Brisbane

A basic 1 pole timer is the cheapest and easiest. A Hager DIN mount EHN011 is a 24 hour analogue timer with battery backup that'll do 16A and will set you back about $75+GST. Or you can go with a cheap Tesla (no relation, I dont think) SUL180A or similar brand which will be more like $50+GST. An hour of a sparkies time will be more than enough to install one assuming you have a free pole left in your board.

The "better" way is to use a smart relay connected to your inverter. It gets programmed to only switch on when the inverter is exporting more than the hot water system will draw - i.e. if you have a 3.6kw element in your hot water system, it would be programmed to only turn on if you're exporting >4kw. That covers you against cloudy days, and will turn the hot water system off if something like your AC or stove or whatever turns on to make sure your usage is always 100% covered by solar. Not all inverters support this, and the cost of installing it will be higher so you may not make back the difference in cost on the extra power you pay on the edge cases that a basic timer can't protect against.

Also sometimes people will swap out the 3.6kw element for a 1.5kw element. This means the hot water system stays running longer, but draws less power so it is more guaranteed to stay below solar export. Not worth the cost of doing if everything is working fine, but if your element fails and you have the choice its not a bad idea to swap.

I've swapped to OVO on their EV plan, and they give a 3 hour window in the middle of the day (11am to 2pm) of free power. I changed our timer to only run between 11:30am and 1:30pm which with a 3.6kw element is plenty of time to get the water toastie again. That way I dont have to worry about smart meters, or cloudy days, or anything! Its always free!