Compare quotes using the same assumptions: total upfront cost after any applicable incentives, expected annual bill savings, export income, maintenance and panel degradation. A simple estimate is net system cost divided by estimated first-year savings, but savings can change with your electricity use, tariff, shading and feed-in rate. Assess batteries separately, as their payback period is often different from the solar panels. Ask each installer to show their assumptions, then check current incentive information on energy.gov.au and confirm tariff details with your retailer.