With the federal battery rebate kicking in from July 1, demand for home battery quotes has gone ballistic.
It’s not hard to see why. Thousands of dollars in subsidies. Rising grid prices. More households with solar. And batteries finally starting to feel like part of the modern home, not a science experiment bolted to the garage wall.
So the question flying in thick and fast is:
“What size battery should I buy?”
But I reckon that’s the second question you should be asking.
The first is this:
“Do I want a battery that makes financial sense, or one that makes me happy?”
Because the battery that gives you the best financial return is likely smaller than you expect. For fastest payback, you want it to fully discharge most nights, so you’re wringing every drop of value from those lithium cells. For many homes, that’s a battery as small as 5 kWh. In fact, some well-meaning government departments have even recommended a teeny tiny 5 kWh battery as optimal for most homes.
But there’s a catch.
It is not hard to tell which house on the street is mine when there’s a blackout.
Blackouts & Battery Degradation
Go too small and you’ll regret it. Not in your payback spreadsheet, but in real life.
You’ll be the one in a blackout with no reserve and no lights (if you are worried about blackouts, you should reserve at least 2 kWh for backup duties).
You’ll be the one kicking yourself when winter nights demand a top-up from the grid during the peak pricing period.
And don’t forget: batteries degrade. Most are only warranted to retain 70% of their original capacity after 10 years. So that “just enough” 5 kWh battery could shrink to 3.5 kWh by the time it hits double digits. Still technically working, but barely. You’ll spend more time topping it up and less time feeling like it’s pulling its weight. At that point, you’ll ask yourself: why did I even bother?
A Different Kind Of Rational
Which is why, if you can afford it, adding a few extra kilowatt-hours now makes more sense than ever. With the new rebate, those extra kWh come at a serious discount. What felt like a splurge a few weeks ago might now be the smarter, longer-term move.
It’s not irrational. It’s just a different kind of rational.
So go ahead, crunch the numbers. But before you do, be honest about what’s more important to you, the shortest payback period, or the peace of mind that your battery’s actually got your back.
About Finn Peacock
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Yes, government/s are sending mixed messages, ie a small battery is all you need, but we want you to sign up to a vpp. Sort of mutually exclusive there isn’t it?
Then they say you can’t have a battery subsidy unless you install solar…
but if you want me to sign up to a vpp, wouldn’t a bigger battery and no solar give better bang for that buck? After all they are curtailing solar right now, so plenty of solar generated power available from other people to charge that battery available right now…
Good advice again. Also, with ability to discharge to grid when FiT is double digits…quadruple if you count the cents, the payback time for a bigger battery is not as long as just with personal home use.
The return for this week from mine is $160 with a day to go. Must admit though, the choice to turn more heaters on or keep discharging at $15.00/kwh isnt always easy.
Consequently, am considering g adding another 2x 3.2kwh modules [$5,000] to a notional 19.2kwh system, thats down to 97% state of health after 30 months.
I think those 2 new modules would be dragged down to 97% too right from install date, which is holding me back.
Neil – $5K (if after federal rebate?) sounds a lot for only 6.4kWh of extra battery
My current 24kWh of battery was only installed a few months ago, but I have just ordered another 16kWh (2x8kWh modules) to add to my Sigen stack – making use of the new federal rebate after July 1st I’m only paying just over $4K for the extra 16kWh
Yes, our 3-year-old 8 kWh of storage struggles during Winter.
The combination of shorter days, cloudy days and more AC use has us importing more energy in June and July.
I am contemplating adding another 3kWh module but still debatng need vs want. Luckily the kWh price has fallen by 1/3 since buying the original batteries.
Which is the other variable to factor in. How far are prices going to fall after the initial flurry.
Rubbish. 5kw is way too small. I’m an electrical engineer also and if you want to be self sufficient and fully electric and not rely on the grid you need 20kw/hrs of battery storage. Minimum.. I’ve got 25kwhr batteries and upgrading to 50kwhr shortly.
If you’ve got a MODERN home and a few normal people in it and run a couple of Air conditioners (or fully ducted AC.), a dishwasher, clothes dryer and washing machine a Fridge or two and a 3.6Kw Hot water heating element. ( An element That is on every night for 3 to 4 hours in winter to heat a 315 litre hot water tank back up to temperature {12kw/hrs just there in hot water alone). Go big or pay the Grid. Or you’ll regret it.
Rubbish really?
I think we are normal.
Our house runs on 3kWh per day most of the year, We have solar HWS boosted by off grid PV and the 3.5kW RCAC is hooked up to the batteries
I have never understood wasteful consumption. It is an indication of low intelligence.
We Burn NO Fossil fuels at our place. Everything in our house is fully electric. Including Lawnmower, line trimmer,power tools, Cooktop, Car (A car that I do not have to buy Fossil fuels to run) etc etc. It all runs from Energy supplied by the SUN. ,( an infinitely renewable Source of energy. Just Ask the plants) via 15kw of Static Solar panels on the roof ( some approaching 20 years old) and a battery in the garage.
I am connected to the grid, but I do Not buy electricity from them, (Grid energy that is mainly produced by burning Coal and Gas). However I often supply green renewable electricity back to the Grid.
My bedroom ceiling fans running on Low when running overnight ( 10 to 12 hours) use more electricity than 3.5kwhrs.?
Parsimony is a Horrible and crippling disease that alienates people and friends sadly.
Good Luck when you get an Electric Car. Running it on 3.5kw hrs a day??
Had to look up what parsimony is. Very apt!
When we get an EV, we will add more solar. Simples. Only a fool would buy a battery to fill an EV.
We took 100kWh from the grid in 2024. And this year will take even less. Sitting at about 40kWh for 2025. Given we are in SA, not a lot of fossil fuel consumption happening.
I would love to see the business case for another battery on that basis. LOL.
Once upon a time frugality was considered a virtue.
Then Capitalism came along with clever marketing to remove the sense (and cents) from ‘consumers’.
And here we are. The Planet destroyed so people can consume more and more.
Sad really.
Gas is good, cooking and hot water heating you can’t beat it. Electricity isn’t the answer to everything.
3.5 kwh a day is hugely low, barely 1300 kwh a year! We use 10 000 kwh a year from our solar system and Reposit and in Canberra any grid supplied electricity is renewable, ie at least is offset by renewable.
Obvs it is your choice, but once you have taken care of the climate change impact, then it is just an economic choice. Canberra is hot on summer and cold in winter, so why suffer just to save a bit?
Adelaide is also hot in Summer, not quite as cold in Winter.
I can assure you we don’t suffer. If I need to import from the grid the 10 days a year the solar and battery don’t quite cut it I do.
Rod, You are not alone. According to the app my Current, Bought from the Grid is 1310kWh, Daily is 3.59 kWh average. Range from 2 kWh to 20 kWh a day purchased
5 Kw solar system, No battery, Heat Pump Heating, cooling, Heat Pump Hot Water, Induction Hob, Dishwasher, Air Fryer.
Small amount of gas central heating at night, after 11:00 pm to 6 am
Most of this usage is used in the daytime using solar
Yup … fully agree. We have 2 PowerWall 2 which started at 27 kWh when new .. now about 22 kWh and supporting most of the house with heat pump heating and hot water … we use it up during this cold winter. The solar panels 12 kWh don’t provide much this time of year. Melbourne..😬
David, intelligently used, a HWS is not treated as a load, but as a thermal battery. Your 315 L unit, heated from 20 to 60 degrees, stores just on 13 kWh of low grade energy. It is wasteful to rob high grade energy from a battery to boost the HWS at an inopportune time. Panels are cheap. (I have 65, for 27 kW, allowing 2.3 kW into the HWS in medium overcast.)
If the HWS isn’t big enough to provide for two days, a second one is cheaper than batteries. Then let the oversized PV array heat them as clouds permit.
That said, I do often heat at night with RCAC, instead of lighting the wood heater. That has similarities, although with a 4:1 efficiency advantage. It only uses 5 or 6% of my 46 kWh battery, whereas 4 x 3.6 = 14.4 kWh, 28.8% of a 50 kWh battery. Ok, that works, if budget permits, but hammers the bad weather reserve.
I’ll rarely raid the battery for the EV, in a long run of overcast, at least for a top-up. Getting home again is worth it.
David, have you thought about a Heat Pump Hot Water System. That would possibly save 10 kwh a day, based on your figures. A 280 litre HPHWS would use around 2 kwh a day with family use. When heating this unit uses between 700 to 800 watts. in summer the unit operates for 1h 30 min, in winter 3 hrs. Might be cheaper than extra batteries by reducing some load. I run my HPHWS during the day, timer set from 12:30 to 15:30 to optimize the heat transfer rate and hopefully use solar. I do not have a battery yet.
A Heat Pump Clothes Dryer may be slightly more efficient as well.
Or put in a Catch Solar Relay & charge existing Electric HWS from solar.
Paul can U explain please how a “catch solar relay” works. We have rental cabin hooked up to our house meter with off peak electric HWS and would like to manage the power input more economically. AGL off-peak from 1 July 39.38c/kWh. Small 15 yo PV system in operation and considering upgrading to larger system plus adding Sigenergy battery. 10kWh. Our personal power usage minimal but tenants don’t worry. .
Paul, So using your method 12kW a day with an element hot water service or 2 kW using a Heat Pump hot water service.
Does Solarquotes have any thoughts re. Australian made batteries from Empower?
An interesting question is,what they mean by Australian made. We just bought an Australian made sofa bed. The mattress and the mechanism were made in China, looks like it was put together in Australia with fabric etc.
Not to mention expanding the battery later will not be eligible for more battery rebate, so would cost full price (and another call-out fee). That’s if the specific battery modules are even still available and haven’t been discontinued / superseded.
Bigger battery = bigger rebate. Do it once, do it right. Choose the largest, most future-proof battery size your budget sensibly allows.
Thank you, Finn. Another great article.
Worried I’ll never recover the cost of battery installation. We work away from home in daylight hours and draw an average of 31kWh every night. With a 3-phase house with a high current draw, options are limited. More frustrating is our 15 year-old 5kW SMA and Trina combo still works fine, (reluctant to replace it). The latest price increase is however a strong motivator:
~ Service 203.5c/day
~ Peak 49c/kWh
~ Off Peak 32.8c/kWh
~ CL1 22.2c/kWh
~ FiT Step1 -4c/kWh
~ FiT Step2 -3c/kWh
~ Solar FiT -4c/kWh
~ Smart FiT -16.7c/kWh
A Sungrow SH20T inverter and SBH400 battery with 15kW of new panels seems best. With a decline to 70% over 10years, the battery should still be delivering the 31kWh I need at the end of the warranty.
Even so, with Daily Service Charge of a quarter of my ave monthly bill ($260), I can’t see my Sungrow costs being recovered over 10 years.
Value your thoughts.
Thanks,
Andrew (Tamworth – Red Energy on Essential Energy & no gas)
Hi Andrew,
I have a 5 kW system, no battery. Electricity yearly bill of $600 and some Gas expenses at the moment. Have quotes for a battery and new solar system but inclined to run the existing system for a little longer as the financial payback not too good. if the system fails, I will proceed with the new solar and battery regardless of payback. I have an all electric home, have gas but not used.
I currently use as much energy during the day using timers if I am not at home. Energy consumption is the highest during the 3 month of winter.
In winter the home is heated during the day to reduce the energy demand at night. Much better to use solar and pay some 0.22 cents during the day than more of the 0.36 cents at night. Also the daily connection fee is 0.96 cents a day. Fit of 0.033 cents soon to drop to maybe 0.004 cents
Maybe use more energy during the day if you can and investigate if there is a better plan to suit you needs
We have gone fully electric, heating, heat pump hot water and induction stove top. Our battery is 19 kWh but I have push up the reserve to 15%. Most morning now in the winter the battery is empty and we use the grid to get the home back to 20 degrees Centigrade. We installed double glazed window so even this morning when it was 1 degree Centigrade outside the inside was still 16 degrees Centigrade.
Hi, nice article. I have a 13.7 battery which i can run a 5kwh ducted air conditioner all day off my 17kw solar system with no grid input. However at night when running my aircon off the battery, i get 2 hours then i have to rely on grid supply. Have set my reserve at 15% for blackout protection. So looking at adding another 13.7 battery, seeing the federal Gov is being soo generous. In my opinion a 5kwh battery just is not enough. Regards Hans
Hans, are there big loads other than the aircon? 5 kW heating may be 1.5 kW input in cold weather = 3 kWh for 2 hrs, or 9 kWh for 6 hrs. Inverter losses would add maybe 6%. (Last night here, 3.5 kWh RCAC run for 6 hrs dropped the 46 kWh battery to 91%, i.e. 4 kWh used. OK, 6 stars, double glazing, etc. helps lots. Fridges ‘n stuff then took it down to 89% by 8 a.m.)
Ah, “ducted”. That’s lossy, heating sparrows. Mine’s split system – in the open living area. No waste. More battery-friendly.
My EVE LiFePO₄ batteries are warranted for 70% SoH after 8,000 cycles, or 20 yrs. Li-Ion may do half that, cost more, and risk fire, due to flammable electrolyte. The 3 banks have done only 61, 51, 34 cycles respectively in 1.5 yrs, due to battery oversize. (I need to emery two busbars on the lazy one.) I also charge to 100% SoC every day – not good for Li-Ion, but OK for LiFePO₄, according to current wisdom.
And I agree – 20 kWh of battery is good – but young folk have so many outgoings. 🙁
Hi Erik , your batteries are diy yes ?
Yes, 3 DIY battery banks, 48 x 304 AH LiFePO₄ cells bought locally @ $350/kWh 1.5 y.a., for an aussie warranty. EVE advertise on YT, and I’d maybe buy direct now, if freight cost is OK, but add in 10% GST on import, and customs agent fee + local freight. (Better option at end.)
The Jikong JK-B2A24S20P BMS has a 2A active balancer for ongoing cell health, and a fine phone app. It’s around $500. Default setup is quick – see relevant “Offgrid garage” YT videos by Andy in Queensland.
There are battery box kits, which might be safer for many – ask for the JK BMS – not all have it. Assembled maybe?
BUT, Rodney reports paying only $4k for 16 kWh after rebate. That’s only available for approved batteries – and it’s hassle & risk free! As long as the rebate lasts, or if A$2.3B expenditure accelerates price decline, then DIY is only worth it for an EV conversion, like my brother’s side x side farm buggy, done last week. (10 kW motor, 72v 100 AH battery – OK for fetching firewood on 4.5 Ha)
Hey Erik , yes lve watched quite a few of Andy’s videos , l made a 280ah 12v battery for our caravan a few years ago , running a 250a Daly bms and seperate 2a active balancing , it works fine .
For the house I’ll probably utilise the government rebate and keep the insurance company happy , lm thinking of going big to cut cycling down and also have more independence and blackout cover just in case .
I’m considering a Neovolt battery system as well as sigenergy , no hurry , but the sparky that l used for solar can do the neovolts so cheaply it’s crazy , and they review well
We currently have around 18kw of solar which in our grey winters ( Ballarat Victoria ) doesn’t go that great compared to our northern friends on here lol
The rebate scheme seriously rewards going big. In my quote, 24kWh of Sigenstore (3 x 8kWh modules) costs about the same as 13kWh. 16kWh (8 + 8) somehow costs even less than 13kWh (8 + 5). The controller bit, installation, etc all cost about the same, after which a few additional stacked 8kWh modules come very cheaply. With that, I can not only waltz through a blackout, but get through a wet winter week or do some serious wholesale exports into the peak, cutting years off my payback. Considering that you can also import from low and negative price events in the day, you don’t even have to match it to your PV size – it’s just something you have to have to qualify for the scheme (that would be just as effective for people without solar, but isn’t being allowed simply because it is being coupled to the SRES scheme).
Adding in that you only get one bite ever at the rebate cherry (which will never be bettered), why not go big?
You aren’t wrong Finn. The wife and I ran through many a scenario like you mention with our energy usage now and in the future.
Before that I was aiming for something around 10kWh and that would just cover us overnight and dodge peak pricing. There wasnt much consideration beyond that.
Now that we have brainstormed and ran the numbers, we have pulled the trigger on 16kW/32kWh Solar/Battery & 7kW EV charger for just under $27k installed!
Complete energy independance is what we landed on and enough battery to mess around with Amber.
I currently have a 6.1 kw Goodwee system and a 9.6kw Goodwee battery.
I also have a 5 kw Fronius system that is 8yrs old.
I want to add another 9.6 kw (3 x 3.2)
Can I putchase them myself and install them myself?
I know that both main switches must be turned off on the Goodwee system? Remove the top section and stack the 3 x 3.2 ontop of the existing and then replace the top section.
Will I still be able to take advantage of the rebate
Hi Colin.
No. Electrical work must be pedometer by a licenced electrician.
Rebates can only be claimed by an electrician with additional SAA qualifications in solar and battery.
I’m in the process of converting everything to electricity. We used to spend ~15-20 kWh/day plus gas ~$7/day to $15/day in winter.
I was going to get Sigenergy 24 kWh battery, 10 kW inverter and 11.4 kW solar. But our latest bill in May showed ~36 kWh/day because we changed everything to electricity except stove tops.
So now I’m getting 32 kWh battery, 12 kW inverter and 14.2 kW solar. I worked out the federal government battery rebate is actually 45%, not 30% like in the news. Everyone should be getting the biggest battery they can afford now.
Thanks to the federal government rebate & ACT government 10 year interest free loan scheme we are adding a 32 kWH battery to our existing 10 kW solar. $15,000. It can charge/discharge at 16kWH per hour (2 hours). Even in Canberra’s cold winter, our maximum daily usage doesn’t exceed around 35 kWH.
A modest approach to using the battery could mean purchasing power on a TOU plan at 21c (night) to charge the battery and using that power when electricity is dearer. As opposed to 33c flat rate. On a high usage day saving up to $3.84 a day
An aggressive approach might involve using Amber and charging and discharging the battery twice per day – keeping 10kWH reserve. If their claim to pass through actual wholesale pricing is true, it seems from AEMO data most days there are multiple 2 hour blocks where the cost is 10c or less & multiple 2 hour blocks where the FIT is 30c or more – hence the possibility of abitrage. An income of up to $8.80 per day gives a payback period of 4.6 years.
For your sums, your levelised cost of storage (LCoS) is 15.8c/kWh. That is what you are paying up front for every warranted cyclable kWh.
The numbers are $15,000 divided by 95,080 warranted kWh on a 32kWh Sig battery (23,770 kWh warranted per 8kWh module).
So, whether on a ToU tariff or wholesale with Amber, your spread should aim to do better than 15.8c/kWh.
Your best returns would definitely be on Amber and won’t be on buying for 10c and selling for 30c, even though that is worth doing, but exporting during market spikes. That will vary from state to state; In NSW we get a lot of spikes, in Vic, not so much. When the Market Price Cap gets hit, it is $17.50/kWh!
With 32kWh storage to a 10kW PV array, you will be looking at some imports. There are a lot of times in the middle of the day when wholesale (Amber) prices get very low or even negative to do that with and it also helps the grid and decarbonisation.
Thanks for the information. Does selling into spikes require manual intervention or will the amber app do that automatically?
You get choices between doing it all yourself and using their ‘Smart Shift’, which is where they automate it, and maybe something in between. Hopefully something in future where you get to input 15.8c/kWh as your minimum for export – although people on Smart Shift report doing much better than this on average already. Smart Shift is pretty much like a VPP but the optimisation is more in your favour rather than towards what best makes them a buck.
You would probably only do fully manual if you enjoy the game of it or mostly want to keep to yourself and self-consume (although with a massive 32kWh storage on 10kW of PV it seems a shame not to be doing some arbitrage).
Every thing l read about solar batteries is too vague to be useful and your site included.
My energy usage is completely different depending on the season.
I have a 6.6kw grid tied solar system.
In summer I send most of the energy back to the grid.
In winter l send very little, if any some days.
In winter l use up to 20kwh from 3pm to 9am most days.
Therefore, l will be purchasing a 20kwh battery.
I know the solar panels will not go anyway near charging the battery but l will charge it from the grid between 9am and 3pm at 8c per kilowatt then use it until 9am when the cycle starts again.
It only becomes a challenge if l lose grid power at 9am. But no system is perfect, but if that should happen then l will drag out the generator to keep vital loads connected.
I will still be able to use my panels to charge the battery, in part, as the inverter can be programmed to do that.
Please take the blinkers off and look outside the box.
Cheers
I bought the Powerwall 3 last year with the intention of getting the additional powerwall 3 battery only add on when it comes out this year. I haven’t heard yet if it’s available yet. Can you let me know as the Powerwall on it’s own will only last all night if I shut air-conditioning and heating off overnight and that’s not feasible in Perth for a lot of the time.
RodM,
Yes, the planet would benefit mightily from a modicum of frugality in the consumption, in the west, of clothing and shoes (mountains dumped, much unused), still serviceable furniture, and fossil fuels. And plastic pollution, which almost outweighs all the oceans’ fish. Humans + livestock = 96% of planetary mammal mass, wild = 4%.
However, redirecting wasted ground-heating sunlight to replacement of CO₂ pollutants, domestically, industrially, and in EVs, is what may save civilisation, if we go all out to *maximise* its utilisation.
“Tread lightly on the environment” is important as we approach 9,000,000,000 planet-eaters – BUT throwing sunlight and wind *hard* at every energy need *is* treading lightly. Maximal self-generation is planet-saving, as is paying others to maximise renewable generation, to kill fossil fuels fast. *Mountains* of batteries!
+4B aircons by 2050 add 0.5°C to GH, but save lives. Bad profligacy?: AI energy waste, dumbing down society. Coal, gas, & oil.
There are two distinct schools of thought on how to minimise emissions.
One is the technology fix. Fill your roof with solar and your garage with batteries and EVs.
The other is to focus on improving efficiency and mode shifting for transportation.
I reside in the latter.
Hi Rod,
It’s been said that EVs aren’t going to save the environment but are instead designed to save the car industry.
We’d all be better off riding bicycles but getting a shift in public mindset will be difficult as the car industry has sold us all the romance of motoring and the government has built the infrastructure they need, which often excludes the competition.
In fact “jaywalking” was a law introduced in LosAngeles 100 years ago at the behest of carmakers, so pedestrians no longer had right of way & motorists were no longer liable for running them over.
The same motor/tyre/oil industry went about buying up tramways and scrapping them.
It’s a travesty.
I like watching cities that are doing it right like Paris and to a lesser degree, London.
They show what can be done in a relatively short period with the right incentives and policies.
So frustrating watching Australian cities failing miserably on Transport.
Rodm, I respect both camps – precisely to the extent that each helps drop atmospheric CO₂ & CH₄ – i.e. Stop Burning Stuff for Energy.
Once we do that, needed energy production also drops by 70%, as only 30% of combustion energy yields useful work – the rest farts out the exhaust /smokestack.
Slow learners, who say it can’t be done, fail to realise that electricity use is mostly 90 – 95% efficient, so not only no CO₂, but so much less needed.
I will admit to having to be pushed to double glazing by regulations, but the reduced energy needs are impressive. (Even better with honeycomb blinds, though.) So, yes, efficiency is a major contributor to speedier and cheaper climate rescue.
But zooming down the highway on photons instead of fossil fuels is the Star Trek level transition which most fills my heart with hope for maybe less than a billion climate refugees – if the uptake kills coal & oil *fast*. (OK, gridscale batteries will amaze before long.)
“RodM says
… Only a fool would buy a battery to fill an EV.”
Consider: A commuter, who cannot charge his BEV at work, recharges it overnight at home, using a government-subsidised domestic battery to time-shift self-generation, without cost beyond the initial investment.
Benefits: Planet-saving CO₂ emissions reduction through elimination of ICE travel and coal-fired electricity consumption. Elimination of fuel costs despite no daytime charging access.
Alternative: Buying CO₂-free night-time electricity from grid-scale batteries.
That is renting “a battery to fill an EV.” (Coal is not a viable alternative.)
Absolutism aside, the optimal choice will vary with retail battery prices, subsidies, and grid-scale battery energy costs, but planetary and individual benefits do accrue to those who do not remain obstinate slow learners.
Of course, once there are electric buses, both the car and battery investments might be avoided, but our commuter is not on a bus route.
Yep battery prices are no where near making that a sane choice
I’ve had 6.6kW of solar now for 6 months. I’ve run the numbers for some summer and now winter days. Based on my worst day I estimate I’ll need 31.5kW of battery storage to get through a winter day. Obviously my solar capacity may not even recharge the battery on some days. However, taking into account battery charge degradation and battery recharge and discharge limits, I’d be looking at a 39kW+ battery to do the job.
Assuming battery cost is $1260 per kWh that would cost $45000 with the rebate in WA. Given the battery life is only about 16yrs and the payoff about 56 yrs. Batteries are simply not viable. Not even with the rebate.
The battery cost per kwh seems very expensive, the battery size huge and the payback period enormous. With a smaller system,say 20kwh, more realistic cost and factor in that you can use battery every day to buy low and sell high, the payback should be down to five years or so. Somebody on this site could give some actual numbers
Ted, I can match your daily consumption on a day that I charge the BEV – but generally skip deep overcast, and charge the next sunny, or even cloudy, day. Do you really have 31.5 kW of non-discretionary daily loads? (How many teenagers is that?)
Generation is an order of magnitude cheaper than batteries, so it is much better design to max the panels first. I have 27 kW, but would add another 15 kW on the old house, if needed. OK, in deep overcast, 6 hrs of only 2.5 kW from 27 kW panels is only 15 kWh, but that avoids the cost of half your proposed battery, and yields again tomorrow, while the battery does not.
As you’re on-grid, you can be paid to take energy at midday, or at least have it for free. Even if you do not export into madly lucrative price spikes, with even a little 20 kWh battery, you could avoid buying (75c/kWh?) evening power, saving maybe $7.50 daily an your usage. Rodney’s $4k for 16 kWh is then paid off in 18 months, 32 kWh in 3 yrs. Carpe diem. It pays.
Fair enough the numbers are subjective. If I look at the outcome using a 20kW battery at an assumed cost of $8,075 ($468/kWh), the minimum payoff period would be 12 years. Still unviable. (Grid input tariff of $0.28)
My original calc was based for a worst case winter day at 36kWh consumption. Mainly for heating, pool pump, HW boost and cooking, etc. no teenagers that are human at least.
Yes perhaps my first job is to increase the solar panel area, however my understanding is that in WA we are limited to a 5kW inverter on single phase (6.6kW panels).
Ted, there seem to be more complaints from WA, than any other state.
I’m off-grid, so no limits, but my on-grid brother in NSW has 19 kW PV & 25 kWh of LTO batteries. He exports 10 kW, for a cash-positive connection. (All the rain up there has played merry hell with that, though.)
Now even if in a NSW area where the export limit might be 5 kW (single phase), then that is the limit, not the PV out the back, it appears. The export limit is then just a matter of software parameter configuration. (“programming”) New installs will have dynamic limiting, I understand. Then what’s on the roof is even more irrelevant.
OK, many networks allow less PV array oversize (wrt inverter output power) than manufacturer’s spec., but he has 4 inverters, as do I. Even a couple might be a way round your local limits? Maybe worth some research.
Failing that, take pool pump, HWS, garage, etc. off-grid with separate PV & inverter? The HWS is a thermal battery. A change-over switch to grid may be good.
Note that my ~$4K (it’s actually $4250) cost for an “extra” 16kWh is for the addition of two extra 8kWh modules to my existing stack so potentially not exactly a fair comparison – since I already paid for the initial install of my Sig Inverter + Gateway (for whole home backup and AC coupling my existing solar, etc) + my initial battery modules plus all the installation costs (physical stack install, wiring, main switchboard, cement sheeting, bollard, etc) – none of that I have to pay again to add my extra battery modules – basically they bring them and lift up the EC/inverter from the top of the stack and drop in the two new modules and return the EC/inverter module at the top – no wiring involved, etc
Not sure what is happening in WA but we are getting 32 kWh battery installed for $15,000
Ken it looks like Neovolt Home Battery System is CEC listed, so the good price gets even better, with the rebate. And yes, going big can drop the battery cycle rate so low calendar life is the limit. (61 cycles in 1.5 yrs here, 7.5 MWh consumed, mostly directly.)
If you’re off-grid, then an 18 kW array seems marginal. If extending my 27 kW, I’d just hook the new array up to an additional MPPT, feeding the battery. That avoids the problem of PV inverter power in excess of the gridforming capacity of the battery inverter(s), that easily arises with added AC coupled solar. (Selectronic is stronger there, than Victron, but you pay for it.)
DC coupled solar energy can flow across the battery terminals, straight to the battery inverters, for direct consumption without battery cycles, so only a small efficiency penalty to consider. I find it the go-to expansion mode in a well-designed modular system.
Hi Eric , we’re grid tied here , winter is pretty bad here for generation unfortunately and lm not sure how much longer we’ll live here so payback won’t be part of the equation, the battery will be handy while we have it and hopefully completes the package as a sweetener for when we sell
I went ott with my system and i am so glad I did, I bought 2 Tesla power walls and two DC expansion units for a total of 54kw/H peek capacity battery. I have 25 460 watt Aikko Solar panels for a total of 11.5kW production capacity.
I don’t get anywhere near that in winter but at the tail end of our first summer when the system was installed I was getting 54kWh a day production.
I have deliberately over sized so that in an emergency we have more than a days worth of power it’s currently winter and we are chewing through 33 to 40% per day of those batteries.
We are running 1 Server and 3 high end PC desktops, 1 fridge, 1 freezer, 1 stove, a convection oven, two oil column heaters, 2 air conditioning units, a router a top end laptop, washing machine and a drier that gets used for 1 1/2 hrs every day and an electric wheelchair that gets charged for eight hours every night. Some of this is periodic obviously but that’s not everything, there can be a C.N.C mill and 8 3d Printers as well.
Chasse, most important first; which CNC mill? A desktop, a Tormach, or I dare not dream? (If the USD falls appreciably, I’m sooo tempted.)
What seems OTT in good times may be just the goods when runaway climate and other exigencies rudely intrude. (My 11 kW north-facing array, here at 38° S, is tilted up 40° for today’s solar nadir. It outperforms the -17° summer arrays by 3:1, even in August, well toward the equinox.)
It may be a little harder on-grid, but going OTT on PV first is still cheapest, and reduces the battery requirement for energy independence. (Perhaps more important off-grid.)
With surplus long cycle-life battery capacity, if on-grid, playing variable FiT and even negative ToU tariffs is an alternative strategy, I figure. But it seems most viable if it is automated, especially given the equipment you have to occupy available attention.
Being able to make stuff is only independent when energy supply is too, as your example demonstrates.
Re battery degradation – that’s the big issue re cost savings, your timeline is very generous, I feel.
Think of it – how long does your phone battery last? They’re all lithium tech, aren’t they? I’m happy to stand corrected –
A 5kw battery, with a Sungrow hybrid inverter limiting the output to only 3.5kW brand new (probably safer to only charge to less than 80% so that’s fine).
For one year it wasn’t even discharging at all (inverter swapped under warranty).
After 7 years use, it is only at 20% of capacity – slightly over 1kW –
Solax provide a warranty that their batteries will still hold 75 % of capacity after five years. it is a good point that it is not clear what the 10 year warranty is worth if there is no capacity warranty beyond five years, and five years is below the payback period on the system.
Neil, warranting only 75% after 5 years, suggests the battery is not fit for purpose, unless it was half the price of the good ones. That is a mere 1825 cycles, even if cycled daily, i.e. only 1/4 of now normal cycle life for 70% SoH. Calendar degradation of 3% is a worst-case figure often heard, but that would still leave 85% after 5 years – not great, but OK if the battery was dirt cheap.
No warranty at 10 years means only one thing, to my mind – run the other way. Years of capacity is the actual product required by the market, not an abandoned bucket of chemical goop. Vendors, warrant your product, or make way for superior goods!
Thanks Erik
Yes, Solax batteries are at the cheaper end, though it is difficult to tell exactly how much as we got our batteries through Reposit Power, and free electricity for five years is part of the deal.
I was not aware before that the performance guarantee was only for five years, even though there is a ten year warranty. Reposit now offer seven year contracts, so they are now confident they will perform well for that period at least. It seems that Reposit want to replace the batteries at the end of the contract, which is less than the payback period, so that is an issue.
It depends partly on what other battery manufacturers actually offer in terms of performance guarantee at the ten year mark, and whether the Solax guarantee is just a bad guarantee rather than an actual expectation that the battery will degrade by that amount. Manufacturers also have to actually still be there in ten years time, and not fall back on small print about how the battery has been used.
Sharon, our phone batteries are Li-Ion = high energy density, but not particularly long life, especially when charged to 100%. Make sure your solar battery is LiFePO₄ (LFP), then cycle life is double at least. They’re also much more tolerant of 100% SoC. The automotive grade LiFePO₄ cells in my house battery are warranted for 70% SoH after 8,000 cycles, or 20 years calendar life. Battery retailers will probably reduce that, for their comfort.
There are Tesla BEVs which have done half a million miles (800,000 km), and still have around 80% SoH. It is becoming accepted that in general, a car will wear/rust out before its battery. (I’ve only done 12,000 km in my base model MG4 BEV (LiFePO₄ battery), but there’s zero perceptible range loss so far.)
Your focus on avoiding more than 80% SoC suggests the Sungrow is Li-Ion, i.e. shorter life, SoC vulnerable, and not remotely as fire-safe as LiFePO₄, which is cheaper. Going for a high cycle-life warranty is the best guide, I think.