This guidebook explains every technical detail, assumption, and formula used by the Brunei Solar Savings Calculator. It is intended for professional users who wish to understand the underlying models (PVWatts® v8), tariff structures, and how each input or constant influences the final energy and financial estimates.

The calculator has two modes:

• Quick Calculator: Draw your rooftop, provide your monthly electric bill, district, and roof orientation. The system auto‐computes estimated DC capacity, then queries the PVWatts® API to obtain hourly energy output, sums up monthly/annual values, and calculates potential savings under Brunei’s tiered Tariff A.
• Engineer Mode: Provide exact latitude, longitude, DC capacity (kW), tilt, azimuth, and system loss percentage. This mode is intended for system designers who already know the physical PV system specs.

2.1 What is PVWatts®?

PVWatts® is an NREL web application (and API) that simulates the hourly performance of a grid‐connected photovoltaic array. It uses typical meteorological year (TMY) data and a collection of physical modules/inverter thermal models to estimate DC‐to‐AC energy output.

• Hourly Simulation: PVWatts® first computes plane‐of‐array irradiance each hour based on latitude, tilt, azimuth, and an irradiance/meteorological dataset (in this case, NSRDB v3 TMY or Himawari v4 for Brunei region).
• Module Thermal Model: Uses INOCT assumptions (45 °C for open‐rack; 50 °C for roof mount) to translate irradiance to cell temperature, then applies a cell‐temperature derate factor (–0.37 %/°C for standard modules).
• DC System Size Input: The user’s DC capacity (kW p) is multiplied by hourly irradiance and derate factors to yield hourly DC (W DC) production.
• Inverter Model: Applies DC‐to‐AC size ratio (default 1.2), clipping any DC output above the inverter’s nameplate power, and then applies inverter efficiency (96 %) to compute actual AC production.
• System Losses: A single percentage (default 14 %) accounts for soiling, shading, mismatch, wiring, connections, LID, nameplate uncertainty, weathering, and availability.

2.2 Default “Advanced” Parameters

DC-to-AC Size Ratio

1.2 (i.e. AC inverter rating = DC/1.2).
Allows inverter clipping during peak irradiance hours.

Inverter Efficiency

96 %.
Applied to DC→AC after clipping logic.

Ground Coverage Ratio (GCR)

0.4 for one-axis tracking.
Represents the fraction of ground area covered by modules when they lie flat. Used only for self‐shading calculations in tracking mode. Not relevant for fixed–roof installations.

Module Type

Standard crystalline Si (19 % nominal efficiency, –0.37 %/°C temperature coefficient).
Other options: Premium (21 %), Thin‐film (18 %).

Array Type

Fixed (open rack) assumed; you may select “Fixed (roof mount)” if desired.
For one‐axis or two‐axis tracking, GCR and backtracking options apply.

We assume Brunei’s “Residential Tier A” block‐rate schedule:

Monthly Tariff Calculation Algorithm:

  function calculate_tariff_A_cost(kWh):
      if kWh ≤ 600:
        cost = kWh × 0.01
      else if kWh ≤ 2000:
        cost = (600 × 0.01) + ((kWh – 600) × 0.08)
      else if kWh ≤ 4000:
        cost = (600 × 0.01) + (1400 × 0.08) + ((kWh – 2000) × 0.10)
      else:
        cost = (600 × 0.01)
             + (1400 × 0.08)
             + (2000 × 0.10)
             + ((kWh – 4000) × 0.12)
      return cost
                  

In code, we use this function hourly for each month’s projected kWh to compute estimated B$ savings. The sum of all months equals Estimated Annual Savings.

The “System Losses” percentage in PVWatts® accounts for all losses not explicitly handled by the module/inverter model. By default, it is 14 %, composed of:

These multiplicative losses combine as:
100% × [1 – (1–0.02) × (1–0.03) × (1–0.02) × (1–0.02) × (1–0.005) × (1–0.015) × (1–0.01) × (1–0.03)] ≈ 14 %

5.1 Tilt Angle (°)

The tilt angle (θ) is the angle between the module surface and horizontal plane (0° = flat, 90° = vertical). - Quick Mode default: 15° (practical for Brunei near‐equatorial installations).
- Engineer Mode: User enters any value (e.g. 20° for a pitched roof).

Rule of thumb: tilt ≈ latitude to maximize year-round energy.

5.2 Azimuth Angle (°)

The azimuth angle (ϕ) is measured clockwise from True North, so 0° = north, 90° = east, 180° = south, 270° = west. - Default (northern hemisphere): 180° (south-facing). - For mornings vs. afternoons: slightly adjust ±15°–30° from due south as needed.

Conversion table for cardinal directions:
North: 0° East: 90° South: 180° West: 270°

When you draw a polygon around your rooftop on the satellite map, the code computes the area (A) in m². We then assume a standard crystalline‐silicon module efficiency of 15 % as follows:

Formula:
DC Capacity (kW ₚ) = ( Rooftop Area (m²) ) ÷ 6.5 ( m² per kWₚ )

Why 6.5 m² per kWₚ?

• Typical crystalline‐silicon modules are ~15 % efficient, which means 1 kWₚ requires ~6.67 m² (because 1 kWₚ ÷ 1 kW/m² ÷ 0.15 = 6.67 m²). We round to 6.5 m²/kWₚ as a convenient, conservative assumption that includes some spacing for wiring, racking, and – if roof‐mounted – limited airflow beneath modules.
• Example: For a 479 m² rooftop, DC Capacity ≈ 479 m² / 6.5 m²/kWₚ ≈ 73.7 kWₚ. (We round to two decimals, e.g. 73.70 kWₚ.)

If you wish to override this assumption (e.g. you know your module efficiency or panel dimensions), simply switch to Engineer Mode and enter your own DC capacity.

In JavaScript:

  // 1) User draws rooftop → compute DC capacity:
  const capacityUser = parseFloat(document.getElementById("simple-capacity").value) || 5;
  // 2) User selects District → fallback lat/lon:
  const coords = districtCoords[selectedDistrict]; 
  // 3) User enters monthly bill and roof orientation:
  const bill = parseFloat(document.getElementById("simple-bill").value);
  const roofDir = parseFloat(document.getElementById("simple-roof").value);
  // 4) Default tilt/losses:
  const tilt = 15;
  const losses = 14;
  // 5) Build the PVWatts® API payload:
  const payload = {
    lat: coords.lat, 
    lon: coords.lon,
    system_capacity: capacityUser,
    module_type: 0,      // Standard crystalline
    array_type: 1,       // Fixed (open rack)
    tilt: tilt,
    azimuth: roofDir,
    losses: losses,
    inv_eff: 96,
    dc_ac_ratio: 1.2,
    gcr: 0.4
  };
  // 6) Send to Flask:
  fetch("/api/calculate", {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify(payload),
  })
  .then(res => res.json())
  .then(data => {
    // data.ac_monthly, data.ac_annual, data.capacity_factor, etc.
    // data.monthly_savings = data.ac_monthly.map(kwh => calculate_tariff_A_cost(kwh));
    // …
  });
                  

The Flask endpoint then returns all relevant fields:

• annual_kWh, capacity_factor
• estimated_monthly_usage, total_savings
• coverage_percent, suggested_capacity
• monthly_kWh, monthly_savings
• All static “Assumed PV System Specifications” (latitude, longitude, DC size, module type, array type, etc.)

When the user clicks “Export PDF,” a client‐side or server‐side routine collects:

1. All user inputs (latitude, longitude, DC capacity, tilt, azimuth, losses, etc.).
2. All constants & formulas (tariff tiers, losses breakdown, rooftop‐area → DC sizing factor, etc.).
3. The reasoning behind each constant (e.g., “Why 6.5 m²/kW p?”, “Why assume 96 % inverter efficiency?”, “Why assign 14 % to system losses?”).
4. All PVWatts® metadata (version 8.4.1, weather data source, NSRDB v3 TMY info, etc.).
5. A summary of the computed results (annual_kWh, capacity_factor, coverage, etc.).

These items are formatted into a professional PDF using a library such as pdfkit (Python/Flask) or jsPDF (client‐side). The PDF layout uses a clean, serif heading font (e.g. Times New Roman or Georgia) for titles and a legible sans‐serif (e.g. Arial or Open Sans) for body text. The color palette is kept grayscale (dark headings on white background) for optimal print/export quality. Margins are set to 1″ on all sides, and table-of‐contents and page numbers are included.

Example PDF Layout:
• Cover page: “Brunei Solar Savings – Assumptions & Formulas”
• Section 1: User Inputs (with a small table)
• Section 2: Constants & Reasoning (detailed bulleted list + equations)
• Section 3: PVWatts® Model Description (summary from Section 2 above)
• Section 4: Tariff A Calculation Logic (copied from Section 3 above)
• Section 5: Rooftop Estimation Formula (copied from Section 6 above)
• Section 6: Computed Results (annual vs monthly vs coverage)
• Section 7: References (NREL PVWatts® docs, Brunei tariff official document)