Each candidate battery size is evaluated as a multi-year cashflow stream.

• Year 0: net upfront cost = fixed_install + variable × size − tiered_federal_rebate − state_rebate
• Years 1..life: year-specific saving from the degraded effective battery size − standby cost
• Import tariff growth scales avoided-import savings, grid-charging cost and standby cost in nominal dollars; default is 0%.
• Feed-in tariffs are held flat over time by default, so lost export value and battery export revenue do not grow unless the model is extended later.
• Optional residual value in year=life: residual % × net_cost
• NPV = sum of cashflows discounted at the hurdle rate
• IRR = the rate at which NPV = 0
• Discounted payback = month at which cumulative discounted cashflow turns positive

Dispatch behavior matters as well as tariff structure.

• The battery always prioritises household self-consumption first: it stores excess solar and later offsets household imports.
• Ordinary solar export can still happen when the battery is full or charging is constrained.
• If a plan includes a configured free charging window, the battery may top up from the grid only when that window is actually priced at 0c/kWh.
• If a plan includes a configured export window, the battery may export only residual surplus after household use in that interval is already covered and enough energy is reserved for likely remaining household use until the next meaningful solar recharge.
• That reserve is based on this NMI's historical demand pattern, uses a recent-history blend with a stable fallback, and treats the next meaningful solar recharge as the reserve horizon.
• Battery-export revenue is valued using the current FiT windows for the plan, and those FiT values are held flat over time in the current model.

• Exact — flat or TOU plans modeled directly within the dispatch path.
• Bespoke — extended logic for free-electricity blocks and time-varying FiT.
• Approximate — simplified TOU; may omit retailer-specific free blocks, conditional fees, dynamic FiT.
• Custom — user-defined plan.