Battery storage feasibility


Crazy idea. Probably way too much effort to implement but.

Whats your thoughts on being able to have an option where you input a battery size and cost and it can work out from your previous years consumption/generation and tariffs how much you would save and the roi?

So say you select an 8kw (usable) battery system. on first day (selectable) it works out when your exporting and fills the hypothetical battery instead of exporting allowing for the fact it takes more kw to charge battery than your getting due to losses. Then works out if your consuming more than generating it takes it from battery storage instead and calculates it for the day.


Try -
If you have monthly consumption history and solar production history to get the input conditions right, the ROI and payback calcs should be pretty close.


There are so many factors affecting battery ROI it really takes a lot of technical info and a complex model to calculate it correctly. That one willisave posted looks pretty awesome. NREL’s System Advisor Model is also a relatively accessible tool, and there are good tutorials and templates for different energy use scenarios. You can download your utility’s tariff structure directly from inside the program. It’s free at


i had a look but cant see where i can input my monthly data or is it at the top where you just average out your year as a daily figure?

im not a huge user of power but if having a battery basically replaces my grid usage and covers me for blackout and is somewhat viable ill definitely look into it.

cant seem to get any decent info whether its worth it.
SA has an up to $5000 rebate but looks like everyone has upped the price of the systems. if i know a battery can suit my usage then i will pursue it more


hi all have a look at this,


Near the top, underneath the "weekday’ cycle chart, is a white selection box labelled ‘Use monthly consumption figures?’ which can be set to YES or NO, and defaults to NO.
click in it, change it to YES, and the twelve monthly average consumption figures become enabled for you to type those in. Then it will ignore whatever is in the ‘average consumption’ fields above the charts.
Its basically a big online hosted Excel spreadsheet, so sometimes takes a bit of clicking around to get it set up right.


Or, if you like, they have anotherone tailred to calculatin gthe effect of adding a battery to a pre-existing solar system:

Additional homework - would be interesting to compare the two calculators using identical system and battery inputs


I built my own battery simulator.

It uses the 5-min interval data I have (via my Fronius smart meter) for all of my energy flows, plus calculates impacts of different retail plans, be they flat rate or TOU tariffs.

The battery logic and specifications most batteries use isn’t too difficult to program. Capacity, max charging/discharging rates, power reserve level, round trip efficiency. If a long term model then factor in capacity decline rate but a full year of real energy flow data is enough to know whether it’s worthwhile.

If you expect to play arbitrage games from/to the grid, that’s a bit more complex, although not many have a FIT sufficiently higher than cheapest grid import price for that to be worthwhile.

I don’t yet have a full year of data so it’s a little early to say, but over this summer quarter a 13.5kWh Powerwall 2 would have saved me just $0.65/day off my bill.

It would need to cost just $1,900 installed to result in an 8 year payback.


any chance you could share your battery simulator or if not i could give you the figures for you to run mine?

finally ran the other one with my monthly figures but its still so inaccurate.
wont let me choose a 2/3 west 1/3 north system and tilt is 30° not the 22.5° mine are set to.

gave an 11.3 year payback. at 8k battery cost on a 6kwh battery.
i was trying to edit the cost and it froze and im not filling all that info out again! for the 4th time

im thinking a smaller battery would suit me and i have a mate that can install batteries so if i can keep my costs down especially with the rebate if i can find someone supplying them without inflated prices it might work out for me.


My simulator runs off smart meter data, in my case from my Fronius smart meter. If you have that and the electricity plan rates (all the tariffs, discounts and other charges applicable). I’d also need the timing of TOU billing rates in your distribution area if not on a flat rate plan.

But unless you have very high import tariffs, a low FIT, and a decent subsidy so that the battery installation cost is going to be less than $300-400 per kWh of capacity, it is highly unlikely it will be financially viable.

I did simple financial models of battery based on aggregate energy data and used some reasonable assumptions. Turns out the aggregate data doesn’t reasonably reflect how the battery operates based on actual energy flows and financial returns in my case were significantly worse than the simpler models would suggest.


i have 5 minute consumption data and generation. consumption goes for a few years generation is about 6 months.

there’s a good chance it’s not viable but if i don’t look at it i wont know


The quick and dirty calculation assumes you fully charge the battery during the day using solar then fully deplete it overnight. Assuming 100% efficiency and factoring in that you don’t get paid export on the power going to the battery the value is peak power cost minus feed in tariff. Assuming 11c FIT and 30c peak cost a 10kWh battery saves 10 x (0.30-0.11) = $1.90 / day or $0.19/day/kWh in the best case scenario.

If battery is ~$1,000/kWh that means 1000/.19/365 = 14.4 years for break even.

I’ve made these numbers up a little bit, but if you substitute in your own estimates it’s going to be near enough to show it’s nowhere near worth the investment at this point.

It’s possible that you actually pull more than 100% capacity from your battery on some days thanks to the fluctuating nature of shade and power draw varying over the day, but then it’s also very likely that some days you don’t generate enough excess power to fully charge your battery thanks to cloudy weather…. and that’s before you factor in efficiency loss. Bottom line the reality is that it’s likely to make the above pay back period worse by some factor.

I like this post which says all that in more detail:

On a side note, I did put together a spreadsheet to calculate my own situation down to 30 minute blocks with my real data. Now I only have a full data set from November to now, so it’s likely to be significantly optimistic since it’s over a summer period with nice long bright days, but even that is only pointing to about $1 / day with a 10kW battery (or about half my first estimate above).


Or using a real-world NSW example, with peak rates on TOU tariffs at 59.29c inc GST, FIT of 11.1c, and a battery price of ~$930/kWh, that comes out to $4.82/day savings, and payback over 5.3 years. But yes, its a fairly simplistic calculation where the output depends entirely on how realistic your input data is.

That article talks about QLD’s ‘Tariff 11’, which are flat-rate same-price-all-day plan. But thats a silly comparison - anybody with a battery will switch to a TOU plan (a ‘Tariff 12’ plan in QLD), with much higher peak rates they’ll never be charged (as power in those hours will be coming from the roof or the battery), and much lower off-peak rates at night time that they will be charged once the battery runs flat - or to charge the battery ready for the next day if solar will be low. Its an apples-for-oranges calculation.

The article also fails to consider the savings from arbitraging the off-peak price, charging the battery up off off-peak power on cloudy days and then using that to eliminate peak power rates, even on days with little solar generation.

In my case, I calculated a payback from the battery alone at ~9.4 years. My electricity bill has reduced by ~$800 - $1000/quarter, which pays back my entire solar+battery system in around 7 years. I’m comfortable with that.


Wow, I didn’t realise the NSW rates were quite that high, I’m on a TOU (peak and offpeak) rate with peak being $0.451 before a 39% discount bringing it down to less than half that which blows out the viability massively.

But your underlying sentiment is bang on, you have to do the numbers for your own situation and in your case that’s a massive saving. Nice work!

Interestingly my dirty calcs match your’s give or take with the “best case” numbers turning out to be about half when translated to real world. I wonder if that will hold true as a generalisation for most people.


That’s not real world. NSW rates are nowhere near that high. If you are paying a peak rate of 59c/kWh on a TOU plan, then you need to do some shopping around. Peak tariffs are well under 40c/kWh. Mid 30s is achievable.

As you note, choosing inflated electricity plans to make a battery look better is false economy.

Even so, if I plug in a peak rate of 59.29c/kWh in my simulator, over the summer period a PW2 would save me $1.04/day compared with not having the battery.

This is a payback period of over 35 years!

On a sensible TOU electricity plan in NSW, the payback period for me is nearly 57 years!


Indeed. Even using very optimistic assumptions that are generous to the battery’s financial performance demonstrate they are mostly going to result in paying a lot more for electricity.

I did a short survey at Whirpool of PW2 users to find out their real life experience of the average daily capacity utilisation of the battery, i.e. the amount of energy out of the battery per day as a ratio of the battery’s capacity.

The average capacity utilisation was 61%, lowest was 45% and the highest was 75%. n=13.

Efficiency was 88-89%.

These are more realistic numbers to use for any basic financial assessment based on aggregate energy data.


I found the same. Every time i found a better plan financially it was making my solar payback time worse. Not that im complaining as im still better off and a payback of under 2 years for the solar system is pretty good


Bottom line in the article I linked, you don’t buy battery because you want to save money, you buy it for other reasons… like a flaky power supply that the battery can fix for you. Our old place used to go out for a few seconds at least once a week and it would have been really tempting just to keep everything running, mind you it was always midnight-ish so the battery would likely be dead by then anyway unless you wired it up purely as a UPS.


There are much less costly solutions for that than a PW2.


Yeah i would like it as a ups as such to keep things running and if the power goes out during the day it could be self sustainable with the solar but if its not financially viable or close to it its not worth it