| Item | Rooftop Example | Ground Example |
|---|---|---|
| System size | 50 kW | 50 kW |
| Yield | 1450 kWh/kW‑yr | 1550 kWh/kW‑yr |
| Total losses | 8% | 6% |
| Installed cost | 850 per kW | 750 per kW |
| Extra site cost | 0 reinforcement | 20,000 land + 6,000 fencing |
| Power density | 180 W/m² | 120 W/m² |
| Area required | ≈ 278 m² | ≈ 417 m² |
| Electricity rate | 0.12 per kWh | |
| Project life | 25 years | |
1) Area required
Area (m²) = System kW × 1000 ÷ Power density (W/m²)
2) Yearly energy
Energyᵧ = System kW × Yield × (1 − Losses) × (1 − Degradation)^(y−1)
3) Yearly net cashflow
Netᵧ = (Energyᵧ × Rateᵧ) − O&M − Replacementsᵧ
Rateᵧ = Rate₁ × (1 + Escalation)^(y−1)
4) NPV, IRR, LCOE
NPV = Σ Cashflowᵧ ÷ (1 + Discount)^y •
IRR = rate where NPV = 0 •
LCOE = PV(Costs) ÷ PV(Energy)
- Enter the system size and project life.
- Fill electricity rate, escalation, and discount rate.
- For each layout, enter yield, losses, and cost details.
- Set power density and available area to confirm feasibility.
- Press Calculate to view results above the form.
- Download the report using CSV or PDF buttons.
Site Constraints and Area
Rooftop arrays monetize otherwise idle roof space and can shorten cabling runs to the service entrance. Typical rooftop power density is 150–220 W/m², yet code setbacks, skylights, HVAC equipment, and fire access lanes reduce usable area. Check structural reserve capacity, wind uplift zones, and waterproofing details before locking a layout, especially for large commercial spans, and always verify roof warranty requirements with the manufacturer.
Energy Yield and Losses
Ground mounts offer alignment freedom, consistent row spacing, and simpler shading control. Where topography is favorable, specific yield commonly lands around 1,300–1,800 kWh per kW-year, while rooftops often deliver 1,200–1,700 after obstructions. Model losses explicitly: shading, soiling, mismatch, and wiring. Apply long-term degradation near 0.3–0.8% per year to avoid overstating lifetime energy.
Upfront Cost Drivers
Installed cost must be compared on a total-project basis. Rooftops may add reinforcement, roof penetrations, crane time, and coordinated outage windows. Ground systems may add land purchase or leasing, grading, fencing, drainage, and trenching to the point of interconnection. A 5–15% difference in cost per kW can be erased by a single civil scope item, so track each cost line.
O&M and Replacement Planning
Operations and reliability differ by access and environment. Rooftops can have limited maintenance routes and higher safety controls, but low vegetation work. Ground sites may budget mowing, pest control, cleaning frequency, and security monitoring. Inverters typically need replacement once in 10–15 years; include the replacement year, logistics, and any downtime assumptions. Factor stormwater, snow drift, and dust exposure into O&M planning.
Interpreting Financial Metrics
Decision-making should convert energy to monetary value and compare finance metrics. Annual savings equal energy times the electricity rate, escalated yearly if tariffs rise. Annual net cashflow equals savings minus O&M and replacement costs. Compute NPV using the discount rate, IRR as the return where NPV equals zero, and LCOE as discounted cost per kWh. Choose the feasible option with best risk-adjusted value.
What does “feasible by area” mean?
It checks whether required array area is less than the usable area you entered, based on power density. If it shows No, the layout likely needs a smaller system, higher density modules, or more available surface.
How should I choose the yield value?
Use a site-specific estimate from production software or historical irradiance data. If unsure, start with 1,200–1,700 kWh/kW-year for rooftops and 1,300–1,800 for ground, then adjust for tilt, azimuth, and shading.
Why include shading and soiling losses separately?
Separating losses helps you reflect real site behavior. Rooftops may have equipment shading; ground sites may see dust or vegetation. Keep combined losses reasonable and validate against measured performance or reputable design assumptions.
What discount rate should I use?
Use your organization’s hurdle rate or weighted average cost of capital. Higher discount rates penalize long-term savings more strongly. For many commercial screens, 6–10% is common, but the right value depends on financing and risk.
Does the calculator include incentives or net metering limits?
No. The model values energy at the entered electricity rate and escalation. If incentives, export credits, or caps apply, convert them into an adjusted effective rate or add them as separate yearly cashflow items offline.
How is CO₂ avoided estimated?
Lifetime energy is multiplied by the grid emissions factor in kg/kWh, then converted to tonnes. Choose a factor that matches your region and utility reporting; results represent avoided emissions assuming solar offsets grid generation.
Why might IRR be blank?
IRR requires cashflows to cross from negative to positive and back in a way that creates a solution. If savings never recover the initial cost, or multiple sign changes occur, IRR may not exist or may be unstable.