Description

This PR fixes a bug in the stochastic-optimization.ipynb example notebook where the parameters to the annuity() function were swapped, causing incorrect calculation of annualized capital costs.

The Issue

The annuity() function signature is:

def annuity(r: float, n: int) -> float:
    """
    Parameters
    ----------
    r : float
        Discount rate (as a decimal, e.g. 0.05 for 5%).
    n : int
        Lifetime of loan or asset (in years).
    """

However, the notebook was calling it with reversed parameters:

# INCORRECT (before this fix)
cfg["fixed_cost"] = (annuity(LIFE, DR) + FOM / 100) * cfg["inv"]

The Fix

Changed the parameter order to match the function signature:

# CORRECT (after this fix)
cfg["fixed_cost"] = (annuity(DR, LIFE) + FOM / 100) * cfg["inv"]

Impact

With the example values from the notebook (DR=0.03, LIFE=25):

• Incorrect calculation: annuity(25, 0.03) = 268.48 (vastly overestimated!)
• Correct calculation: annuity(0.03, 25) = 0.0574 (correct value)

The bug was causing a massive overestimation of annualized capital costs by over 300,000%.

Verification

All other notebooks in the repository correctly use annuity(discount_rate, lifetime), including:

• capacity-expansion-planning-single-node.ipynb
• myopic-pathway.ipynb
• 3-node-cem.ipynb
• example-3.ipynb

The test file test/conftest.py also uses the correct parameter order.

Fixes #issue_number

Fixes #1397

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