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Does it make sense to buy solar battery storage in Perth?

The first consideration when calculating the payback time on solar storage batteries is your starting point.

If you have already purchased a hybrid inverter and it's associated 'smart meter' then it's going to be substantially cheaper to add batteries than it would be for someone who has a non hybrid solar inverter.

For example, let us say you have a Huawei hybrid inverter and smart meter already.
You purchase an LG Chem 9.8kW high voltage battery for $7,800 and a further $400 getting it installed.
(Yes, you probably could get it cheaper than that, but those are the regular 'list prices')

So, you are now up and running having spent $8,200 on your battery.
You use all 10kWh of the battery storage available every day and at the end of year one it has saved you $800 (more on how $800 was calculated in a moment).

In year two, Synergy put their prices up a bit and the battery saves you $840
Year 3, another power price rise and you saved $882

Early in year 9 you have recovered the investment and are now making a profit.

The battery warranty expires at the end of year 10 but let's assume it lasts another 5 years, albeit working perhaps at 75% of its original efficiency.

It's not exactly the World's greatest investment, but it isn't all that bad either.

Now throw in a Goverment subsidy on the battery of $2,000 and the picture looks a lot better. $6,200 investment gets paid off in 6.5 years.

However, if you don't have a hybrid inverter then the equation looks less good.

Your non hybrid inverter can either be replaced with a hybrid and smart meter at a cost of about $2500 (installed) or you can keep your existing inverter and leave it to run the solar panels, and then purchase another inverter, connected to your switchboard, to run the batteries.

Goodwe, SMA, Growatt, Sonnen and no doubt several more have products to do exactly this, and even the famous Tesla Powerwall 2 is in fact an inverter too, it's just that the big white box also includes the battery. Either way, you won't see much, if any change from $2,500.

Now the total cost is $2,500 + $8,200 for the battery, making $10,700 and a 10 year payback or just under 9 years with a $2,000 battery subsidy.

How did we arrive at $800 return in the first year?

We took the current cost (inc GST) or a kWh of electricity from Synergy...28.3272 cents.
Deduct 7.135 cents as this is what you would have been paid for surplus solar without a battery That makes a difference of 21.192 cents per kWh
The battery delivers 10 kWh per day, so 10 x 21.192 = $2.192 a day or $800.08 a year.

Simple maths?
Yes, this is a simplistic method of calculating payback.
Ideally we should factor in things like what interest you would have earned on the money you spent on the battery, or the fact that you might go on holiday once or twice a year so the battery would not be fully utilised and much more.

However, as we are making guesstimates about power prices projecting years into the future anyway, which could all be totally wrong, we might as well just keep it simple.