Has anyone ever used a computer battery backup for lights?

They charge with the grid and then you can easily run a light or two (on/off not constant) for at least a day or two. They are very affordable compared to other solutions, and really plug-and-play.

I wonder, because I have 3 of them in use for my computer/network system and I think for the $ they would could be easily moved if power goes out to continue seeing in the house at night.

One of my systems says it will power my monitor for 11 minutes and I think it's 220 watts (BIG 30" Dell LCD Monitor (not tv)). So that would be a very long time for lights on/off

If I test this out I`ll do a write-up with pictures too. My one backup has a count-down on it for consumption vs.storage too.

Probably not the ideal solution but it could work. I seem to remember one UPS that refused to power-up if it did not initially have a live AC input. I think that was because it was a rather small UPS intended to keep the comptuter alive only long enough to quickly save WIP and "safely" shut down. You wouldn't want to turn on a computer that was already off because it would only last long enough to boot. Long story short, you might want to test yours.

I have an old UPS with worn out gelcells I'm thinking of replacing with some large deep-cycle batteries. It could be used to power my furnace during short power outages.

Mine are all over 1200 (whatever they measure them in) and I've run my dual monitors, and two computers off them for a few minutes after power went out. My newest is digital and starts at like 300 minutes then when the power fully drains and it draws 100% from it it goes down to 15 or 11 depending what monitor I have. Either way, I think you are right "not the best" but it could work with some string lights, or LED christmas tree lights

You have to watch for the low end ones, they are not well designed thermally and past the 15-30 minutes they are designed to power a small pc they could overheat the inverter. But the well known brands such as APC or tripplite will work fine.

Couple things to think about. Those consumer UPS's are the switching or offline type. This means they send power straight through and until the voltage is high or low enough to pass the thresholds they then switch over to battery. Depending on how good the filters and such are this still allows noise, spikes, surges, etc through.
Now there are online type, medical, data center, etc which have a bigger battery charger and always run the inverter off the battery and charge the battery from the input so there is no direct path and the battery acts as a big filter. You could buy one of these or look at ham radio folk who run a big 12v supply and power their radios from the battery. Look at the thread last week dealing with 12v lighting, this is how I'm doing my emergency lighting and gear, all 12v dc off a battery which is always charged. This eliminated the losses in the inverter as well as issues with the aproximated sine wave and noise and such due to offline ups's. My next step is to use a solar charge controller and solar panel to charge the battery and use the charge controllers low voltage disconnect to switch back to the line powered supply as needed.

With LED lighting it would be better to go that direction then to go with UPS. UPS supplies are not made for constant duty. I would think they may not handle it well.

UPS units can certainly be used to power lighting or other small loads in case of powercuts, but as others point out they may not be the best solution.

Some UPS units do indeed require a grid supply to start, these could be used for backup lighting, BUT COULD NOT be shut down (to conserve battery power) and then re-started.
APC units CAN be shut off and then re-started.

If a UPS is used at one qaurter of it rated load, the run time will be more than qaudrupled, since the battery is more efficient at the reduced load.

If however the load is very low, say 2% of the rating, then you WONT get 50 times the run time. Thats because the internal circuitry of the UPS consumes some current, no matter how low the load.
The run time is unlkely to exceed 12 or 24 hours even with no load att all.

With basic electrical knowledge one can fit an external battery of much greater capacity in order to extend the run time, if you do this, remember that the invertor was probably only designed to run at full power for a few minutes.
To avoid overheating, I would limit the load to 50% of the rating if useing an external battery.
Also remember that there may be NO ISOLATION between the battery connections and the grid supply. There is therefore the potential for a fatal line voltage shock from the battery terminals. These must be regarded as line voltage connections and protected against being touched.

I recently installed a 2,500 VA UPS at my mothers home, this is fitted with a large external battery and powers essiential lighting, fridge, freezer, TV, cellphone charger, and base unit for cordless phone.
With everthing turned on at once, the run time is 6 hours, in more typical use it is about 24 hours.

fwiw I've had good luck with this small generator with a built in 12v inverter, it powered lights, radio etc in the evening a couple years back when we were without power for 9 days. Add a $12 car adapter and you can recharge it from your car, or take it to a friend's house with power and plug it in. When the lights go out, your ability to deal with an overheated UPS could be compromised, I recommend using something safer by design.

http://beprepared.com/product.asp_Q_pn_E_CM%20P400_A_name_E_ALL-START%AE+Emergency+%27One%27+Power+Generator

That "power generator" is not a generator, its simply a battery in a box, no different than jumo start packs.

Not quite. UPS batteries are 12v, 24v, 48v DC, etc they are not the 120v AC line voltage so if there was no isolation between line voltage and the battery the battery would explode due to the too high of voltage. You need to be careful wround ups's due to multiple reasons, one is the battery charger may have chargec capactors that can zap you, second is the battery itsself can source a large amount of current (ever welded with a battery or seen a screwdriver vaporize due to being dropped across a big battery terminals) and the fact that even when unplugged there is still power at the output since thats the primary function of a UPS.

Not to pick nits, but he doesn't mean that the battery is connected across the 120VAC line, but rather that one of the terminals might possibly be referenced to the AC line.

Had it happen just yesterday - we were at a customer troubleshooting our 90VDC motor equipped product running on a third-party motor controller. I was measuring the encoder feedback signals with the customer's brand new four channel o-scope. I was hesitant to connect the probe ground lead to the negative side of the 5V encoder supply. They assured me that the encoder supply was isolated from the AC line or at least at referenced to ground. I threw the switch and ZAP-POW and smoke wafted off the board. Luckily it burnt the "ground" trace off the motor control PCB rather than blowing the scope. Turns out the encoder supply was referenced to the +380VDC bus on the board which was referenced to the AC line which was referenced to the same ground as the scope.