They have a max wattage specified to them, such as 3500 W.

Control

Surge protection and filtering

Surge suppression is usually provided by one or more metal-oxide varistors (MOVs), which are inexpensive two-terminal semiconductors. They act as very high speed switches, disconnecting at the designed voltage. The most commonly used are built to trigger at a voltage somewhat above the local mains supply. In the US, this is (nominally) 115 VAC. It should be borne in mind that this voltage is RMS, not peak, and also that it is only a nominal value.

In most of the developed world, mains electrical circuits are (supposed to be) grounded (earthed), so there will be a live wire, a neutral wire, and a ground wire. Power strips often come with only one MOV mounted between the live and neutral wires. More complete (and desirable) power strips will have three MOVs, mounted between each possible pair of wires. Since MOVs degrade somewhat each time they are triggered, power strips using them have a limited, and unpredictable, protective life.

More elaborate power strips may use inductor-capacitor networks to achieve a similar effect of protecting equipment from high voltage spikes on the mains circuit. These more expensive arrangements are less prone to silent degradation than MOVs.

Overload protection is different from surge protection. Some boards only have overload protection, which does not protect from electricity spikes (surges); it only means that the device will trip itself when too many devices are plugged into it. The standard rating for overload protected powerboards is 2400W in Australia, and exceeding that power will make the board trip. An overload protected board cannot cause damage to trip distribution boards that are plugged into them, that damage can only be done by surge protected board by the way the MVO's inside them are manufactured. Both boards have a reset switch. Overload protection boards generally do not have a power light.

Green energy-saving features and standby power

Some power strips have energy-saving features, which switch off the strip if appliances go into standby mode. Using a sensor circuit they detect if the level of current flowing through the socket is in standby mode (less than 30 watts), they will turn off that socket. This reduces the consumption of standby power used by computer peripherals and other equipment when not in use, saving money and energy. Some more sophisticated power strips have a master and slave socket arrangement and when the "master" socket detects standby mode in the attached appliance's current it turns off the whole strip.