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Turning on electric lights is included in the prohibition of lighting a fire. It is also prohibited to turn on or activate electric appliances and devices, such as telephones, doorbells, microphones, air conditioners, and computers.
Shabbat prohibitions take effect only with the onset of Shabbat, so it is permitted to set a timer before Shabbat to turn on the lights, heat, or air conditioner at the desired time on Shabbat. For example, before Shabbat, one may leave on lights and set a timer that will turn them off when people plan on going to sleep, turn them back on for lunch, turn them off again after lunch, and turn them on again for se’uda shelishit. Similarly, one can set a timer so that an electric heater or air conditioner goes on and off at the desired times.
If on Friday night someone wants to delay the time at which the lights/heat/air conditioner will turn off, one may adjust the timer to do so (assuming it is mechanical). Similarly, if in the morning someone wants the lights/heat/air conditioner to turn on later than planned, one may adjust the timer to do so. In other words, it is permitted to take action to extend the current state of affairs. However, the timer may not be adjusted to make the lights/heat/air conditioner go on or off earlier than scheduled. This is only permitted in a case of great need, and only on condition that at least ten minutes will pass before the change comes into effect, as then it is considered to be indirect (section 5 above).
