DRAFT

A UPS is an Uninterruptible Power Supply.

It is a type of continual power system that provides automated backup electric power to a load when the input power source fails. In the event of a power cut, the UPS's batteries supply energy to your equipment for a while. UPS autonomy varies according to battery capacity and power requirements. 

A UPS is also called an inverter, although there are slight differences between them. A UPS contains built-in batteries whereas for inverters batteries are external. Another difference is the time it takes for them to switch on. A UPS switches ON immediately whereas inverters use some delay to switch ON. 

You need a UPS for sensitive systems such as servers which cannot afford the sudden power loss. Not only will they provide energy during the power outage but also can they shut down your crucial equipment properly.

A UPS has different sub-assemblies. The rectifier which transforms alternating current (AC) into direct current (DC), to charge the batteries. The batteries, to store the energy. The inverter that transforms the DC voltage from the rectifier or battery back to an AC output that powers the critical load. The UPS might have one or more fans. The assembly offers outlets to plug your devices.  

Depending on the UPS model and quality, some UPS have a screen to manage them. At least it is good to choose UPSs that are manageable either through a screen, or a software using a USB cable, or via a web portal, if it has a network integrated card. This will help you check the battery status, the UPS autonomy, and sometimes schedule maintenance tasks, or alerting.

There are three types of UPSs:

• Off Line UPSs are the most basic and economical ones, typically for computers, which have switch-mode power supplies.The “Off line” UPS takes over rapidly but not immediately. There's a short break in supply of a few milliseconds while the inverter switches on and powers the load from the batteries.
• Line Interactive UPSs control and filter the input voltage, through their inverter before sending it to the equipment. Current quality is better, but the disadvantage is that voltage variations are not well regulated. This type of UPS is not recommended for mission-critical servers.
• On Line Double Conversion UPSs constantly convert incoming AC power to DC power and then back to AC power, ensuring a clean and stable output regardless of the quality of the input power. The On Line inverter offers the most stable output voltage and the best immunity to interference. They are the best choice for all mission-critical servers. According to Paul Z, SIL Power systems, Online UPSs are made to handle poor electrical systems. Online means that the inverter is always running off the battery power and at the same time there is a separate battery charger which can accept a wide range of voltages and frequencies to charge the battery. They are more expensive because the inverter needs to run 24x7 and high capacity battery chargers are not cheap. Commercial Online UPS systems are usually very expensive and large, but it is possible to assemble smaller ones from good quality inverters and battery chargers. They have one listed on their website https://power.sil.org/product/ac-ups-300w/. The 15a costs around $400 without a battery. They are working on an article that will be published on their website that describes how to build various sized Online UPS systems using Victron components. There are Victron dealers throughout Africa. Victron makes very good quality equipment in the Netherlands which is very reasonably priced.

Ideally the UPS should be able to shut down your server(s) after a chosen time, before the UPS runs out of power. This will guarantéé you against an improper shutdown and possible data loss or corruption. 

Speaking about Synology NAS, the manufacturer provides you with a dedicated webpage that lists compatible products, including UPSs. So it is recommended to check that page beforehand and choose a compatible product, which has been tested, and can communicate well with your NAS model.

According to Paul Z, SIL Power systems, one of the most common problems people have with UPS systems is caused by the generators producing the correct voltage, but the incorrect frequency. The UPS will refuse to accept the generator input, while all the lights and other equipment works just fine. Many people don't know how to correct that by adjusting the speed of the generator.