Long-run cost model
Tariff and panel behaviour below power the payback chart. The headline ROI above still uses year-one dollars (unchanged).
Tariff inflation is not a forecast from your retailer. The chart applies the same compounded annual rate to retail and feed-in (and to petrol in EV mode) as a simple sensitivity: nominal prices often drift with general inflation and network or policy changes. The default 2.5%/yr is in the ballpark of long-run Australian CPI and headline electricity retail price movements in national statistics (e.g. ABS Consumer Price Index series for electricity; ACCC and AER publish market-level trends that vary by year, state, and tariff). Use it as a dial—raise or lower it if you expect sharper bill rises or flatter tariffs in your area.
Estimated annual grid payments by scenario
Each curve is $/yr: modeled annual benefit (orange); grey is all household load at retail + supply if you had no solar; teal is net cash to the retailer (household imports + EV grid charging kWh × retail − exports × FiT, then daily supply). Net can go negative when feed-in exceeds imports. EV charging split: “solar charging share” targets kWh from exportable PV; any shortfall is modeled as grid imports. Green vertical = payback from cumulative savings vs system cost. Optional dashed lines = other stacks. GST excluded. No time-of-use; petrol inflation uses the same dial.
Scenario Comparison
| Scenario | Annual Benefit | Years to ROI | 10-yr Net |
|---|---|---|---|
| Solar Only | — | — | — |
| Solar + Battery | — | — | — |
| Solar + EV | — | — | — |
| Solar + Battery + EV | — | — | — |
Your Setup
Auto-calculated from presets. Edit to override.
Your Energy Profile
NSW default. Edit to override.
NSW default. Edit to override.
Fixed amount on your retailer bill (often labelled supply or service charge). Some retailers pair a high daily charge with lower c/kWh import rates—enter both so the chart matches your plan.
Target share of EV energy from PV that would otherwise export; actual kWh is min(target, available export). Any remainder is grid charging at retail.
Live average from ProjectZeroThree. Edit to override.
Methodology & Sources
Daily load vs self-consumption
The self-consumption slider is % of annual load met directly from solar in this model. Direct kWh/year are min(load × slider%, generation). If typical daily generation is below daily load, you cannot cover more load than the array produces: the cap is min(slider %, 100 × daily_generation / daily_load).
Solar generation
Average daily generation per kW of installed capacity, by capital city. Based on annual averages from Energy Matters and PVWatts.
| State | kWh/kW/day | Source |
|---|---|---|
| NSW (Sydney) | 4.3 | Energy Matters |
| VIC (Melbourne) | 3.5 | Energy Matters |
| QLD (Brisbane) | 4.4 | Energy Matters |
| SA (Adelaide) | 4.5 | Energy Matters |
| WA (Perth) | 4.6 | Energy Matters |
| TAS (Hobart) | 3.5 | Energy Matters |
| NT (Darwin) | 5.0 | Energy Matters |
| ACT (Canberra) | 4.4 | Energy Matters |
Electricity rates
Default retail rates in cents per kWh. Sourced from AER Default Market Offer (DMO), ESC Victorian Default Offer (VDO), and state regulators for non-NEM states.
| State | c/kWh | Source |
|---|---|---|
| NSW | 33 | AER DMO 2024-25 |
| VIC | 28 | ESC VDO 2024-25 |
| QLD | 29 | AER DMO 2024-25 |
| SA | 40 | AER DMO 2024-25 |
| WA | 32 | Synergy A1 tariff |
| TAS | 28 | Aurora Energy regulated |
| NT | 27 | Jacana Energy regulated |
| ACT | 30 | ICRC determination |
Feed-in tariffs
Minimum or benchmark feed-in tariffs by state, in cents per kWh.
| State | c/kWh | Source |
|---|---|---|
| NSW | 5 | IPART benchmark |
| VIC | 3.3 | ESC minimum |
| QLD | 5 | QCA benchmark |
| SA | 5 | ESCOSA benchmark |
| WA | 3 | Synergy DEBS |
| TAS | 9 | Aurora Energy |
| NT | 9.3 | Jacana Energy |
| ACT | 6 | Retailer avg |
System costs & rebates
Indicative installed costs after Small-scale Technology Certificates (STCs). Based on Solar Choice price index and installer quotes.
| System | Cost (after STCs) | Source |
|---|---|---|
| 6.6 kW solar | $5,500 | Solar Choice 2024 |
| 10 kW solar | $9,000 | Solar Choice 2024 |
| 13 kW solar | $11,000 | Solar Choice 2024 |
| Powerwall 3 (13.5 kWh) | $12,500 | Installer avg |
| BYD HVM (16.6 kWh) | $10,500 | Installer avg |
| Sungrow SBR (16 kWh) | $10,500 | Installer avg |
Battery calculations
The battery shifts energy that would otherwise be exported (at the feed-in tariff) to self-consumption (at the retail electricity rate). A 90% round-trip efficiency factor is applied to battery capacity.
shifted_kWh = min(battery_kWh × 365 × 0.9, exportable_energy)
battery_savings = shifted_kWh × (electricity_rate - feed_in_tariff)
Exportable energy is reduced by any solar used for EV charging.
EV bonus methodology
For a standalone petrol vs EV comparison (no rooftop model), see Plug or Pump.
Compares annual petrol cost against the cost of charging an EV. Ordering on the annual export pool: EV first (capped), then battery shift, then what remains exports.
petrol_cost = annual_km × L/100km / 100 × petrol_price
ev_energy = annual_km × kWh/100km / 100
requested_from_export = ev_energy × solar_charging_share
ev_from_export = min(requested_from_export, pv_export_after_household)
ev_from_grid = ev_energy − ev_from_export (adds to retailer imports on the payment chart)
From-export charging is costed at the feed-in tariff (opportunity cost); grid charging at retail.
net_saving = petrol_cost − fi_cost(ev_from_export) − retail_cost(ev_from_grid)
What we don't model (yet)
- Inflation / rate changes (headline) — the large ROI number is year-one dollars. The long-run chart applies adjustable tariff inflation to retail & feed-in (and petrol in EV mode).
- Panel degradation (headline) — headline benefit ignores it; the chart applies adjustable loss on solar output each year (~0.5%/yr typical).
- Time-of-use tariffs — single flat rate assumed; TOU can improve battery ROI
- Battery degradation — warranted capacity loss varies by product
- Financing costs — assumed upfront purchase; loans add interest
- Government rebates beyond STCs — state-specific schemes vary
- Virtual power plant (VPP) income — available in some states
- Daily supply charge — chart grid-cash lines include an editable AUD/day estimate (same with and without solar). Anything beyond supply + c/kWh (GST, discounts, demand charges) is not modeled
- Network export limits — no inverter/host limit on export; real systems may clip or cap feed-in
- Orientation & shading — one kWh/kW/day figure per state (roof specifics and shade not applied)
- Load vs solar timing — daily load caps self-consumption on an energy balance only; intraday mismatch (morning peak vs solar noon) is not simulated
- Marginal vs average tariff — retail rate is a single number, not block/step tariffs