>>>>>>> I'm thinking I'd want to run a 25 watt tv, a 20 watt fluorecent light, and a fan (don't remember watts). It'd be great if I could get three or so hours out of battery with this load.
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Good questions…first, some disclaimers. Hopefully all my math is correct and coherently organized. If not, feel free to point it out. I’m no electrical expert or math whiz by any stretch of the imagination. To me it's still PFM how the light comes on when you throw a switch. <img src="images/graemlins/grin.gif" alt="" />

The short answer is approximately 4 hours and 15 minutes.

Here's how to figure it out for any configuration:

Assume you’re using a 12-volt, 115-amp-hour, deep-cycle marine trolling battery (about $65 from Wally World) to supply DC power to the inverter, which converts it to AC power for your TV, fan, PS2, and light.

A device that uses 1 amp per hour will in theory deplete your battery in 115 hours. An inverter/battery powering a 5-amp device would last in theory 23 hours (115 ÷ 5 = 23); 25 amps would be 4.6 hours. Notice all the in theory disclaimers? Using an inverter causes a power loss of some amount. Most experts quote a 10-20% reduction in the run time. Just be aware that you aren’t going to get that 115 hours of battery use if you’re running an inverter.

If you have all DC appliances and can run them directly off your marine battery, then that power loss won’t be quite so bad. But if you’re like me, you don’t happen to have a 12V DC fan lying around (I should, they’re pretty cheap - under $10)

So, you wanna hook up your PS2, a 25W (watt) TV, a 20W light, and a fan. As long as you have a good-sized inverter, you could throw caution to the wind, plug everything in, and see if it all works or if your inverter just vaporized itself….not recommended unless you have a 2000 watt inverter (or higher)!

So get a piece of paper & a pen, and check out the specs label on each electrical device you want to run. Write it down 'cause you'll forget it. Some will have the watts listed…others will have only the amps. It doesn’t matter ‘cause we have the (easy) math to figure it out.

Convert everything into one unit, preferably watts.

My little floor fan only lists .9 Amps, but I want to know the wattage it uses.

The basic formula is Volts x Amps = Watts.

So 120V x .9A = 108W for the fan using AC power (we’ll figure in the DC part in a sec).

108W (fan) + 79W (PS2) + 25W (TV) + 20W (light) = 232W total.

At the very least you’ll need a 500W inverter (about $45 or so). Double the needed watts for the start surge, and you’ll know how big an inverter to use – at a minimum.

(BTW, it’s more efficient to run a 500W inverter for a 232W draw than a 1000W inverter, but I wouldn’t sweat bullets over it.)

Now, to figure out how long your 115 amp-hour marine battery will power your goodies before needing a recharge:

remember V x A = W?? We’ve got volts (12 -- remember, it’s a 12 Volt DC battery), we’ve got watts (232), so just jiggle the formula around to solve for amps: W ÷ V = A.

232 watts ÷ 12 volts = 19.3333 amps. Be conservative and round up to 20.

The battery is in theory a 115 amp-hour battery, so:

115 amp-hours ÷ 20 amps = 5.75 hours of battery life.

BUT, don’t forget the inefficiency penalty for using an inverter. I’d knock off about 1.5 hours, just to be safe. So you can figure about 4 hours and 15 minutes (4.25 hours) of battery time.

After I got hooked on battery power, I walked around the house & garage and made a list of all the devices I might possibly want to use. The first thing to go was my hair dryer --1500 watts, a real hog!! <img src="images/graemlins/crazy.gif" alt="" /> <sigh> What price beauty……

If you need more battery time and your inverter can handle it, get another marine battery and hook them up in parallel. Then instead of only having 115 amp-hours, you’d have 230 amp-hours (in theory, again, don’t forget the slight energy loss) to use up.

Just to beat a dead horse even more, I’m a big fan of inverters & batteries. Do you already have a generator? I decided against getting one because it’s a gas hog (4-5 gallons/day IIRC) and extremely noisy. The noise aspect bothers me the most. First-hand accounts (I think on ETS as well as other sites) of long-term power outages quoted many survivors as saying they had to sit up day & night with a shotgun to ward off burglars. People drove around neighborhoods in Homestead, FL, after Hurricane Andrew looking for gennys to steal. Pretty ugly. However, if someone gave me a Honda genny, I sure wouldn’t turn it down <img src="images/graemlins/grin.gif" alt="" />

Battery power is quiet, clean, cheap, and reliable. My next goal is to get some solar panels for trickle-charging the batteries.

Sorry this is so long – hope it starts to answer your questions.

(PS) On reading this before posting, I see that you weren't asking about using a PS2 or Nintendo in the equation, but I'm too lazy to rework all the math. <img src="images/graemlins/smile.gif" alt="" />