This guidebook explains every technical detail, assumption, and formula used by the Brunei Solar Savings Calculator. It is intended for users who wish to understand the underlying models (PVWatts® v8), tariff structures, and how each input 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 your estimated DC capacity, queries the PVWatts® API for energy output, and calculates potential savings under Brunei’s tiered Tariff A.
• Engineer Mode: Provide exact latitude, longitude, DC capacity (kW), tilt, azimuth, and other technical parameters. This mode is intended for system designers who already know the 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 using typical meteorological year (TMY) data.

• Hourly Simulation: Computes plane‐of‐array irradiance each hour based on location, tilt, azimuth, and a meteorological dataset.
• Module Thermal Model: Uses INOCT assumptions to translate irradiance to cell temperature and applies a temperature derate factor.
• DC System Size Input: The user’s DC capacity (kWp) is used to calculate hourly DC production.
• Inverter Model: Applies a DC‐to‐AC size ratio (default 1.2), accounting for clipping and inverter efficiency (default 96%) to compute AC production.
• System Losses: A single percentage (default 14%) accounts for soiling, shading, wiring, degradation, and other real-world factors.

2.2 Default Parameters in Engineer Mode

The following defaults are used but can be overridden in Engineer Mode:

DC-to-AC Size Ratio

1.2

Inverter Efficiency

96%

Module Type

Standard (crystalline silicon)

Array Type

Fixed (open rack)

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

Consumption (kWh)	Rate (B$/kWh)
0 – 600	0.01
601 – 2,000	0.08
2,001 – 4,000	0.10
> 4,000	0.12

Monthly Tariff Calculation Algorithm:

...
cost = (600 * 0.01) + (1400 * 0.08) + ((kWh - 2000) * 0.10)
...

This calculation is performed by the backend server for each month's solar generation to determine the estimated savings in B$.

The “System Losses” percentage in PVWatts® accounts for all losses not explicitly handled by the module/inverter model. By default, it is 14%. You can override this in Engineer Mode.

Category	Default Value (%)
Soiling	2%
Shading	3%
Mismatch	2%
Wiring	2%
Connections	0.5%
Light‐Induced Degradation	1.5%
Nameplate Rating	1%
Availability	3%

5.1 Tilt Angle (°)

The angle between the module surface and the horizontal plane (0° = flat).
- Quick Mode default: 15° (practical for Brunei).

5.2 Azimuth Angle (°)

Measured clockwise from True North (0°=N, 90°=E, 180°=S, 270°=W).
- The default in Quick Mode is 180° (south-facing).

When you draw a polygon on the map, the code computes the area (A) in m². We then use a standard assumption to estimate the potential DC system size.

Formula:

DC Capacity (kWp) = Rooftop Area (m²) / 6.5

This assumes a typical crystalline-silicon module efficiency of around 15% plus spacing requirements, resulting in an estimate of 6.5 m² of roof area needed per kWp of capacity.

When you click "Calculate", the following sequence occurs:

1. Data Collection: The user inputs from the selected mode (Quick or Engineer) are collected in the browser.
2. Payload Construction: A JSON payload is built using these inputs and any necessary system defaults (e.g., tilt=15°, losses=14% for Quick Mode).
3. API Request: The payload is sent from the browser to the application's backend server endpoint (`/api/calculate`).
4. PVWatts® Call: The backend server makes a request to the NREL PVWatts® v8 API with the technical parameters to get hourly solar generation data.
5. Savings Calculation: The server uses the returned generation data and the Tariff A model to calculate the estimated monthly savings.
6. API Response: The server sends the final results (annual kWh, monthly savings, etc.) back to the browser to be displayed in the results cards and charts.

When you click “Export Assumptions PDF,” the application uses the jsPDF library to generate a report directly in your browser. This report captures the state of your most recent calculation.

The generated PDF contains the following sections:

• User Inputs: A table listing all inputs for the calculation (e.g., District, Monthly Bill, Capacity, Tilt, etc.).
• Constants & Reasoning: Key constants like the rooftop area-to-capacity factor and the Tariff A tiers.
• PVWatts® Model Description: A summary of the NREL model used for the simulation.
• Tariff A & Rooftop Formulas: The specific algorithms used for calculations.
• Computed Results: A summary table of the key outputs, such as Annual Output (kWh) and Annual Savings (B$).
• References: A list of data sources and technologies used.

The calculator uses several key financial metrics to assess the viability of a solar investment. These are based on the estimated system cost and the projected savings from the Tariff A model.

9.1 System Cost Estimation

The estimated cost of the system is derived from a financial model based on local market data. The application uses two methods:

• Interpolation: For the "Starter (33%)" and "Mid-Range (66%)" packages in Quick Mode, the cost is interpolated from a data table that maps monthly savings targets to system costs.
• Linear Formula: For the "Full Offset (100%)" package and for all systems in Engineer Mode, the cost is calculated using a linear formula: Cost = (B$844.94 × System Size in kWp) + B$2,210.50.

9.2 Payback Period

This is the time required for the cumulative financial savings to equal the initial investment. It provides a simple measure of how quickly the system pays for itself. The formula is:

Payback Period (Years) = Estimated System Cost / Estimated Annual Savings

9.3 Net Profit (20-year)

This metric estimates the total profit generated over a 20-year operational lifetime, after the initial cost has been recovered. Crucially, this calculation includes an annual panel degradation rate of 0.5%. The logic is as follows:

1. The savings for each year are calculated, with each subsequent year's savings being 0.5% less than the previous year's due to panel degradation.
2. These 20 years of "degraded" savings are summed together to get the total lifetime savings.
3. The initial system cost is subtracted from the total lifetime savings to determine the net profit.

9.4 "Is Solar Worth It?" Verdict

The summary verdict shown in Quick Mode provides an at-a-glance assessment based on the payback period of a system that would fully offset your electricity bill (a 100% offset system). The thresholds are:

• Excellent Investment! - Payback period is 10 years or less.
• Good Investment. - Payback period is between 10 and 15 years.
• A Long-Term Consideration. - Payback period is more than 15 years.