Solar energy offset is not the same as electricity-bill savings
Separate annual solar production, self-consumption and exports before estimating a reduction in your electricity bill.
Method
Annual energy offset compares estimated solar production with annual electricity use. It does not say when the energy is generated or used. A home can produce almost as much electricity as it consumes over a year and still buy electricity from the grid after sunset.
Start with the solar-offset calculator’s production estimate: array capacity multiplied by peak sun hours, 365 days and a performance ratio. Use local, realistic assumptions. Peak sun hours describe equivalent full-strength sunlight, not the number of daylight hours or the panel’s operating time.
An illustrative 8 kW array with 5 peak sun hours a day and an 80% performance ratio produces 11680 kWh a year. Against 12000 kWh of annual use, the energy offset is about 97.33%. This is an energy ratio, not a promise that the bill falls by 97.33%.
View data
- Used at home
- 4672 kWh 11680 × 0.40
- Exported to grid
- 7008 kWh 11680 × 0.60
Worked example
An illustrative 8 kW array with 5 peak sun hours a day and an 80% performance ratio produces 11680 kWh a year. Against 12000 kWh of annual use, the energy offset is about 97.33%. This is an energy ratio, not a promise that the bill falls by 97.33%.
Suppose 40% of that production is used at home and the rest exported. Self-used energy is 4672 kWh and exports are 7008 kWh. With hypothetical import and export values of 0.25 and 0.08 per kWh, their combined value is 1728.64 currency units. Valuing all production at the import price would instead give 2920 and overstate this example.
Production = 8 × 5 × 365 × 0.80 = 11680 kWh/year
Annual energy offset = 11680 / 12000 × 100 = 97.3333%
Self-used = 11680 × 0.40 = 4672 kWh; exported = 7008 kWh
Illustrative value = 4672 × 0.25 + 7008 × 0.08 = 1728.64
Use your actual tariff, export agreement and time-of-use periods. Fixed charges may remain, exports may be limited, and batteries introduce losses and costs. Shading and seasonal variation change production. This simple annual estimate does not model hourly flows, battery dispatch, financing or guaranteed payback.
View data
- Estimate production
- 8 × 5 × 365 × 0.8
- Separate use and exports
- 4672 + 7008 = 11680 kWh
- Apply the relevant tariffs
- 4672 × 0.25 + 7008 × 0.08
Checks and limits
Use your actual tariff, export agreement and time-of-use periods. Fixed charges may remain, exports may be limited, and batteries introduce losses and costs. Shading and seasonal variation change production. This simple annual estimate does not model hourly flows, battery dispatch, financing or guaranteed payback.
Related calculators
Sources: Australian Government — solar self-consumption and bill savings