# System Costs

## Overview

Calculates the capital (CAPEX) and operational (OPEX) costs for building energy supply systems under a specific what-if scenario. Uses the supply configuration and peak loads from the final-energy results to size equipment and calculate costs.

## What It Calculates

**Capital Costs (CAPEX)**:
- Equipment purchase based on peak capacity
- Annualised using equipment lifetime and interest rate

**Operational Costs (OPEX)**:
- Fixed O&M as percentage of investment cost
- Variable energy costs per carrier

**Total Annualised Cost (TAC)**:
- TAC = Annualised CAPEX + Fixed OPEX + Variable OPEX

Costs are calculated for building-scale systems, district plants (including pumping and piping), and solar installations.

## Prerequisites

- **Final Energy** completed for the what-if scenario
- Cost database with component parameters

## Key Parameters

| Parameter | Description |
|-----------|-------------|
| **What-if name** | Which final-energy scenario to calculate costs for |

## How to Use

1. **Run Final Energy** first

2. **Run System Costs**:
   - Navigate to **Life Cycle Analysis**
   - Select **System Costs**
   - Select the what-if scenario
   - Click **Run**

3. **Processing time**: Under 1 minute for typical districts

## Output Files

All outputs under `{scenario}/outputs/data/analysis/{what-if-name}/costs/`:

| File | Description |
|------|-------------|
| `costs_buildings.csv` | Per-entity cost summary (one row per building/plant) |
| `costs_components.csv` | Per-service component-level cost breakdown |

## Understanding Results

- **DISTRICT-scale buildings** show zero costs at building level; their costs appear on the plant row
- **Plant rows** include conversion equipment, pumping, and piping costs
- **Multi-component assemblies** (e.g. chiller + cooling tower) produce multiple rows in `costs_components.csv` for the same service
- **Solar costs** (PV/PVT/SC) are included if solar technologies are configured

## Troubleshooting

| Issue | Solution |
|-------|----------|
| Costs seem too low/high | Check cost database is appropriate for your region; verify currency and units |
| Missing cost data | Update cost database with local costs; check component codes exist |
| Zero CAPEX for district buildings | Expected - DISTRICT-scale buildings have costs at the plant level |
| Plant piping costs missing | Ensure thermal-network Part 2 has been run |

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## Plot - Cost Sankey

### Overview
Creates a Sankey diagram showing cost flows from individual cost components through services to total building/district costs.

### What It Shows
- Left side: Individual cost components (equipment, energy carriers)
- Middle: Cost categories (CAPEX, OPEX) by service
- Right side: Total cost per building or district
- Flow width proportional to cost magnitude

### Key Parameters

| Parameter | Description | Options |
|-----------|-------------|---------|
| **What-if name** | Which scenario to plot | Multi-select |
| **CAPEX view** | How to show capital costs | Annualised, Total |
| **Cost categories** | Which categories to include | CAPEX, OPEX, or both |
| **Y unit** | Currency unit | USD, kUSD, MUSD |
| **Normalise by** | Normalisation | None, Gross floor area |

### Examples

Cost sankey showing flows from services through equipment to cost categories:

![Cost Sankey](images/06-life-cycle-analysis/06-3-costs-sankey.png)

Cost breakdown comparing building-scale and district-scale costs per m2:

![Cost Breakdown](images/06-life-cycle-analysis/06-3-costs-breakdown.png)

### CAPEX View Options

- **Annualised CAPEX**: Shows annual equivalent cost (spread over equipment lifetime). Title: "System Costs (Annualised CAPEX)".
- **Total CAPEX**: Shows full upfront investment cost. Title: "System Costs (Total CAPEX)".

### Chart Interpretation

- **Wide flows** = large cost items (focus here for cost reduction)
- **CAPEX vs OPEX balance** shows if costs are investment-heavy or operation-heavy
- **Service breakdown** shows which services drive costs
- **Energy carrier costs** in OPEX show fuel cost sensitivity

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## Related Features
- **[Final Energy](06-1-final-energy.md)** - Prerequisite (provides supply configuration and peak loads)
- **[Emissions](06-2-emissions.md)** - Environmental analysis of the same scenario
- **[Heat Rejection](06-4-heat-rejection.md)** - Environmental heat impact

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[<- Back: Emissions](06-2-emissions.md) | [Back to Index](index.md) | [Next: Heat Rejection ->](06-4-heat-rejection.md)