| Scenario | Size (kW) | Cost | Rebates | Rate | Sun Hours | Self-Use | Payback (yrs) |
|---|---|---|---|---|---|---|---|
| Starter home system | 3.5 | $4,800 | $600 | $0.16 | 5.0 | 65% | 6–8 |
| Mid-size household | 5.0 | $6,500 | $1,000 | $0.16 | 5.0 | 70% | 5–7 |
| High-usage + strong export credit | 8.0 | $9,800 | $1,200 | $0.18 | 5.5 | 55% | 4–6 |
- Net upfront investment: Installed Cost − Incentives
- Annual energy (calculated mode): kWh₁ = Size(kW) × SunHours × 365 × PerformanceRatio
- Degraded production: kWhᵧ = kWh₁ × (1 − d)^(y−1) where d is degradation rate.
- Escalated tariff: Rateᵧ = Rate₁ × (1 + g)^(y−1) where g is tariff escalation.
- Value per kWh: Valueᵧ = Rateᵧ × [SelfUse + Export × Credit] with Export = 1 − SelfUse.
- Gross savings: Savingsᵧ = kWhᵧ × Valueᵧ
- Net cashflow: Netᵧ = Savingsᵧ − O&Mᵧ − Replacementᵧ
- NPV: NPV = −Upfront + Σ(Netᵧ / (1+r)^y) where r is discount rate.
- Simple payback: First year where cumulative net cashflow ≥ upfront.
- LCOE: (Upfront + PV(Costs)) ÷ PV(Energy)
- Enter installed cost and any rebates to estimate net upfront investment.
- Choose production mode: calculate using size and sun hours, or enter annual kWh directly.
- Set your electricity rate, escalation, self-consumption, and export credit assumptions.
- Add operating costs, degradation, and any expected inverter replacement event.
- Pick a discount rate and horizon, then click Calculate to view payback, NPV, and cashflows.
- Use Download CSV or Download PDF to save results for proposals.
Production modeling aligned to site conditions
Use the calculator to translate system design into annual energy. In calculated mode, energy equals size times sun hours times 365 times performance ratio. A 5 kW array with 5.0 sun hours and 0.80 ratio yields about 7,300 kWh in year one. If you already have a PV study, switch to manual kWh to avoid double counting losses from shading or tilt.
Tariffs, self-consumption, and export value
Electricity savings depend on where each kilowatt-hour goes. Self-used energy offsets the full retail rate, while exported energy is valued at the export credit. For example, with 70% self-use and an 80% export credit, each solar kWh is worth 0.70×rate plus 0.30×0.80×rate, or 0.94×rate. Adjust these fields to reflect net metering, time-of-use, or feed-in tariffs.
Cost structure, maintenance, and replacements
Operating costs should be realistic but not inflated. Many residential systems budget 1–2% of installed cost per year for cleaning, inspections, and minor repairs. The tool lets you apply an escalation rate so O&M grows with inflation. Include a midlife inverter replacement if applicable; setting year 12 and a fixed cost captures common warranty windows. This replacement can delay payback even when energy savings look strong.
Return metrics for investment decisions
Simple payback shows when cumulative net cashflow crosses net upfront investment. Discounted payback adds the time value of money, using your chosen discount rate. NPV summarizes discounted profitability; positive NPV indicates value above the discount hurdle. IRR estimates the annualized return that makes NPV zero. LCOE compares lifetime cost per kWh; if LCOE is below your projected retail rate, solar is economically competitive.
Quality checks and sensitivity testing
Before sharing results, run a quick sensitivity sweep. Test tariff escalation at 0%, 3%, and 6% to see how price growth drives savings. Vary self-consumption by ±15 percentage points to reflect battery additions or load shifting. Increase degradation to 0.8% for conservative output, and add higher O&M if your site has dust or difficult access. Consistent outcomes across these cases indicate a robust project overall.
Q1. What is the difference between simple and discounted payback?
Simple payback uses raw yearly net cashflows until they equal the upfront investment. Discounted payback applies your discount rate to each year’s cashflow, so future savings count less and the payback year is usually later.
Q2. How should I choose the performance ratio?
Use 0.75–0.85 for typical rooftop systems. Choose lower values if shading, high temperatures, or dirty environments reduce output. If you have a site report or monitoring data, enter annual kWh directly instead.
Q3. How do I estimate self-consumption percentage?
Start with 50–80% for homes without batteries. Daytime loads like cooling, refrigeration, and business hours increase self-use. If you plan a battery or smart load shifting, increase self-consumption and reduce exported energy accordingly.
Q4. What discount rate should I use?
A practical choice is your long-term borrowing cost or required return, often 5–10% for household projects. Use higher rates for riskier assumptions or limited warranties. Lower rates emphasize long-lived energy savings.
Q5. Why might the IRR show N/A?
IRR can fail when cashflows never turn positive enough or when multiple sign changes create unstable solutions. Check that net cashflows become positive, reduce extreme inputs, or rely on NPV and payback for decision-making.
Q6. How is CO₂ avoided estimated?
The calculator multiplies total solar generation by your grid emission factor in kg CO₂e per kWh, then converts to tonnes. Use a local factor if available; otherwise treat the result as a directional estimate.