By Russ and Tiña De Maris
If you’re considering adding an inverter to your rig, or already have one, it’s important to keep in mind the battery power you have available. Nothing can ruin a spot of boondocking quicker than killing off your “house” batteries. Replacing deep-cycle batteries is an expensive proposition, and a bit of quick math can help you prevent the problem.
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).
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Something seems wrong in your calculations of 572 amp capacity, actually you only have half of that at best, like 286 amps available! Thats for lead acid batteries, if you Lithium you get to use all of the amps available! But I’m not exactly an expert on batteries, hopefully my calculations are correct!
Snoopy
Very good article. I think in terms of watts being used at any given moment as that is the immediate limiting factor for an inverter,,, until the batteries are depleted. ; ) Still, by use of these calculations and their battery capacity, one should be able to get a useful approximation of the minimum length of time they can expect power to be provided, if they operated at the constant full capacity of the inverter. However, just like cooking, nothing beats keeping an eye on things.
I’m not sure where the “NO LESS THAN 4” comes from in the load/ capacity calculation. I understand that an AGM deep cycle battery has only 50% of the stated capacity available, but I’m not sure where the other 2 times comes from. Perhaps you could be more specific. I have 2, 100 amp hour AGM house batteries and a 2000 watt inverter. I have a few minutes of capacity to run my micro wave to heat up that cinnamon bun for morning coffee. Certainly not enough for cooking a full dinner while boon docking.
In the calculations for the microwave oven, it would use 143 amps in one hour. Most people are not running a microwave that long. Please correct me if I’m wrong.
It uses 143 amps continuously while it’s running. Over one hour, that’s 143 amp-hours, which nobody does because few of us have a complement of batteries to do that. That’s like running your air conditioner. So, over that one hour it’s used 143 amp-hours.
“Amps in one hour” (or, “amps per hour”) is misleading in that it may indicate to some people that these amps are adding up to be 143 over the course of one hour. Not the case.
100ah @48v lithium. So yeah.