Please read the following. Note there are 3 sections. A device maybe designed as just one of these things or use 2 or all 3 components in its design.
Is simply a circuit in which a charger is used to maintain voltage on a battery, a transfer switch to go from line voltage to battery and back, and an inverter that takes battery voltage and converts it to line voltage.
Please note: Cheap UPS systems use a modified sine wave output (also called quasi sine wave). It is more similar to a square wave than a sine wave; the level of distortion in the wave is usually proportional to the cost of the system. Some sensitive electronic equipment will have real problems with being feed crappy power. Have mostly seen this with cheap network gear. True sine wave systems are expensive by comparison but provide a perfect output comparable to the electrical utility.
A double conversion system is where there is no transfer switch used. The output is always run off the inverter.
Remember with battery backup there is maintenance. Every 3-5 years you are going to be replacing batteries, which on large UPS systems and be expensive. And not environmentally friendly.
Note: Battery backup is a separate system and doesn’t have anything to do with surge protection. Although most battery backups usually have a surge protection circuit built into them – this however is a separate circuit and nothing to do with the battery backup circuit.
Can be in the form of a battery backup system, as mentioned above, for a cheap solution. A true voltage regulation system however has some type of large autoformer with multi taps. The system automatically switches the taps on the autoformer in relation to the input voltage, to maintain a specific output voltage. Most cheap units make a lot of noise (clicking) as they switch the taps. When the input voltage gets too high or low (beyond what the taps can do) you will also get hum. Also cheap units that only use a few taps, and the voltage is right in between 2 taps, you can get a lot of clicking and hum.
The cheapest form of surge protection is to just throw a few MOV (Metal Oxide Varistor) across the line, neutral, and ground. A MOV will conduct electricity when a certain voltage is reached. As you can imagine there is a huge variation among MOVs. They can be cheap ($0.05) like found in power strips costing $5-$10 which afford next to no protection what so ever. Or they can be very large, expensive units that will handle large sinks of energy quickly (generally found in the more expensive models by manufactures). The problem with MOV based systems is they are sacrificial. Once a MOV conducts, it has wear on it. If it sinks a lot of energy (like from a surge) it can be destroyed. Usually you will find a bank of MOVs in a surge protector. A lot of more expensive MOV based protectors are a MOV hybrid as well.
With MOV based units, it’s a good idea to replace the unit every so often to maintain maximum protection. Do NOT rely on the lights on the unit to tell you when it needs to be replaced, those lights NEVER work. The only time those lights are accurate is when the unit is new and when it has been very obviously hit and destroyed. The unit could be on its last leg and offering very little protection; the diagnostic lights will still show good. If you live in an area that sees frequent surges, you might consider replacing these type units every 5 years or so. If you live in a place that rarely ever sees surges and generally has good power, then replace when you know the home has been hit with a large strike or every 10 years or so; whichever comes first.
Most good protectors will include transformers as well to impede the path of surges and filter noise. However, this can be counterproductive with high current amplifiers as the increased inductance can starve an amplifier for power - we’re talking about high current amps here, ones that usually need their own dedicated electrical circuits. Or at the least, sucking down 10-12 amps at peak. Some higher end protectors have dedicated outlets for such equipment, that bypass the inductors (transformers, coils).
More expensive and better protectors with usually have a smart circuit with relays to disconnect power when the voltage goes above or below a set point (over voltage and brownouts). And will not reconnect power until voltage has been stabilized for a set amount of time (usually 1-3 seconds).
At the top of the surge protection chain you will see designs that get away from MOV based units and have non-sacrificial components. These designs generally will also include zero ground contamination. They are expensive by comparison to MOV designs and are generally large in size.