def power_calc(flow_m, head, eff):
    """Calculate power.

    Parameters
    ----------
    flow_m : pint.Quantity, float
        Mass flow (kg/s).
    head : pint.Quantity, float
        Head (J/kg).
    eff : pint.Quantity, float
        Efficiency (dimensionless).

    Returns
    -------
    power : pint.Quantity
        Power (watt).
    """
    power = flow_m * head / eff

    return power.to("watt")