Estimator Inputs
Enter your numbers, then submit to calculate costs, savings, and long‑term economics.
Example data table
Sample inputs and typical outputs to help you sanity‑check your entries.
| Scenario | Monthly usage | Rate | Offset | Peak sun hours | Perf. ratio | System size | Panels | Net cost | Year‑1 savings |
|---|---|---|---|---|---|---|---|---|---|
| Home baseline | 900 kWh | $0.20/kWh | 90% | 4.8 | 0.80 | 5.80 kW | 11 × 550W | $7,450 | $1,780 |
| Higher rate | 900 kWh | $0.28/kWh | 90% | 4.8 | 0.80 | 5.80 kW | 11 × 550W | $7,450 | $2,492 |
| Lower sun | 900 kWh | $0.20/kWh | 90% | 3.8 | 0.80 | 7.30 kW | 14 × 550W | $9,100 | $1,780 |
Formula used
1) Annual usage
Annual Usage (kWh) = Monthly kWh × 12
or
Annual Usage (kWh) = (Monthly Bill ÷ Rate) × 12
2) Recommended system size
Target Energy (kWh) = Annual Usage × Offset%
System Size (kW DC) = Target Energy ÷ (Peak Sun Hours × 365 × Performance Ratio)
3) Annual production
Annual Production (kWh) = Actual Size (kW) × Peak Sun Hours × 365 × Performance Ratio
4) First‑year savings
Value per kWh = Self% × Rate + (1 − Self%) × Export Rate
Year‑1 Savings = Annual Production × Value per kWh
5) NPV and IRR
NPV = Σ(Cashflowᵧ ÷ (1 + Discount)ᵧ)
IRR solves NPV = 0 for the discount rate
How to use this calculator
- Pick your usage input type: monthly kWh or monthly bill.
- Enter your electricity rate and offset target percentage.
- Set peak sun hours and performance ratio for your region.
- Fill in panel wattage and area to estimate panel count and roof space.
- Enter cost items, taxes, and any rebates or incentives.
- Choose cash or loan and provide financing terms if needed.
- Adjust degradation, escalation, and discount rate for long‑term results.
- Press Estimate Cost to see results above the form.
FAQs
1) What are peak sun hours?
Peak sun hours approximate daily solar energy as “full‑sun” hours. They depend on location, season, shading, and panel tilt. Use an annual average for planning.
2) What does performance ratio include?
Performance ratio captures real‑world losses: heat, inverter conversion, wiring, dust, mismatch, and downtime. Many residential systems fall between 0.75 and 0.85.
3) Why does panel count change after rounding?
Panels are whole units, so the estimator rounds up to meet your target. The actual DC size becomes slightly larger than the raw recommendation.
4) How is self‑consumption used in savings?
Self‑consumed energy is valued at your retail rate. Exported energy uses the export credit rate. If your plan credits exports at full rate, set export credit equal to the retail rate.
5) What does simple payback mean?
Simple payback is net cost divided by first‑year savings. It ignores escalation, degradation, financing structure, and discounting. Use NPV and IRR for a more complete view.
6) How do NPV and IRR help decisions?
NPV estimates today’s value of future cashflows using your discount rate. IRR is the implied annual return. Higher NPV and higher IRR typically indicate a more attractive investment.
7) Does the estimator include batteries?
Not by default. You can approximate a battery by adding its cost to balance‑of‑system and increasing self‑consumption. For detailed battery modeling, add separate inputs for capacity and cycling.
8) Why might my quote differ from this estimate?
Quotes vary due to roof complexity, electrical upgrades, permitting, equipment brands, financing fees, and local labor rates. Treat results as planning guidance, then validate with multiple installer proposals.