05 / THE BILLS & THE ECONOMICS
The same season.
A clearer starting point.
The household’s paid 2025 electricity bills cover the same-season comparison window across a retailer change. Prorating the billed energy and supply charges to the matching dates gives an average of $14.85 per day, compared with $4.97 per day from the supplied 2026 bill after commissioning. The calculation method is detailed below.
Same-season comparison · AUD · Based on household bills; 2025 charges prorated to matching dates
| Measure |
2025 bill-based comparison |
2026 after commissioning |
| Comparison dates |
7 Aug – 6 Oct 2025 |
7 Aug – 6 Oct 2026 |
| Days |
61 (35 AGL + 26 ENGIE) |
61 (ENGIE) |
| Period cost |
$905.66 (prorated) |
$303.13 |
| Electricity / day |
29.78 kWh (prorated) |
35.38 kWh |
| Average cost / day |
$14.85 (prorated) |
$4.97 |
~66.5%
Indicative daily cost reduction
($14.85 − $4.97) ÷ $14.85 × 100
A bill-based comparison using prorated 2025 charges. The difference includes tariff changes and cannot be attributed solely to the battery; aligning bill credits may change the result.
~$9.88
Indicative difference per day
$14.85 − $4.97
The supplied 2026 daily electricity use is about 18.8% above the prorated 2025 figure. Weather, household use, billing allocation and battery losses may contribute.
~2.8 years
Arithmetic illustration only
$10,000 ÷ ($9.88 × 365) ≈ 2.8
Annualising the indicative difference gives about $3,606. This is an unverified simple-payback illustration, not a forecast or guarantee.
THE STORY BEHIND THE NUMBERS
Two electricity companies. One family’s bills.
The family changed from AGL to ENGIE in September 2025, so last year’s comparison brings together bills from both companies. These were bills the household actually paid.
7 August–10 September · AGL
AGL’s final bill covered 69 days. To match the 35 days in our comparison, we divided its electricity and supply charges by the number of billing days, then multiplied by 35. That gives $598.33 including GST. AGL used an estimated meter reading, but the bill itself was real and the family paid it in full.
11 September–6 October · ENGIE
ENGIE listed this period separately on its bill. Adding the electricity and supply charges, with the plan’s 21% discount and GST, gives $307.33. Government bill-relief credits are not included in this figure.
Together: $905.66 over 61 days — an average of $14.85 a day.
This is a comparison calculated from paid bills, with AGL’s charges prorated to the matching dates. The calculation method is available below.
How we calculated the figures
AGL: $598.33 for 35 days. We took the electricity and supply charges from the paid 69-day bill, added GST and prorated them to 35 days: ($357.89 + $637.20 + $77.24) × 1.10 ÷ 69 × 35. We used the same method for electricity use: 2,102 kWh ÷ 69 × 35 = 1,066.23 kWh. This spreads the bill evenly across its dates; it does not show what the home used on each individual day. AGL based its meter reading on an estimate; the family paid the bill in full.
ENGIE: $307.33 for 26 days. We used the charges listed for 11 September–6 October, added 26 days of supply charges, then applied the 21% discount and GST: ($119.78 + $204.25 + 26 × $1.1395) × 0.79 × 1.10. The $75 government credit on the longer bill has not been included in this 26-day calculation.
The daily comparison: $14.85 versus $4.97. The 2025 calculation totals $905.66 over 61 days. The supplied 2026 bill totals $303.13 over 61 days. Using the unrounded daily amounts gives a difference of about 66.5%. The plans changed, and the 2026 bill’s credits and adjustments still need to be checked on the same basis, so this is not a measure of battery savings alone.
The 2.8-year figure: $10,000 ÷ ($9.88 × 365). This simply extends the daily difference across a year. Future bills will vary, and the calculation does not include maintenance, battery ageing, replacement or financing costs. It is not a guaranteed payback.
