Inputs
Example Data Table
Sample scenarios showing how irradiance assumptions and losses affect yearly energy and value.
| Scenario | Size (kW) | Irradiance (kWh/m²/yr) | POA | Adj PR (%) | Year‑1 Energy (kWh) | Rate | Year‑1 Value |
|---|---|---|---|---|---|---|---|
| Rooftop, low shading | 5.0 | 1,700 | 1.00 | 78.0 | 6,630 | 0.15 | 994.50 |
| Commercial, higher temperature | 50.0 | 1,900 | 0.98 | 74.5 | 69,335 | 0.12 | 8,320.20 |
| Bifacial ground-mount | 100.0 | 2,050 | 1.03 | 76.0 | 160,180 | 0.10 | 16,018.00 |
Formula Used
1) Adjusted irradiance (POA):
Hadj = HLT × (1 + Dev%) × POA × (1 + Bifacial%)
2) Adjusted performance ratio (PR):
PRadj = PRbase × Π (1 − Lossi)
3) Annual energy:
Eyear1 = SizekW × Hadj × PRadj
Ey = Eyear1 × (1 − Deg%)(y−1)
4) Finance:
Value = E × Rate ·
NPV = −NetCost + Σ (NetCashflowt / (1 + Discount)t)
How to Use This Calculator
- Enter the long-term annual irradiance for the project location.
- Use deviation (%) to reflect a specific year or forecasted variability.
- Set the plane-of-array factor to account for tilt and azimuth.
- Fill in realistic losses (shading, temperature, wiring, downtime, and others).
- Add finance inputs to estimate value, NPV, payback, and LCOE.
- Press Calculate. Export your results as CSV or PDF.
Irradiance baseline and variability
Annual plane-of-array irradiance commonly ranges from 1,200 to 2,200 kWh/m². The deviation field models year-to-year weather swings of ±5% to ±15% without changing the baseline dataset. A 1,700 kWh/m² site at +8% becomes 1,836 kWh/m² before orientation effects for preliminary feasibility checks.
Tilt and bifacial adjustment
The plane-of-array factor summarizes tilt and azimuth impacts. Values near 1.00 reflect optimized roofs, while 0.90 can represent east–west layouts or flat mounting. Bifacial gain adds an uplift, often 3% to 12% for bright ground surfaces. The calculator combines these multipliers to produce adjusted sunlight.
Loss stacking and performance ratio
Base performance ratio typically falls between 75% and 88% for grid-connected systems. Individual losses are applied multiplicatively to avoid double-counting. Common planning ranges include shading 0%–6%, soiling 1%–5%, temperature 4%–10%, inverter/clipping 1%–3%, wiring 1%–3%, and availability 0.5%–2%. The resulting adjusted PR reflects net deliverable energy.
Degradation and multi-year yield
Module degradation is frequently assumed at 0.3% to 0.8% per year for modern equipment. The selected operating year uses (1 − Deg)^(year−1). For example, 0.5% degradation reduces year‑10 output to about 95.6% of year‑1. This matters when comparing contracts, warranties, or refinancing timelines.
Financial outputs and decision signals
Energy value equals annual kWh multiplied by your rate. With 5 kW, 1,700 kWh/m², and 78% PR, year‑1 energy is about 6,630 kWh. At 0.15 per kWh that is roughly 995 per year. For long-horizon decisions, NPV discounts each year’s net cashflow and can be paired with a 1%–4% escalation assumption. Payback is the first year cumulative net exceeds net cost. Set annual O&M near 1%–2% of installed cost, and discount rates of 5%–10% for screening. LCOE divides discounted lifetime costs by discounted lifetime energy for clean comparisons.
FAQs
1) Which irradiance value should I enter?
Use the best available long‑term annual irradiance for your site, ideally plane‑of‑array or a reliable GHI dataset converted to POA. If you only have monthly data, sum it to an annual value and keep deviation near zero.
2) Why are losses multiplied instead of added?
Losses often interact. Multiplying each (1 − loss) keeps totals realistic and prevents exceeding 100%. It also matches how many yield models apply independent derates such as soiling, wiring, and availability.
3) How do I choose the plane-of-array factor?
Start with 1.00 for near‑optimal tilt and azimuth. Use 0.90–0.98 for east–west or shallow roofs, and 1.02–1.08 for well‑tilted arrays with favorable orientation. Refine using site simulations when possible.
4) What does adjusted PR represent?
Adjusted PR is your base performance ratio after applying the selected loss stack. It summarizes how efficiently the system turns irradiance into delivered AC energy, excluding the irradiance adjustment itself.
5) How is payback calculated in this tool?
Payback is estimated when cumulative annual net cashflow first exceeds net upfront cost. Net cashflow equals energy value minus annual O&M. The result interpolates within the crossing year for a smoother estimate.
6) What does LCOE mean and how should I use it?
LCOE is the discounted lifetime cost per discounted kWh produced. Compare it against your expected grid price or alternative generation options. Lower LCOE generally signals better long‑term economics, assuming similar risk and reliability.