Model/domain/sap10_calculator/worksheet/utilisation_factor.py

"""SAP 10.2 Table 9a — heating utilisation factor η.

η reduces the contribution of internal + solar gains when they outpace the
dwelling's heat-loss rate. A well-insulated dwelling with large solar gains
in October can't fully use those gains — it's already warm enough.

Formula per Table 9a:

    a  = 1 + τ / 15                   where τ is the dwelling time constant (h)
    γ  = G / L                        gain-to-loss ratio (W / W)
    if γ > 0 and γ ≠ 1:  η = (1 − γ^a) / (1 − γ^(a+1))
    if γ = 1:            η = a / (a + 1)
    if γ ≤ 0:            η = 1

The time constant τ = TMP / (3.6 × HLP) comes from the dwelling's thermal
mass parameter and heat-loss parameter; computed by the orchestrator and
passed in here.

Reference: SAP 10.2 specification (14-03-2025) Table 9a (page 184).
"""

from __future__ import annotations


def utilisation_factor(
    *,
    total_gains_w: float,
    heat_loss_rate_w: float,
    time_constant_h: float,
) -> float:
    """SAP 10.2 Table 9a heating utilisation factor η.

    γ = total_gains_w / heat_loss_rate_w; η ∈ (0, 1]. When the heat-loss
    rate is non-positive (dwelling already balanced or gaining), η = 1
    so the gains are fully credited.
    """
    if heat_loss_rate_w <= 0:
        return 1.0
    gamma = total_gains_w / heat_loss_rate_w
    a = 1.0 + time_constant_h / 15.0
    if gamma == 1.0:
        return a / (a + 1.0)
    return (1.0 - gamma**a) / (1.0 - gamma ** (a + 1.0))