Never seen a adsl 2+ modem work on 5v before.
The normal 9/10/12v modems consume around 10watts
Don’t screw with your laptop USB ports. Consider an average nokia/motorola mobile adapter. Most of them are rated 5v. You can pick one up from the streets for 50 bucks. Cut the wire in the middle and solder the adapter plug to it. 5v is not enough power to fry your modem. There are just two wires you need to experiment with.
^^
What & all this UTL will be able to power up ?
take any standard UPS and use a computer PSU to power your gadgets thru UPS !!
of course, you need a casing for the PSU and take care to not touch bare wires …
Please do yourself a favour and get an AC adapter and UPS for the modem.
A 500VA ups should offer enough backup time for your modem. Given that the UPS is only for the modem, you could also check out some of the ‘popular’ local makes.
With a UPS, you get Power Surge Protection and Battery Backup. 
Thank you.
I like to have backup for Airtel 220BX & Asus N13U Wireless, So can have 24 hours downloading & no worry to start the download incase of power cut.
I guess totally it will be around 20v requirement. Do I need 500va UPS for that ?
I have one 500VA UPS for my Wireless ADSL modem at home, gives 2 to 3 hrs backup. A cheap one costs 1.2k, install and forget
I see that archish has given you a clear idea.
Thanks for info.. Which brand do you have ?
I had APC few months back 500 VA, It used to make so much of noise when power goes.. (Loud beep beep sound)… It really disturbs people sleeping at night.
Thats the reason I sold off.
Is it a good idea to have small UPS not even 500 VA ?
By “not even 500 VA”, if you meant lesser capacity, it is obviously going to give you lesser backup time compared to the 500VA model, which in turn will give you close 3 hrs of backup.
Err..don’t they all. If at all, the pitch, type of sound and frequency may vary across different manufacturers.
It has been implemented for a reason, which is to continually warn you that you are running off the battery backup.
^ an addition to above post :
UPS with pc monitoring sw (eg. APC) have mute option to toggle the warning alarm
^ IIRC, they only come with 600VA + models, right? Basic ones don’t feature PC monitoring.
Yes, only the 600VA ones and the ones above it come with the option to toggle the alarm through software. No such functionality for the 500VA models. However, if someone is adventurous, the ups can be opened and the alarm be disabled.
I dont mind alarms but APC 500 VA has too much cray laud alarm.
I have killed alarm(speaker) and have fitted a LED instead(along with 500ohm resistor in series) 
^^
Kool, Warranty gone right ?
Built-in amplifiers, perhaps? 
I have the same model and do not find it annoyingly loud/high pitched.
It’s almost 6years old UPS. Even if it was new, I would have gone ahead with it, as I don’t bother with warranty for UPS( atleast the cheap ones).
Hmmm. so just to back up Airtel modem & wireless need to take 600 VA UPS..
why do we need an UPS for router and modem backup. since UPS generally convert stored DC back to AC again and power adapters for the router and modem converts this AC back to DC, this is not at all a good solution I think. The best would be with my limited knowledge, get a 12Volt bettery and connect a charger adapter with it. And tap out the necessary DC directly to the router and modem. Here we may need two extra circuits other than the battery.
12 Volt charger from 230 volt AC mains. for the battery.
12 volt DC to 10V, 6V converters.
I think the the battery and the charger is easily available in the market.
any body can help in making a DC to DC convertors. A series resistor will do the stepping down?
Anand