Separate panel capacity from electrical energy

Array capacity in kilowatts describes rated power under test conditions. Energy in kilowatt-hours measures production or use over time. The calculator multiplies array kW by a user-supplied daily yield per kW and 365 days, then removes the curtailment or unavailability percentage.

The default 6.6 kW array at 4 kWh per kW per day produces 9,636 kWh before the 2% curtailment input and about 9,443 kWh after it. If 40% is self-consumed, 3,777 kWh avoids grid purchases and the remainder is exported, subject to the annual-use cap. The example values self-use at 35 cents per kWh and exports at 7 cents per kWh.

Site yield should already reflect location, roof orientation, tilt, shading and system performance assumptions. Do not use the best sunny-day output as a year-round average. Government guidance points consumers to specialist tools such as SunSPOT, which can use roof and electricity data to produce a more detailed estimate.

Self-consumption usually carries the higher bill value

Every solar kilowatt-hour used behind the meter can avoid buying one kilowatt-hour from the retailer, provided the household would otherwise have imported it at the entered rate. Excess generation exported to the grid receives the feed-in tariff where the retail plan provides one. Feed-in rates are often lower than import rates, so timing loads into solar hours can increase value.

The self-consumption percentage is not the percentage of household demand covered by solar. It is the portion of solar generation used on site. The coverage output separately divides self-used solar by annual electricity use. A household can self-consume most of a small system while covering only a modest share of annual demand, or export much of a large system while still offsetting the same daytime load.

The model caps self-consumed kWh at total annual usage. That prevents a mathematically impossible result, but annual totals still hide timing. Evening demand cannot be directly supplied by midday solar without storage, and seasonal generation may not match seasonal heating or cooling. Interval meter data gives a stronger estimate than a single annual bill.

Use tariff values from the actual electricity plan

A flat tariff can be entered directly. Time-of-use plans require a weighted avoided rate based on when solar is self-consumed, not the average price paid for all imports. Demand tariffs and controlled-load rates need separate treatment. The page accepts one import value and one feed-in value, so document how each was derived.

Retail tariffs, discounts and feed-in offers can change. Some plans pair a high feed-in tariff with higher usage or supply charges, export caps or eligibility conditions. Compare the whole bill rather than selecting a retailer on feed-in rate alone. Run lower feed-in and import-price scenarios to see how the payback depends on current pricing.

Distribution networks can impose static or dynamic export limits. Generation above the limit may be curtailed, used on site or stored depending on system controls. Use the curtailment field for an estimate supplied by the designer or network-informed modelling. It is not a substitute for connection approval.

Simple payback is a screening measure

The first-year payback indicator divides installed out-of-pocket cost by first-year net benefit. It does not discount future cash flow, borrowings, opportunity cost, tariff change, component replacement or tax. It is useful for detecting which assumptions drive a quote, not for ranking every investment.

The analysis-term benefit improves on a single year by reducing generation with the entered annual degradation rate and recalculating self-use, exports and benefit each year. Tariffs and annual service costs remain constant in nominal dollars. The final “net position” subtracts installed cost but does not include residual value.

Panel performance generally changes gradually, while inverter or system failures can create discrete costs and downtime. Obtain product, performance and workmanship warranty details, responsible entities and claim procedures. A long warranty has value only if its terms apply and the responsible business can honour it.

Run a conservative case with lower yield, lower self-consumption, lower feed-in tariff, higher curtailment and an inverter allowance. Compare it with the seller’s result. Ask for the quote’s generation profile and tariff assumptions rather than accepting a single savings number.

Treat STCs as part of the quote, not free cash twice

Eligible small-scale solar systems can create small-scale technology certificates under the Australian Government scheme. Entitlement depends on system type and capacity, location, installation year and other rules. The Clean Energy Regulator provides the authoritative calculator and eligibility guidance.

Solar retailers commonly offer an upfront discount in exchange for assignment of STCs. If the installed-cost input is the customer’s quote after that assignment, do not subtract a separate certificate value. STC market value and retailer treatment can differ, and eligibility requires approved components and compliant installation.

The solar PV deeming period decreases as the scheme approaches 2030. A certificate estimate for an earlier installation year should not be copied into a 2026 quote. Use the Regulator’s current calculator and check exactly who creates and sells the certificates.

A financial model cannot design a safe compliant system

Roof condition, orientation, shading, wind loading, fire access, waterproofing, electrical protection, inverter location, cable routes, switchboard capacity and network connection need competent assessment. System size can be limited by available roof, inverter specifications and distribution-network rules. Batteries add chemistry, usable capacity, power, operating mode, losses and backup design not represented here.

Use an appropriately accredited designer and installer, approved products and applicable Australian standards. Obtain multiple itemised quotes with panel and inverter models, layouts, monitoring, warranties, estimated annual generation, exclusions and connection work. Confirm approvals and metering responsibilities.

After installation, keep contracts, certificates, manuals, shutdown instructions, network approval and warranty details. Monitor expected production and address faults through qualified providers. Do not access rooftops or open electrical equipment to investigate an underperforming system.

A disciplined quote-comparison workflow

Start with at least a year of bills and, where available, interval data. Model current loads and foreseeable changes such as electric vehicles, heat pumps, induction cooking or reduced occupancy. Obtain an independent size estimate before inviting sales quotes.

Enter every quote using the same site yield, tariff and analysis term, then replace those assumptions with the vendor’s documented values for a second comparison. A cheaper system with unrealistic generation is not necessarily better; a larger system may create more exports but less value per kWh.

Finally, compare total installed cost after the same rebate treatment, product quality, design, expected self-consumption, export constraint, warranties and service. Save the calculation beside the quote so later performance can be compared with the promise.