What This Guide Covers
- How to calculate your ideal solar system size (in kW)
- How many panels you’ll need based on wattage, usage and roof size
- Impacts of EV charging, home battery systems and export limits
- Common constraints: roof orientation, shade, single vs three‑phase supply
- 2025‑specific data for Australian households
- A downloadable worksheet to help you estimate your system size
Step 1: Understand Your Current and Future Energy Usage
Review Your Electricity Bills
Evaluate your usage for the past 12 months: monthly kWh records, peak vs off‑peak usage, and changes you expect.
The Australian Government’s SunSPOT calculator guides you through this.
Plan for Future Energy Needs
Are you adding an EV, upgrading appliances, or installing a battery? These will increase your energy demand.
Step 1: Determine your electricity usage
The first step is to know how much electricity you use:
- Check your electricity bills — find your daily average usage in kilowatt-hours (kWh). Use 12 months of bills to include seasonal variation.
- If you have a smart meter or interval data, use that—it shows when during the day you consume the most.
- Decide what proportion of your electricity you want solar to cover. Is it just daytime loads, or also evening? Maybe you plan for EV charging, or want to include future increases (like a pool, AC, etc.).
Example: Suppose your home uses 25 kWh/day on average, and you want solar to cover 80% of that (20 kWh/day) during sunlight hours.
Step 2: Calculate required system size in kW
Step 3: Determine how many panels
Solar panels are rated in watts (W) under standard conditions. Common residential panels are 400-510 W.
To find how many:
Number of Panels = (System Size in watts) ÷ (Panel Wattage)
Example:
- A 5 kW system = 5,000 watts
- If using 350 W panels:
5,000 ÷ 350 ≈ 14.3 panels → round up to 15 panels
However, you also need to factor in inverter oversizing rules. A 5 kW inverter can usually be paired with up to 6.6 kW of panels (about 33% more) if there’s no battery. With batteries, the allowance can go up to 200%. Going beyond these limits means you won’t qualify for government rebates.
Step 4: Adjust for roof space & layout
Even if your calculations suggest you need, say, 15 panels, the available roof area may constrain you.
Key considerations:
- Panel size: a standard panel is approx 1.7 m² (often about 1 m × 1.7 m)
- How many panels can physically fit, considering roof pitch, obstructions (chimneys, skylights, vents), orientation (north, east, west)
- Whether part of the roof is shaded for part of the day—this reduces output significantly.
If roof area is tight, higher-efficiency panels or panel placement on multiple roof faces may help.
Step 5: Consider other constraints and future growth
When sizing, it’s wise to think ahead:
- Will you add major appliances (EV charger, pool pump, heat pump etc.) that increase daytime load?
- Will you want a battery, or to go off-grid?
- What are the local grid connection rules? For example, inverter size limits, export limits, phase (single-phase vs three-phase) limitations.
Feed-in tariff (rate you get if you export surplus electricity to the grid) matters. If tariffs are low, you’ll get more value by self-consuming your solar rather than exporting.
Example Scenarios
Here are two example households and how many panels they’d need:
So, how many solar panels do you need? The answer is: it depends. It depends on your daily electricity usage, how much sun your roof gets, how much of your power you want to supply from solar, your roof space, and budget. By following the steps above, you can calculate a system size (in kW) and then determine how many panels will deliver that.
