- Enter your roof area and estimate the usable percentage after setbacks and obstructions.
- Confirm panel size, panel power, and adjust tilt, shading, and performance ratio.
- Set sun hours and your import/export rates, then choose a realistic self‑consumption percentage.
- Fill in system costs, rebates, and optional financing terms if you plan to borrow.
- Press Calculate. Review the summary and projection, then download CSV or PDF.
| Scenario | Roof (m²) | Usable (%) | Panel (W) | Sun hrs/day | Import rate | Self‑use (%) | Est. size (kW) | Est. Y1 (kWh) |
|---|---|---|---|---|---|---|---|---|
| Small home | 50 | 65 | 410 | 4.2 | $0.18 | 55 | ~7.4 | ~9,900 |
| Medium home | 80 | 70 | 410 | 4.5 | $0.18 | 60 | ~12.3 | ~16,800 |
| Small business | 200 | 75 | 550 | 5.0 | $0.22 | 80 | ~45.6 | ~63,000 |
Roof geometry and usable area assumptions
Start with the roof’s total accurately measured area, then apply a usable percentage that excludes safety setbacks, skylights, vents, and maintenance paths. For example, 80 m² at 70% usable yields 56 m² available for modules. This value directly limits capacity and prevents overly optimistic system sizes when the roof is complex or heavily obstructed.
Panel packing and capacity estimation
Capacity is derived by dividing usable area by panel footprint, then rounding down to whole panels. With 1.8 m² panels, 56 m² supports floor(56/1.8)=31 panels. At 410 W each, the estimated DC capacity is 31×410/1000 = 12.71 kW. Using real panel dimensions keeps results consistent across brands and wattages.
Energy yield drivers and loss factors
Year‑1 production uses peak sun hours and three multipliers: tilt/azimuth factor, shading factor, and performance ratio. With 4.5 sun‑hours/day, tilt 100%, shading 95%, and performance ratio 85%, production is 12.71×4.5×365×1.00×0.95×0.85 ≈ 16,858 kWh. Annual degradation, such as 0.5%, reduces output gradually for long‑term planning.
Financial valuation and cashflow modeling
Value is split between self‑consumed energy and exported energy. If self‑consumption is 60%, self‑used energy is about 10,115 kWh and exports are about 6,743 kWh. At an import rate of 0.18 per kWh and export rate of 0.08 per kWh, the first‑year benefit is roughly 1,821 + 539 = 2,360. Escalation inputs then grow tariffs annually for a realistic forecast. Financing can be enabled to model a down payment and amortized loan over time.
Interpreting NPV, IRR, and payback outputs
Upfront cost is estimated from a per‑kW rate plus fixed costs; for 12.71 kW at 1,100 per kW plus 500 fixed, gross cost is about 14,481 before incentives. The model subtracts maintenance (for example, 1% of net cost) and scheduled replacements, such as an inverter change in year 12 at 1,200, then discounts net cashflows using your discount rate to compute NPV. IRR is the rate where NPV approaches zero, while simple payback marks the first year cumulative cashflow turns positive.
FAQs
How do I estimate usable roof percentage?
Measure obstructions and required clearances. Subtract areas near edges, skylights, vents, and access walkways. Typical usable ranges are 60–85%, depending on roof complexity and local fire or safety setback rules.
What should I enter for peak sun hours?
Use a location average from a reliable solar resource, then choose a conservative value. Peak sun hours represent equivalent full‑sun hours per day, not daylight length. If unsure, test a low and high scenario.
What is performance ratio and a typical value?
Performance ratio captures system losses from temperature, wiring, inverter conversion, soiling, and downtime. Many residential systems fall near 75–90%. If you use quality components and regular cleaning, values toward the upper end are reasonable.
How does self‑consumption affect savings?
Self‑consumed energy offsets your import rate, which is usually more valuable than export credits. Increasing self‑consumption—through daytime loads, timers, or batteries—raises savings and typically improves payback, NPV, and ROI.
How are NPV and IRR calculated in this tool?
The model builds yearly net cashflows: benefit minus maintenance, loan payments, and replacements. NPV discounts those cashflows using your discount rate. IRR is the discount rate that makes NPV approximately zero, when a solution exists.
Does the calculator support net metering?
Yes. Enter the appropriate export rate or credit value for your program. If export credits match import rates, set them equal. If credits vary by time, approximate with an average or run multiple scenarios for comparison.