Let's look at how much electricity and money can be saved by using solar thermal to heat a cylinder of hot water, rather than by using an electrical resistance heater.
Assuming a 100% efficient resistance heater (that's near enough to real values), and ignoring tank losses during the heating period.
180 litres of water is near enough 180kg.
The specific heat capacity of water is approx 4.2 J / g.K
The increase in temperature is 45 Kelvin (60-15)
So the energy needed is 180,000 x 45 x 4.2 = 34MJ
1kWh = 1000W x 3600s = 3.6MJ
Hence, the energy needed is 34/3.6 = 9.5 kWh. At $0.30/kWh that's $2.85
If you want to account for the heating efficiency, just divide by it.
So if your heater is 100% efficient, you divide by 1 - which is why we ignore it in the above calculation. If your heater is 95% efficient, you divide by 0.95, to get 10 kWh. This calculation works whatever the type of heater it is.