I have managed to put together some information, which will hopefully 'level the playing field' re UPS's. The information is up to date & is as a result of changes to the latest IEC/European standards for UPS's. The document does not tell you how to work out the size of UPS. The document is attached at the bottom.

Batteries & Back-up Time.

Contrary to popular belief, the VA rating of the UPS does not strictly control the amount of UPS back-up time. Back-up time (the amount of battery power) is determined by the capabilities of the UPS battery charger, which in many models, is only extended in a larger UPS - thus the belief that bigger back-up times means a biger UPS. Quite a few (but not all) small UPS brands allow for extra modules to be added onto the UPS. One of these modules is usually a bigger battery charger, which means that you can increase the number of connected batteries without blowing up the charger/UPS.

UPS sizing.

Electrical equipment that supplies power is mostly rated in Volt-Amps (VA). Common examples are transformers, generators & UPS units. The reason for this is that the manufacturer has no control over what will be connected to the output of these devices.

Example: A 220 volt single phase transformer is rated at 50kVA. How much current can it supply?

I = VA/E

= 50 000/220

= 227.3 Amps

What is the largest load in Kilowatts it can supply if the Power Factor is unity (1)?

P = E x I x Cos~

= 220 x 227.3 x 1

= 50 kW (rounded off)

What is the largest load in Kilowatts it can supply if the Power Factor is 0.8?

P = E x I x Cos~

= 220 x 227.3 x 0.8

= 40 kW (rounded off)

PLEASE NOTE - the current is the same for both Power Factors.

The VA rating is therefore used to indicate the maximum current & voltage limitations of a supply device.

It is therefore necessary to convert kilowatts to VA in order to be sure. If the Power Factor of a device is unknown, assume that it has a low Power Factor of 0.5. This may result in slight oversizing of the supply equipment.

APC have a good selection utility at this link APC, they are also readily available in thailand, i have used APC for years now and they are probably about the best all round, but not as cheap as some other brands.

Remember tho if you dont want to suffer corrupt data, then a shutdown cable is recommended that shuts the PC/Server down gracefully before the batteries run out of power. Splitters can be purchased for the cables to allow multiple PC/Servers or software can be used, but then the switch needs to be on UPS as well as its sends the shutdown command over the LAN.

Baz

APC have a good selection utility at this link APC, they are also readily available in thailand, i have used APC for years now and they are probably about the best all round, but not as cheap as some other brands.

Remember tho if you dont want to suffer corrupt data, then a shutdown cable is recommended that shuts the PC/Server down gracefully before the batteries run out of power.

Agree 100% with APC, never buy anything else.

The bigger models have an ethernet connection to allow them to shutdown any number of computers (Windoze, Mac, Unix etc) without additional cables etc, just need to install the necessary software. For a smaller unit supplying a single PC you usually end up wth a serial or USB connection.