I know nothing about them apart from they can smooth out the power and provide power during a power cut. Their ratings mean nothing to me. Its for domestic use with the following attached. I yet to get the power ratings of them all as I'm yet to purchase some of it or it requires a crawl about in the attic, but just to give a rough idea of what I'll need will help:

Virgin Superhub (input rated at 1.5A@12v)
Asus RT-N66U router
TP-link 8 port gigabit switch (cheapy plastic model)
ReadyNAS Duo 2x1TB (input rated at 5A@12V, PSU rated at 1.5A/150VA @ 100-240V)
Hikvision 8 channel NVR with 2x4TB drives powering 8 POE IP cameras
Voltek exterior PIR system (100mA, plus a 20amp contactor..100mW? ).

The purpose of this is to maintain CCTV and internet operation when the power is cut. Also I want the NAS connected at it doesn't auto-power on after a power cut and has to do a resync when I manually turn it back on, which is a pain in the ****.

As for how long it can work for...would 2 hours be a reasonable time to aim for?

...the average power cut would be 30mins, and if it was a break-in maybe an hour with overlap just incase (last break-in it took about 3 hours to get the power restored as they stole the main fuse).

Also, what is the useful life of the batteries? Do they need changing after every 5years or so like house alarm batteries?

 

The VA rating is volts multiplied amps. 1200 VA rating will give 240ac at 5 amps for 1 hour. Work how much current total, times 240 , times 3 hours to give your VA rating UPS. Then add 20% to build in tolerance margin. Batteries will need replacing from time to time. Some of the UPS have sophisticated electronics to manage charge and monitor battery performance. Cost plenty of cash for big current demands

APC seem popular with various models

 

one of the APC 3000 should do it, but shop around there still a grand... there are cheaper alternatives, as you prob wont use the bundled software

also try these

http://www.powerinspired.com/kr-ups-...c-100_107.html

 

yes they do

as said APC - and the software will tell you when the battery needs replacing and it is usually a pretty simple task

 

I worked out the running wattage as about 165watts total (that includes the drives inside the NAS and NVR etc), so call it 200watts. PA with a power factor of 0.9 ( (W=V*A), so VA=W/pf I remember that bit from college) roughly makes the VA=222.2

Now this is where I get confused...UPS are rated in peak output: eg the APC SMT3000 is rated 3000VA peak...yet at full load (3000VA/2700watts) it will run for 5minutes! But at a 250watt load it'll run for about 120minutes. Fine, its up to the job (run time charts here: http://www.apc.com/products/resource...total_watts=50

BUT APC do extended run time battery units....and this is what gets me....I can have a SMX750 which maxes out at 750VA (hence the name) but with additional battery unit (http://www.apc.com/resource/include/...ku=SMX48RMBP2U ), which gives me running time of about 4 hours! That combined cost is cheaper than the retail price for the SMt3000 and has a far better run time for my application.


So I "can" have a lower rated UPS with an additional battery unit.

Another option, which is cheaper still: APC BR1500 (1500VA UPS) http://www.apc.com/resource/include/...e_sku=BR1500GI with a BR24BPG (http://www.apc.com/resource/include/...se_sku=BR24BPG ) battery unit...that gives around 2.5hours..about the same as the SMT3000...but at more than half the retail price of the SMT3000.

Hence my head exploding...and thats with just looking at one brand!

 

Just get the cheapest with the necessary runtime whether it be one unit or with additional battery. Obviously run time is not linear as load increases. Use the graphs to guide you on runtime