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.
Your yearly bill, year by year
Each line is a single year’s dollars. Orange — what your system saves you. Grey — what you’d pay the retailer with no solar. Teal — what you actually pay after exports (can go negative when you export more than you import). The green vertical marks payback. Dashed lines compare other setups. Excludes GST. No time-of-use. Petrol inflation tracks the same dial as electricity.
Scenario Comparison
| Scenario | Annual Benefit | Years to ROI | 10-yr Net |
|---|---|---|---|
| Solar Only | — | — | — |
| Solar + Battery | — | — | — |
| Solar + EV | — | — | — |
| Solar + Battery + EV | — | — | — |
Your Setup
Excludes solar cost from ROI. The calculator uses your existing solar generation and export pool to value a battery add-on.
Models evening wholesale peaks (Amber / AGL VPP) and network flexible export windows — battery exports some throughput at a higher rate instead of shifting to self-use.
Editable default. Ballpark evening wholesale / VPP average in AU. Check your retailer (Amber, AGL, Origin Loop) for current rates.
% of battery throughput discharged to grid at peak tariff. The rest still shifts exports to household self-use at retail.
Auto-calculated from presets. Edit to override.
Amortised loan (fixed annual payment). Interest adds to the effective system cost, pushing out years to ROI. Scenario table below still reflects cash prices for comparison.
Defaults to your system cost. 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 charges from what solar would otherwise export, then discharges to cover remaining household load (load minus what direct solar already supplied) at retail. A 90% round-trip efficiency factor is applied to capacity.
throughput = min(battery_kWh × 365 × 0.9, exportable_energy)
remaining_load = daily_load × 365 − solar_self_consumed
shifted_kWh = min(throughput × (1 − vpp_share), remaining_load)
battery_savings = shifted_kWh × (retail − feed_in)
If remaining load is smaller than throughput, the unused capacity has no extra value (those kWh simply don’t get stored; the solar still exports at flat FiT). With flexible exports / VPP enabled, that leftover capacity does earn a peak-tariff uplift — see the VPP section.
Exportable energy is reduced by any solar used for EV charging before the battery gets access to it.
Flexible exports / VPP (battery peak discharge)
A flat feed-in tariff hides the fact that evening wholesale prices routinely run 15–30 c/kWh on services like Amber and retailer VPPs (AGL, Origin Loop, Tesla Energy Plan). When this toggle is on, a share of the battery’s annual throughput is modelled as discharging to grid at your editable peak tariff instead of shifting to self-use at retail.
shift_kWh = min(throughput × (1 − share%), remaining_load)
vpp_kWh = throughput − shift_kWh (the intentional share plus any leftover that load couldn’t absorb)
battery_savings = shift_kWh × (retail − feed_in) + vpp_kWh × (peak_feed_in − feed_in)
The second term is the uplift over flat-FiT exports — the flat-FiT earnings on those kWh are already credited in the solar export line, so we only add the peak-vs-flat gap to avoid double counting.
Networks are also introducing flexible (dynamic) export limits that lift the static 5 kW residential cap during grid-friendly windows (see the Ausgrid fact sheet). In this model there is no kW cap on export, so the flexible-export benefit shows up entirely via the peak-tariff uplift on battery throughput.
Defaults are editable — the 20 c/kWh peak tariff and 20% VPP share are AU-wide ballparks, not a forecast.
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)
Financing with a loan
When Finance with loan is on, the headline ROI and chart payback marker use your loan-adjusted total (cash upfront plus total loan repayment over the term).
annual_payment = loan × r / (1 − (1 + r)^−term) where r is the annual interest rate.
total_interest = annual_payment × term − loan
effective_cost = (system_cost − loan) + annual_payment × term
ROI_years_with_loan = effective_cost / annual_benefit
Annual payments and benefits are not discounted (no NPV). Interest is fixed for the term; no redraw, offset, or variable-rate modeling. Scenario Comparison table below keeps cash pricing to isolate equipment trade-offs.
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
- 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