Savings and payback model
We first estimate how much exported solar can be charged and later used after sunset. Delivered energy is limited by the usable capacity, stated solar exports, stated evening use and round-trip efficiency.
Charge energy is the lower of daily solar export and usable capacity.
Delivered energy applies round-trip efficiency and cannot exceed evening use.
Daily value is avoided grid imports minus the feed-in tariff sacrificed.
Payback accumulates yearly savings with 2% annual degradation until the price is recovered.
Default efficiency is 90%. The user can change it. We show warranty recovery because “eventually pays back” is not enough if eventually comes after the warranted life.
Federal certificate model
For batteries installed May–December 2026, we use the official factor of 6.8 and the legislated capacity taper: 100% for 0–14 kWh, 60% for 14–28 kWh and 15% for 28–50 kWh. We round down the final certificate count.
Dollar support is displayed as a range of $35–$40 per certificate. That is not a promise of the retail discount: the certificate market, retailer contract and administration affect what appears on a quote.
What this first version does not model
- Half-hour interval data, seasonal solar generation or weather.
- Time-of-use tariff windows or wholesale energy plans.
- VPP payments, curtailment, demand tariffs or grid charging.
- Finance interest, maintenance, replacement parts or residual value.
- Switchboard, metering or site-specific installation complexity.
- State incentives, which can change independently of the federal scheme.
Those omissions make the result a decision screen, not a bankable engineering forecast. The next model version will accept interval data and evaluate multiple tariff strategies.
Primary sources and verification
Rule changes are reviewed before calculations are updated. Material changes will be recorded here.