Understanding lower voltage’s effects
Cords and outlet strips are rated in current capacity which has to do with how large the conductors are and how hot the device can get safely (in addition to maximum voltage which has mostly to do with the insulation).  They are not really rated in watts, even if they are labelled that way.

If the voltage drops, some appliances will draw less power (resistive loads) and some will draw more (switching power supply based appliances, etc).  The total current should not exceed the maximum rated current for the unit, but what that means in terms of appliances depends on which appliances you are using.

Power = Voltage * Current

Current = Voltage / Resistance (for resistive loads like heaters)

or
Power = Voltage * Voltage / Resistance

so as the Voltage drops, the power drops with the square of the change in voltage.