To get started, you'll need to know how many amps your shore power equipment will use. This can typically be found on the manufacturer's ID plate on each appliance. The plate may list amps, but in many cases, power consumption will be expressed in watts. You'll need to know the figure in amps, so here's the method for figuring amps.
Amps = watts/volts
For example, if your TV set uses 160 watts, divide 160 by 120 (for volts) and the result is 1.3. Your TV uses 1.3 amps to operate.
Now, add up the amp usage by all equipment that would be operated at the same time; this will give you the maximum amount of current that equipment will call for from your inverter, at 120 volts. Since your batteries operate at 12 volts, then the actual amount of current drawn from the batteries would be 10 times that of shower power needs – 120 volts is 10 times more than 12 volts. Our television, then, would "ask" the batteries for 13 amps of battery power (10 x 1.3 amps).
Sad to say, power inverters are not particularly efficient; to make the transformation between battery power and usable shore power, they eat up juice, on average, about 10 percent of the total transfer. What this means is, your battery will need to produce more current than actually used by the equipment.
Here's how to figure out how much "real" juice you'll take from your batteries.
Amps required by equipment x 1.1 x 10 = amps demanded from batteries.
So in our TV example, 1.3 amps from the TV times 1.1 (inverter efficiency loss) works out to 1.4 amps x 10 (the difference between the shore power voltage and the battery voltage) equals a total of 14 amps from your battery.
While 14 amps doesn't sound like a lot, think about what happens when you fire up your microwave oven. Our RV microwave demands 13 amps at 120 volts. Let's walk through the math:
Equipment demand x 1.1 (inverter inefficiency) x 10 (voltage difference) means, 13 x 1.1 x 10 equals 143 amps demanded from the RV batteries. That's a considerable wad of power.
A final, and really important factor: Your battery system capacity should be NO LESS THAN four times as large as the maximum current draw placed on it by your inverter. In the case of our microwave then, our battery bank would have to be a minimum of 572 amp-hour capacity. If we tried to run anything else at the same time as operating the microwave, the bank would have to be even larger.
How do you know the "size" or amp-hour capacity of your battery bank? Hopefully the amp-hour statistic is printed on your battery label. If not, you'll have to get the manufacturer and model information of your batteries and do the research to find the amp-hour specs. With that information in hand, here's the rest of the story.
In a simple, batteries-connected-in-parallel, system – for example, you're using all 12-volt batteries, than simply add up the values for each battery. The result is the total amp-hour capacity of your system. If you're using 6-volt batteries, wired in series (and possibly in parallel), things are a bit more complex. Batteries wired in series do not "add" the amp-hour capacity. For example, two 6-volt batteries, each rated at 200 amp-hours, wired in series to provide 12 volts to your rig, would provide 200 amp-hours. If another pair of these same batteries are wired in parallel, then the total would be 400 amp-hours (two sets of 200 amp-hours each, wired in parallel).
Clear as mud? Let us know if we need to go into more detail – russ at rvtravel dot com.