How much solar do I need, and why does the time I use power matter?

There is no single “standard” solar size that is right for every home. A useful design starts with how much electricity you use, when you use it, your available sunny roof area, local network limits, tariffs, budget and likely future loads such as an EV, heat pump or more air-conditioning.

The time of use matters because solar generates during daylight. Electricity used while the system is generating can reduce grid imports directly. Electricity exported to the grid is credited at the applicable feed-in tariff, while evening and overnight use still comes from the grid unless a suitably designed battery supplies it.

Feature, advantage, benefit and limitation

• Design using 12 months of bills or interval data. This shows both seasonal demand and, with interval data, when the home uses electricity. The benefit is a design based on your household rather than a generic package. The limitation is that forecasts still depend on tariffs, weather, shading and stated assumptions.
• Exact panel and inverter model numbers. This makes approvals, specifications, warranty and substitution easier to check. The benefit is fewer surprises after signing. Ask what happens if the specified model is unavailable.
• Roof and network assessment. This identifies useful panel area, shading, phases, connection and export limits. The benefit is a design that reflects the property. A remote estimate may still need an on-site confirmation.

What to ask for in writing

Ask each retailer to state the proposed panel capacity and inverter capacity, exact equipment models and quantities, roof layout and shading assumptions, estimated annual generation, assumed self-consumption and export, electricity tariff used, network/export limits, warranties, inclusions and exclusions. Tell them about future electric appliances before they size the system.

The Australian Government Solar Consumer Guide recommends using smart-meter interval data for the most accurate usage estimate and says system size depends on usage timing, roof area, pricing, sunshine and budget. Use its independent SunSPOT tool as a sense-check; supplier savings estimates are forecasts, not guarantees.