Other less expensive UPS systems are not intended to serve as replacement power systems for an extended period. In these cases the UPS will go to a temporary battery backup and trigger a warning to system administrators and users, stating that mains power has failed and instructing users to close down their systems in an orderly manner. This kind of UPS is intended to prevent accidental data loss or corruption through power failure, by allowing a controlled close down of a system, rather than ensuring that work can proceed on backup power. The other function of UPS systems is to smooth out 'spikes' in power supplies. While power supplies are meant to supply a constant level of electricity, they can on occasions provide an over-supply of power, called a 'spike'.
Spikes can be dangerous to computer equipment and can cause fuses to blow or components to burn out.
A UPS system will intercept a spike and prevent it from reaching sensitive equipment. Another common method of ensuring reliable power supplies is to use a generator. Various kinds of generators can be used, powered by various kinds of fuel, typically petrol or diesel. Generators can be used constantly, particularly where mains power is unreliable or unavailable, or can be used as backup if mains power fails. Generators can be coupled with UPS systems, so that the UPS can handle a transmission from mains power to generator power.