What battery to buy?


Im looking at getting a battery to complement my solar. i have a 10kw solar system and thinking a 7-8kw battery would cover my for overnight use.

i don’t want to change inverter and am happy for it to go down if there is a blackout as it doesn’t happen all too often so i guess i need some sort of inverter/charger for the battery system too?

What should i look at getting?
and how much am i looking at?


Is your inverter grid tie only? By you statement above I believe it is so you need another inverter. You need a inverter that can convert DC (battery voltage) to AC (mains voltage). This Inverter will need to be large enough to support your needs while running on batteries, this could be a lot if you think to run entire house on batteries while a power outage happens. If, on the other side, you want something to store your over production on day (you produce more than you use and instead of export to the grid you store it locally on batteries) and use that energy at night to easy your import necessity overnight, than you are looking to a hybrid inverter or something similar to a Tesla Power Wall.

For the second case, do your company charges you different amount of money for the imported vs exported energy? For instance, here in Brazil, domestic customers pays a flat rate to KWh no matter time of consumption, we don´t have peak hour or shoulder tariffs (in fact, this year it became optional to customers that want to have this kind of billing). Hence, there is no economic benefit invest on a battery bank to import less energy overnight, my bill will always be “minimum + (Imported - exported)” for “imported-exported” being positive number or just the “minimum” otherwise (the export excess will roll over to the next month like a saving account). So, expend money on batteries has no economic reward. However, for power outage it is another matter, it could be important to some people have power backup since our summer is full of thunderstorms and small power outages due trees falling over electrical wires or thunders disarming high voltage break switches at distribution lines.

Regarding prices, a lot depends on your local market so I have no idea.



I have made the decision that I will not get a payback worthwhile if I installed battery.
When I get a quote for batteries, I find the payback is significantly greater than the battery life (typically 10 years, but you can squeeze the life to 15 years, which is my cut off point for ROI).
For me, I spec’d up my solar to generate 3 times more than I need and feed that back int the grid. So, I am using the grid as my battery.
At the moment, my feed in tariff to the grid is about 40% of my usage rate, so I am only out of pocket a small amount + the daily connection charge. Some months I am actually positive, including the daily connection charge. [yay!]
btw, I am based in Sydney, Australia and am with Energy Australia.
Of course, batteries will have their place for some, but purely on an economic argument, they are not for me today.


I have also made the decision that I will not get a payback worthwhile if i installed a battery.
I got a quote for batteries, and found the payback is significantly greater than the battery life (typically 10 years, but if i can squeeze the life to 30 years for the battery then it might work.
For me, I spec’d up my solar to the max size in west auatralia 6.625 kw and feed the excess back into the grid. So, I am using the grid as my battery.
At the moment, my feed in tariff to the grid is about 25% of my usage rate, so I am only out of pocket a small amount + the daily connection charge. Some months I am actually positive, including the daily connection charge.
I am based in 300 kms south of Perth, Australia and am with synergy Australia.
over the last 12 months my total spend for power and the daily supply charge has been $170— so total cost is 0.46 cents per day
Of course, batteries will have their place for some, but purely on an economic argument, they are not for me .
But if you want a Powerwall 2 in SA right now, AGL are advertising them, as part of their heavily subsidised Virtual Power Plant (VPP) trial, for only $5,490 (+ $399 if you want backup).


Yes Batteries aren’t worth while normally but the South Australian Government is starting a scheme in October that gives $500 back per kw of batteries capped at 5 grand.

that could actually make it worth doing. as i use most of my power after the sun goes down

i have a grid tied inverter and have heard that you can put a separate inverter for the batteries between the solar and Grid inverter

Im not phased on having it for blackout protection just looking at the cheapest way to setup batteries with my current system (not the cheapest batteries). so my ROI becomes worthwhile doing.

how do you work out the ROI on the batteries?


The pay back offered is interesting although I don´t know how much a KWh worth of batteries costs there.

More or less the same way you do for the regular system but adding the cost of the batteries. I don´t know any battery set that would work for 15+ years (*for instance, most electric car manufactures give 8~10 years warranty and recommend a change around 10 years - even for hybrids). For comparison, here in Brazil a 100Ah 12V stationary battery costs around 550 BRL (around 130 USD or 180 AUD). Normally battery banks are organized in 48V (there are 60V , 24V and even 12V arrangements too) to result in more efficient conversion from Vdc to Vac on the inverter. This means we need at least 4 of those to form one 100Ah bank, in a very rough account 2000BRL (700 AUD) to have 4800Wh (4.8 KWh = 100Ah * 48Vdc). There are other things one might take into account to size her system but you can´t squeeze all the 100Ah from the battery if you want it to last, hence, there is not 4K8 watts hour worth of juice in the above arrangement. Also, those 4 batteries weights 100Kg; Lithium battery arrangements might weight a lot less (and spends lot less real state too) but I can´t easy get a quote for those here (they are all imported) but I can bet they will be even more expensive.

Regarding adding a second inverter, yes that is true but how is strongly dependent on how things are wired at your place. You might consult someone there for precise information.

PS: I went to Tesla to compare with a “dream only for Brazilians” Tesla Powerwall. The tesla unit has 13KWh worth of energy and weights 125Kg (this includes the internal inverter). Warranty is 10 years… Amazing! Costs “only” 12500 USD (2 powerwall, site only allows quote from 2 to 10). Still need to add the price cost of solar panels. To have same 13KWh energy of one powerwall, draining 60% of normal batteries above we need 5 banks of 4 batteries each weighting 500Kg and costing around 3 thousand USD + inverter costs and some cables/hardware… To match 2 power walls we are around the same ball park but Tesla is way sexier than a ton of batteries and cables stacked somewhere in the yard.


My very rudimentary way of calculating ROI is to take the installation costs and divide that by what I could potentially save from paying for electricity.

For example, for a Powerwall 2 installation, I work on $12,500 installation cost. I currently pay between $60 and $100 per quarter for my electricity.

Simple calc… $12,500/$100 (per qtr) = 125 quarters. That’s over 30 years for payback in the absolute best case.

Now of course tarriff’s will change over that period, but I doubt the payback will ever get to 10 years or under, which is the warranty period (and life) of most batteries.
As tariffs and battery prices and technology changes, I’ll reevaluate.


$60-$100/qurater is very very small usage! congrats. My bill used to be ~$1000/quarter, using ~ 30 - 50 kWh/day.

My rudimentary calc, also with PW2:
My Tesla PW2 holds 13.5kWh, and cost ~ $12,500 installed.

I drain that 13.5 kWh to empty from sunset to (sometime around midnight), which replaces power I would have pulled from the grid as peak @ 54c/kWh and shoulder @ 23c/kWh - lets say a blended average of ~ 41c/kWh.
13.5 kWh x 41c/kWh = $5.53 saved per night in power NOT pulled from the grid.
$12,500 / 5.53 = 2260 days = 6.19 years payback, assuming grid prices don’t rise or fall.

In summer, I don’t actually drain all the battery - when 10pm = end of Shoulder period arrives, the battery still has ~30% charge in it, which it then holds until morning, so I’m not really avoiding all the $5.53/night. In winter running the aircon, the battery does drain quickly, so all the savings apply.

There will also be overcast days, especially in winter, when the battery isn’t full by sunset, so it won’t avoid all the simplistic 5.54 grid usage until 10pm because it will run out earlier.

So 6.19 years is the best theoretical payback possible (for this capacity and price), and the real payback I’m expecting to be ~ 9 years.


willisave hi a question for you, do you get a FIT for the kws you export to the grid ?
i was thinking if you do say get 15cKW then the cost in lost exports per day to charge the powerwall of say 15KW per day would be about $2.25 per day in lost exportes so your ROI would then be $3.28 so $12500/ 3.28=3810days =10.44 years
B-Man here is a list https://www.solarquotes.com.au/battery-storage/comparison-table/

willisave thanks for your helpfull reply,for me my FIT is 7c kw and i have afixed rate of inport all day of 30c kw so the ROI for me would be 14.5kw at 7ckw= - $1.02 day charging cost, $13000 pw2 fitted with stand alone unit,so13.5kw at 30c=+$4.05c day less cost to charge of $1.02 =3.03c day devided into $13.000 =11.75 years payback so not viable for me till the cost comes down at least 50%


I ended up making a ROI Calculator for batteries. And as jimdcollie says once you allow for the loss of fit to charge the batteries that would also have losses thats not accounted even with the 5000 rebate its a 19 year payback. Based on the tesla powerwall 2 13.5kw

The only way i can see it being plausible for saving money is if you have a high tariff, low FIT and basically drain the battery every day.

Thats not saying that a smaller battery to suit your consumption couldnt work better. Especially if it was going to be fully drained every day.
Anyone know of any decent batteries around the 5kw size i could look at getting priced up?


@jimdcollie yes, I do get a FIT of 11.1c/kWh, rumoured to be decreasing to around 6c/kWh. So yes, I guess I’m foregoing around 14.5 x 0.111 = $1.61 in FIT to fll the battery. OTOH, having the battery enables in winter to charge the battery overnight offpeak @ 15c/kWh from grid, and draw that down during shoulder instead of paying 23c/kWhr, for a net saving of 8c/kWh for those. So, lets hand-wave the combination of these two factors to $1.40 average, leaving 5.53-1.40 = 4.13/day, = 8.3 years payback - for me. I’m OK with that.

@B-Man - its the first few kWh of storage that makes the best return, so yes a smaller battery might be a better proposition for many - at the end of the day, its the $/kWh storage that determines the ROI, and a Tesla PW2 comes in these days at less than $1000/kWh. A 5kWh battery for around $4500 fully installed or lower would have a similar or better return. Effectively the sizing question comes down to - in Winter, how many kWh do you use from sundown around 4:30pm to 10pm when off-peak rates kicks in - adjusting for your local timezone, shading, and tariff structure.

https://www.solarchoice.net.au/blog/solar-pv-battery-storage-sizing-payback-calculator#fullcalc also has some useful calculators, you can do a “without battery” and a “with battery” and compare the difference to see if batteries of different sizes are useful.

  1. Have you measured your actual overnight use?
  2. Have you measured the actual use you have when solar is not providing your electricity, which is when the battery will discharge (whether or not it is “overnight”)?

In my experience, a small home with barely enough space on HALF the roof for 5.6kWp solar (so if we get some more solar in morning direction on the OTHER half, it would be a 10kWp system too), we use up the full capacity of 27kWh EVERY DAY during the winter. Part of the problem is that our 5.6kWp isn’t enough to fully recharge the batteries, but the other part is that we actually use a lot. Much of our use is electric space heating; do you have any high draws overnight, such as A/C, electric heater, etc.?

  1. With the batteries I have, there is no concern of the existing inverter, because they already have all their own inverters built in. I have the Tesla PowerWalls. In the case of the Tesla PowerWalls, once you’ve paid for their standard system, in USA you get backup for free, whether or not you want it, although it COULD be engineered out of the system at extra cost for no gain, and you’d have equipment sitting there capable of backing up just not working.

So, I have no idea what you’re talking about when you mention “do I need some sort of inverter/charger for the battery system too?” Yes, of course you do, is my answer, BUT, you MIGHT be able to use the solar inverter IF THE SOLAR INVERTER has unused battery ports ALREADY BUILT in to it THAT WILL BE FULLY COMPATIBLE with the new batteries you purchase, almost none of which is true of off-the-shelf inverters actually installed anywhere at the time I got solar two years ago (maybe some of that has changed). Typically the few inverters that DO have battery capability built in are (a) proprietary, meaning they lock you in to one battery type, and that battery type is usualy bankrupt, out of business, no longer made, or unavailable by the time you want to install, and (b) that battery capability is rarely installed in solar installations when a battery isn’t ALREADY ordered and being installed at the same time.

So, my recommendation is to go spec your existing inverter, what its full capabilities are, how much of its full capabilities are available to you to add use of, and what those specifications are, find out what batteries meet those specifications (likely NONE), AND THEN start to add to your possible battery setups all the other battery configurations you could get that each require whatever they would require given what you have and don’t have.

I.e., you’ll likely need the charger, inverter, and whatever controllers necessary, just for the batteries, depending on what inverter capabilities are not used on your existing inverter(s).

The easiest is to figure out how many 13.5kWh Tesla PowerWalls you need (keeping in mind a buffer at the bottom) to get you whenever the sun isn’t shining, order a Tesla PowerWall or 2 or 3, but the lead time for that is long.

Almost every inverter company has their own locked in battery type of the week; it’s best to specify them as a pair via one of their engineers. Get quotes from each one and see what you can do!