Originally Posted by David in Maryland
I'm no math whiz, so I can't cite the equations, but I do know that anything 12 volt that produces heat by use of a coil, like heaters, hairdryers, coffeemakers, etc. are big drains on a battery
. It's basically a controlled short circuit. That's also why incandescent lights
draw so much more power than LEDs. Appliances with motors aren't much better, if at all.
The math turns out to be really simple - just multiplication. Here's an easy example:
My wife has a 1200 watt hair dryer I want to run on my battery (she doesn't really, but it's an easy example. Why not me? I don't have much hair these days...)
Power is amps times volts (P=IV). If power is 1200 watts, and volts is 120, then, sure enough, that's 10 amps. Glad your trailer has a 30 amp supply if you're also running the air conditioner and a coffee maker at the campground...
Now the bad news. The battery is 12 volts. So I need 100 amps to power the hair dryer from the battery. If the battery is 100 amp hours, I can have a theoretical maximum of one hour run time before I've completely flattened the battery.
In the real world batteries don't respond well to maximum loads like this, and you never want to discharge the battery below about 50% capacity for maximum lifespan. We won't get into inverter efficiency calculations.
The wattage draw for nearly anything can be found out. Often it's on the labeling or in the manual. If not, it's usually pretty easy to measure.
Basically, it's all about the work you're doing. Heating things up is work. Running motors that move something is work. Work over time is total energy used. Running a little LED light
isn't much work, so a battery can do it for a very long time indeed. Example:
The 12V interior LED light
strip I installed in the Casita
draws 250 mA (0.25 Amps) at full brightness. How long will my 60 Amp Hour battery run it?
60 amp hours / .25 amps per hour = 240 hours. So yeah, not a big worry.