How This Calculator Works
We use probability-weighted scenarios to estimate expected value over 25 years. Rather than a single "best case" number, we model pessimistic, median, and optimistic outcomes — then weight them for a realistic projection.
Solar Panel Assumptions
| Installed cost | $1,150/kW | Australian average for tier-1 systems |
| STC rebate | $360/kW | Small-scale technology certificates |
| Panel degradation | 0.5%/year (median) | Pessimistic uses 0.8% |
| System efficiency | 78% | Inverter, wiring, temperature losses |
| Inverter replacement | Year 12 (median) | ~$1,800 cost |
Battery Assumptions
| Federal rebate | ~$311/kWh | Cheaper Home Batteries Program (late 2025) |
| Round-trip efficiency | 85-92% | Varies by battery model |
| Annual degradation | 1.5-2.5% | LFP batteries degrade slower |
| Replacement threshold | 70-80% capacity | When battery is replaced |
| Replacement cost | 65% of original | Accounts for falling battery prices |
| Utilization factor | 75% (median) | Not every day is perfect |
The Utilization Factor
This is where many calculators go wrong. They assume you'll perfectly cycle your battery every single day. Reality is messier: cloudy days mean less charging, some days you're home during the day, seasonal variation affects generation. We model 75% average utilization in the median case, 60% in pessimistic, 85% in optimistic.
Why TOU Pricing Matters
Without Time-of-Use pricing, your battery's value is limited to the spread between your usage rate and feed-in tariff:
Without TOU
~23c/kWh saved
(32c - 5c) × 85% efficiency
With TOU
~40c/kWh saved
(52c - 5c) × 85% efficiency
That's a 74% difference in value. This is why batteries can show marginal or negative returns without TOU, but strong returns with it.
State Rebates (Late 2025)
| State | Battery Rebate | Notes |
|---|---|---|
| Federal (all) | ~$311/kWh | First 50kWh, decreasing annually until 2030 |
| VIC | None | State program closed |
| NSW | VPP incentive only | Up to $1,500 for VPP signup |
| SA | None | Program closed |
| WA | Up to $5,000 | $500/kWh for Synergy customers; stacks with federal |
| QLD | None | Program closed |
| NT | Up to $6,000 | $400/kWh; stacks with federal |
Financial Assumptions
| Discount rate | 6% | Opportunity cost of capital |
| Feed-in tariff | 5c/kWh | Late 2025 market rates |
| Electricity price growth (median) | 4%/year | 2.5% pessimistic, 5.5% optimistic |
| Annual maintenance | $120 (median) | Panel cleaning, inspections |
When Batteries Make Sense
✅ Likely +EV
- • TOU pricing enabled
- • WA or NT (stacked rebates)
- • High evening usage (50%+)
- • VPP participation
- • High electricity rates (SA)
⚠️ Marginal or Negative
- • Flat-rate tariff only
- • Low evening usage
- • Tasmania (cheap electricity)
- • Small solar system
What We Don't Include
- • Blackout protection value — Real but hard to quantify
- • Property value increase — Studies suggest 3-4%, but we don't model this
- • Carbon offset value — Environmental benefits have no direct financial return
- • EV charging optimization — May increase battery value
- • Tariff arbitrage — Charging from grid during off-peak
Sources
- • Solar Choice Battery Price Index (August 2025)
- • SolarQuotes "Are Batteries Worth It?" Analysis (July 2025)
- • DCCEEW Cheaper Home Batteries Program Documentation
- • Australian Energy Regulator Default Market Offer 2025-26
- • AEMC Residential Battery Investment Analysis
Disclaimer
This calculator provides estimates based on modeled scenarios and publicly available data. Actual results will vary based on your specific circumstances, actual solar generation, electricity usage patterns, and future electricity prices. This is not financial advice. We recommend getting multiple quotes from accredited installers and consulting with a financial advisor for major investment decisions.