powertop fails to create saved_parameters.powertop on systems where the value of "current_now" is negative.

[huburb4n]

A negative value is sometimes used for current while discharging.
In "measurement/sysfs.cpp" at line 83 (v2.15) the rate becomes negative leading to the specified failure.

bool sysfs_power_meter::set_rate_from_current(double voltage)
{
	int current;

	if (!get_sysfs_attr("current_now", &current))
		return false;

	/* current: µA
	 * voltage: V
	 * rate: W */
	rate = (current / 1000000.0) * voltage;
	return true;
}

To fix the issue, use the absolute current value in the rate calculation as below:
rate = (abs(current) / 1000000.0) * voltage;