What size solar system do I need?

Sizing solar is a sum, not a sales conversation. You need two numbers: how much electricity you use, and how much a kilowatt of panels produces where you live. Everything after that is about fitting the answer to your roof and your plans.

Step 1: find your daily electricity use

Your bill shows the kilowatt-hours (kWh) you used and the number of days in the billing period. Divide one by the other. Do it for a summer and a winter bill, because use changes with the seasons. Our guide to reading your electricity bill shows where to find each number.

If you have a smart meter, your distributor’s online portal can show when you use power across the day. That matters later, because solar is worth most when you use it on site rather than exporting it.

For a sense of scale, 24 kWh a day is around what a five-person household uses on average, according to Energy Made Easy.

Step 2: find out what 1 kW of panels produces where you live

The Clean Energy Regulator sorts every postcode into one of four solar zones, each with a rating for how much a kilowatt of panels generates in a year. Divided by 365, that gives a daily average. Look up your postcode in the regulator’s zone ratings table.

These are yearly averages for a well-placed system. Winter output is lower and summer higher. East- or west-facing panels on a typical roof can produce up to 15% less than north-facing ones.

Step 3: do the sum

Daily use (kWh) ÷ daily output per kW = panel size (kW) that matches your yearly use.

Matching your yearly use doesn’t mean a zero bill. Panels produce most at midday, so you’ll export some power cheaply and still buy power in the evening. That’s why the next step matters as much as the sum.

Step 4: check it against your roof and the rules

• Inverter and export limits. Your electricity network can limit system size and exports. On a typical single-phase home, systems up to 5 kW are usually no problem; bigger ones may need extra paperwork or may not be allowed. Your retailer should check before quoting.
• The 33% rule. Panels can be up to 33% bigger than the inverter and still earn STCs, which is why 6.6 kW of panels on a 5 kW inverter is so common.
• Roof direction and tilt. North is best. Panels facing different directions need an inverter with two inputs, or microinverters. Keep panels tilted at least 10° so rain cleans them.
• Shade. One shaded panel can drag down a whole string of panels. Ask the installer how they’ll design around trees or chimneys.

Bigger isn’t always better

Modelling for Solar Victoria found 4 to 6 kW systems often had shorter payback periods, and cheaper efficiency fixes, such as insulation, draught sealing and efficient appliances, can pay back faster than extra panels. Size for what you’ll use, then add these if your budget allows:

1. Move daytime-friendly loads, such as the washing machine, dishwasher and pool pump, into the middle of the day.
2. Consider a heat pump hot water system on a midday timer (see heat pump vs solar hot water).
3. Plan for an electric car or a battery now, if they’re likely in the next few years (see is a home battery worth it?).

This guide is general information, not financial or legal advice. Rules, rebates and prices change, so check the linked sources before you decide. Facts checked 18 October 2026.