What kind of battery backup would I need to run a 100 watt light bulb for 8 hours?

What kind of battery backup (volts, wattage, etc) would I need to run a single 100 watt lightbulb for eight hours?

I'm assuming you are talking about a UPS which has an inverter to convert the 12 volts DC from the battery to 120 VAC.

PS, use a CFL, and a 30 watt bulb will give you the same amount of light.

UPSs come in many different sizes and storage levels. I don't think they come as small as 100 watts. But that's good, as they all have limited storage capability.

For example, here is a 330 watt unit

http://www.amazon.com/APC-BE550R-Back-UPS-8-Outlet-Battery/dp/tech-data/B000KBG51W/ref=de_a_smtd

that states "Up To 44 Min Battery Backup Time" but doesn't state at what power that 44 minute number is measured at.

here is another 330 watt unit

http://www.amazon.com/APC-Back-UPS-Outlet-550VA-120V/dp/tech-data/B0019804U8/ref=de_a_smtd

that lists "Full Load Run Time: 3.3 minutes"

which is way too short for your application, even tho at 100 watts you will get 19 minutes.

I's suggest you continue to search the internet. Search for "UPS" and look for the power rating in watts, and run time specs.

.

Your problem description lacks enough information...

But assuming you're running the light bulb directly from the battery (no inverters or other circuitry), the light bulb is DC and the battery is rated for the same voltage as the lightbulb, then you need a battery which is 800/V Amp-Hours in size, where V is the voltage of the battery & light bulb. Typically, batteries can't give up all their current before their voltage drops to unusable levels, so in a real world application, you'd probably want to oversize this by 10% or more, based upon the battery technology (lead acid, NiHM, etc.) you're using.