Solar savings calculator
Size a solar system from your bill, then see net cost, payback and 25 years of cash flow. Change any assumption — the maths updates instantly.
Your home
Costs & credits
Your solar estimate
Cumulative cash flow (25 years)
| Estimated annual use | — |
| Year-1 solar production | — |
| Roof space needed | — |
Model: production = kW × sun hours × 365 × 0.80 (system losses); 420 W panels at ~21 sq ft each; 0.5%/yr degradation; 0.37 kg CO₂/kWh grid factor. Estimates only.
How this solar calculator works
It starts with your bill. Dividing your average monthly bill by your electricity price gives monthly usage; multiplying by 12 gives annual kilowatt-hours. The calculator then sizes a system to cover the share of that usage you choose, using your location's peak sun hours — the number of hours per day the sun delivers the equivalent of 1,000 watts per square metre.
Real systems never produce their nameplate rating all day, so we apply a 20% allowance for inverter losses, wiring, heat, dust and minor shading. That is in line with the default loss assumptions used by the US National Renewable Energy Laboratory's PVWatts model, the industry's go-to free estimator.
Worked example
A $160 monthly bill at $0.17/kWh means about 941 kWh a month, or roughly 11,300 kWh a year. In a 4.6 sun-hour location, each kilowatt of panels produces about 4.6 × 365 × 0.80 ≈ 1,343 kWh a year. To cover 100% you need 11,300 ÷ 1,343 ≈ 8.4 kW — about 20 panels of 420 W. At $3.00 per watt that's roughly $25,200 before incentives.
Why export credit matters so much
Solar you use the moment it's produced saves you the full retail price. Solar you export to the grid is credited at whatever your utility pays — 100% of retail under classic net metering, but often 25–75% under newer "net billing" rules. Set the export credit and used directly fields to match your utility; adding a battery raises the share you use yourself. Read net metering explained for the details.
Getting the inputs right
- Price per kWh: divide the total of a recent bill (including delivery charges) by the kWh used.
- Cost per watt: divide a quote's total price by the system size in watts. Compare quotes on this number.
- Incentives: enter the combined percentage of rebates and credits you're confident you qualify for — check our rebates page.
Frequently asked questions
How accurate is this solar calculator?
It is a planning estimate typically within a reasonable range of professional designs when your inputs are accurate. Roof angle, orientation and shading can move production by 10–25%, which a site survey or detailed modelling captures.
What is a good payback period for solar?
Many homeowners treat anything under about 10 years as strong, given panels are commonly warrantied for 25 years. Your payback depends most on your electricity price, sun hours, install cost and export credit.
How many solar panels do I need?
Divide the system size in watts by your panel's wattage. The calculator assumes 420 W panels, common for modern residential modules.
Should I size solar for 100% of my usage?
Not always. Where exports earn much less than retail, sizing nearer your daytime usage or adding a battery often gives a better return than oversizing.