processes I am over it. My objectives are ease of use and little or no bills.
So getting it set up properly in the first place by the installer is important to me. I am about to have a discussion with the people recommended by Solar Quotes and hope they know their stuff and set it up properly.
5kW is indeed too low for, let’s say, running a ducted AC system. For the EV, if the charger is a DC charger, and directly integrated without having to convert from DC in battery to AC and then back to DC, it could work.
I have ordered a SigEnergy stack which has a 40kWh battery, 10kW inverter and a 25kW DC charger. This goes with a 15kW solar panel array. I also have OVO Energy’s EV plan which allows for cheap charging between 12am and 6am, so it should work well.
I only have 5kW, and run AC splits throughout the whole house totally fine, never had an issue. As well as charge an EV overnight.
Ashley, I can see yours moving from its current exemplary status, showing the imminent future to all, to a common standard in 5 years max. Its utility and fitness for purpose must quickly become generally evident.
Some say AI ought to foster abundance, despite that fact that is produces nothing, but it is overflowing solar energy which is beginning to do that where sufficient panels and batteries are installed – whether privately or gridscale comes to the same – it’s all equally CO₂-free beneficence.
‘But batteries aren’t flat-packs. They’re high-energy machines that only pay off when the design matches the household. Without that, the dream of “no bills” becomes a reality of “mystery imports.”’
I think “no bills” is a myth unless you go off grid. As more batteries come on stream, DNSPs will have to increasingly recover more costs from the fixed daily usage charge than from energy supplied. I’d predict the daily supply/service charge will eventually be a fixed cost greater than the kWh charge. Which is fair(er) to those who have batteries, but not so much to those can’t afford them. My 2 cents worth.
And regarding battery inputs and outputs, something of a surprise to me was that the new Powerwall 3 has max 8kW in (although if it doesn’t have at least one expansion pack it’s stuck at 5 kW) and 11 kW out. As much as I hate Elon, I have to admire Tesla tech.
For me (with a 32kWh battery and 10kW inverter), the big problem with a 5kW inverter would be with exporting excess to the grid within the window of time in which it is profitable. With a 10kWh battery, you wouldn’t care so much because you’d be self-consuming mostly. But there is no point to 30kWh on a normal home if you aren’t going onto a wholesale tariff (Amber) and using some of that capacity to earn some shortening of the payback.
I am finding a 90 minute peak window in which I average 15kWh of exports for a couple of bucks on a daily basis works great (added to which some serious price spikes will get me about $1,000 a year).
Given that taxpayers paid for nearly half of it I also think powering three other homes through dinner in the peak on renewables without involving the transmission network matters as well. a 5kW inverter couldn’t do that job and therefore the rebate shouldn’t allow one on a bigger battery.
I’ve gained a good general understanding of the Australian battery market from this article, but as a non-technical person, I need practical, accessible advice on adding battery storage to my existing solar setup (10.4 kW panels, 8.5 kW GoodWe inverter). An installer quoted me an AC-coupled system: a 5 kW Hiconics inverter and a 30 kWh battery. I’m also planning to purchase an EV early next year and install a heat pump water heater. Is the proposed setup suitable for significantly lowering my electricity bill and maximizing self-consumption across my solar, battery, EV, and hot water needs? I would greatly appreciate guidance or suggestions for optimizing this system design.
I have 40kWh battery with 5 kW inverter. 13.2Kw solar. Running my ducted RC in Winter and pool pump/bore in Summer – rarely exceed 5kW [on startup] and not at all fussed as saved up front the price difference of going to 10kW inverter. Feeding back with Amber enough power to keep my usage in the positive. No EV.
Keeping the purchase price down kept my battery ROI to under 3 years after Fed and State VPP rebates. [SA $2k]
Plus using more power now [AC] and saving $ not using gas for heating.
Daughter with 20kWh battery and 5 kW inverter also very happy {split AC}
I got 2x11kWh BYD batteries installed to pair with 6kW fronius inverter, so it would all work well with fronius ecosystem and wattpilot too. Except no one told me that BYD HVM battery modules 11kWh stacks can only charge or discharge at 4.5kW. And to fix this I need to add more modules to the stacks to raise the voltage. And I used the subsidy for this first install…
Referring to “Even 10kW hybrid inverters often allow 10 kW of solar power but only 5 kW of battery power – always check the specs!”, what do I look for in the specs? Is it Max. Charging Power?
I’m getting the Goodwe GW9900-ET-20 and the data sheet notes Max. Charging Power is 15kW. Not sure what that means because the inverter is 10kW.
My install will also include Goodwe Lynx 28.8kwh battery, Goodwe smart meter GM3000 and a changeover switch. Is the smart meter good enough to manage the system? I have 10kw solar panels (no more room on roof for more, sadly!!).
To further confuse me, the installers won’t remove the original inverter (Goodwe 10kW) and will AC Couple the new hybrid inverter to it. Do you have any articles on managing multiple inverters?