are there any subsidies to insulate your house in WA? (Western Australia, not Washington!)
Over here (Vic) we have had subsidies on and off over the years.
While not wishing to offend anyone, I'd suggest that this isn't really the best place to seek the information you need. Does WA offer any equivalent to Sustainability Victoria, which will give you unbiased information about all this sort of stuff? You could also contact the ATA (Alternative Technology Association) who will be able to point you to advice about how to cut your heating / cooling energy use. I have been an ATA member for over 20 years, I was a committee member in its early years. The ATA publishes RENEW magazine, which is a wealth of information about renewable energy and energy saving. Back issues should be at your local library.
Owner-Builder magazine also has had a lot about energy efficient building practise over the years. A library should have back copies of that, too.
In answer to your original question, I'd be reasonably confident that you would use less electricity to turn off the AC when you aren't around. The real issue you have is (1) poor design and (2) poor insulation.
To address design - you want your building to have most glass on the North (southern hemisphere) with least glass on the South and West. You should have longer eaves and/or shade to the North and West in particular. Shade sails or pergolas are good to provide shade but still let in a bit of light. Shade blinds on the outside are much more effective than inside blinds or curtains. Windows are a major source of heat leakage. (assuming they are single glazed.)
To address insulation - your building is making it difficult but you should think long, hard and laterally about how to better insulate your house. If your house has solid double brick outer walls (which I understand is normal over West) then you could consider putting an outer insulative skin over the outside of the bricks (Blueboard?) with a render or cladding over that. Insulation on the outside of the brick is better than inside it, as it means the brick adds to the thermal mass of your home. It will pay for itself, over time.
You could also consider adding a suspended or false ceiling, with extra insulation inside.
Do you have slab floor or timber floor on stumps? If you can get under your floor, you can add insulation underneath.
Spend some time eradicating air leaks and draughts. Air getting in round ill-fitting doors and windows, and through wall vents (block 'em up!) adds hugely to your heat load. This is the low hanging fruit - the easy stuff with biggest benefit for lowest cost.
The whole aim is to lower your heat infiltration, as this lowers the demand on your aircon.
You can work in stages as you can afford it - do draughts and leaks first; then prioritise one or two rooms for a real effort to insulate and provide shade over windows (suggest your bedroom and your most used living area); plant deciduous trees to your North to provide shade in summer, but allow sun through in winter; provide a solar pergola or verandah to your North.
particularly if money is an issue, try to be innovative about using cheaply or freely available materials in your area. Our house is double skins of mud, with 35mm (1.5 inches) of polystyrene insulation between the two skins. I sourced the polystyrene 2 ways - (1) when polystyrene is made, it is often made as a huge block that is then cut into slices using a giant cutter. The last slice is often wonky or too thin and is waste. I bought a lot of these cheap offcuts. (2) We arranged with my work and other nearby restaurants and hotels to save their polystyrene fruit and broccoli boxes. I cut the boxes up into flat panels using a bread knife. The result is we spent almost nothing to insulate the walls of our house during construction. We also have double glazing throughout (I made the windows myself, bought the double glazed inserts and put them into frames I made) and ceilings have 150mm (6 inches) of polyester batts, made from recycled PET drink bottles. (AUTEX brand from memory.) the result is that despite us living in the cold wet bit down the south west coast of Vic, we have almost no summer cooling requirement and minimal year round heating requirement. Our only heating is a single Rinnai gas room heater. It would normally be used for only one room but it heats our whole house. Generally we run it for a couple of hours on a higher temp (maybe 20 degrees C) to warm the place up, then turn it to the lowest setting (L) which keeps the place warm, and turn it off before going to bed. There is enough stored heat in the slab floor and the mud walls to keep the place comfortable all night and the next day, only needing to put the heater on the next evening. On a "cold but sunny" late winter, spring or autumn day, we don't need any heat, as sun coming in through the north windows warms up the floor and there is enough stored warmth to last the evening. It really works.
So try to look for ways to improve your building's thermal performance. It pays a return both in lower bills, and in a more comfortable home.
good luck with it.
