You need to work out your actual costs.
You say $50-$70 a month does that include your supply charge.
Looking at general plans for Vic that it costs $1.00 a day ($30 a month for supply).
That leaves $20-$40 a month if you are correct.
So at 20c a KW that means you are using 100 to 200 Kw a month or 3.4 to 6.7 Kw a day.
If you are already taking into consideration the monthly amount of your feed in Tariff that means you have no extra solar power to charge the batteries.
For last month I exported 850Kw back into the Grid, purchased 115Kw, consumed 170Kw of my solar output.
So that's +$170-($40+$30) or a Credit of $100 for the month onto my Bill.
That figure is Feed-in-(Elect used +Supply Charge)
Looking at those figures it would take me $5000/$100 or 50 months to make the investment neutral. (but only get that for about 6months of the year, hence the 100 months to get my investment back)
But in winter I would have no credit as I would be consuming all of my solar on the premises charging my batteries to get 3kw of "free" power at night. I already get free hot water from my panels (have a heat pump).
I rang (in SA) a battery supplier and after a very brief discussion he said that it would not be worth it.
We use less than 5Kw a day and generate on average 17kw a day.
So we are about neutral.
A quick calculation by the guy said it would take us about 10 years to re-cover the cost. (The battery would have cost around $5K installed.
Try a phone call with your WINTER bill on hand to give the details. Its all about what you can do in winter.
Hopes this helps
By the way saying you have 20 panels is not a good way of explaining your solar panels.
We have 24 panels capable of a a maximum 6.6 KW output but we only have a 5Kw inverter.
This means on days of less sun we can produce more power but on days of good sun we loose 1.6Kw a day as the inverter dumps it as heat.
The short answer is "No". The reason being is due to the high cost of batteries, you would not see a return on the expenses anytime soon if you figure the cost of the battery. I would say, remain solar only, and wait and see how much batteries cost in 5 years. Then it may be cheap enough to have batteries fitted.