(This entire post is probably painfully obvious to anybody with any electrical savvy, but I'm a fumbling newbie when it comes to electricity, so I had to work this out carefully for myself.) Anyway, out of curiosity, I wanted to know exactly how much power each device is using. So after a little bit of trial and error, I developed this "beginner's cheat sheet" on how to do it. I hope this is helpful! This seems like a big project, but it's not. All you need is an inexpensive multimeter.
1. Make sure everything in the RV is off.
2. Disconnect the battery, using the battery disconnect switch.
3. Put the red (positive) multimeter plug into the 10 amp socket on the front of the multimeter. (I'm assuming that this is a pretty standard set up – I've only seen a few multimeters, and they all had this feature. If I'm wrong, I'm sure somebody will let me know.)
4. Unhook the negative terminal of the battery. Using an alligator clip, clip the multimeter's red lead to the negative cable (the one that would've been connected to the battery). (You could probably also use masking tape instead of an alligator clip.)
5. Using an alligator clip, clip the black lead to the screw post of the negative terminal on the battery. I think that what I've just described is called hooking up the multimeter "in series" – i.e., the current has to flow from the negative battery cable through the multimeter to the negative terminal. (Again, if I'm wrong on my terminology, I'd appreciate it if somebody would set me straight.)
6. Turn the multimeter on to the 10 amp setting. It should read "zero."
7. Reconnect the battery, using the battery disconnect switch. You should get a fairly low reading – this is the background or parasitic draw of the carbon monoxide monitor, the smoke detector, and the propane detector.
8. Turn on each of the electrical devices in your RV, one at a time, and see how the meter changes. So, for example, if the background number is 0.10 amps, and the meter jumps to 0.30, you know that whatever you are testing draws 0.20 amps.
You can then determine how much current each of your electrical devices is drawing. There is a formula for converting amps to watts, but (frankly) it's a lot easier to use the conversion utilities that are provided free on the Internet. See, for example:
Now, armed with all of this information, you can guesstimate how much power you use each day (how many amps for how many hours). And now you can compare that number to the rated "amp hours" of your battery, to figure out how many days you can go on your battery before drawing it down too far. (It's my impression that you never want to use more than 50% of your capacity. So, for example, if you have a 100 amp hour deep cycle battery, you don't want to use more than 50 amp hours. I think that's right, but let me know if it isn't!)
For whatever it's worth, here is a complete list of my electrical usage per device, expressed in amps and watts. (Maybe you can use my findings for comparison!) All of my lights are LED. Most of the low usage lights are those flat-panel "wedge base" units. One of my LED lights is a screw-in that runs off of 110 current, through my inverter. As you'll see below, it draws a lot of current, but it puts out a lot of light – great for reading.
Background parasitic draw: 0.1 A, or 1.2 W
Porch light: same
Bathroom light: same
Light near the back closet: same
Stove light: same
Closet interior lights: even less
Refrigerator: 0.3 A, or 4 W
Front bedroom lights: 0.5 A each
Center overhead lights: 0.5 A each, or 11 W total
Water heater: 0.8 A, or 10 W (only while it's heating)
Inverter: 0.3 A, or 4 W
Inverter plus LED screw-in light (a 60 W equivalent): 0.9 A, or 11 W
Radio: 0.7 A, or 8 W
Radio plus antenna booster: 0.9 A, or 11 W
Vent hood fan: 1.3 A, or 16 W
Furnace: 3 A, or 36 W
(This sounds like a lot of current, but we only run it for a few minutes a day.)
Water pump: 3.7 A, or 44 W
I was surprised at how much current the water pump draws, and I was also surprised that the water heater draws a fair amount. The heating element is propane, not electrical, but I guess the electronic controls for the water heater draw some power.
Overall, I think that we are doing okay, especially with all of these LED lights. I have two group 31 batteries (rated at 110 amp/hours each) mounted on the tongue – one in use and one for a spare. We just got back from six nights in Yosemite in fairly cold conditions (teens and 20s at night), and each battery lasted us three nights with plenty to spare.
With this clamp-on meter you can just clamp around the battery cable to take the reading. Won't have to disconnect anything..
My Posts are IMHO based on my experiences - PM me Roy and Carolyn
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Don't know if they all do, but when I was doing that 10a trick with the multimeter one time, amps were more than 10 and it smoked. Good thing those meters don't cost much. So try to know ahead of time that amps will be less than 10.
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You may have a measurement error. The 3A furnace current seems low.
Your method of measuring current is dangerous to the ampmeter. You can easily exceed the max 10A rating and blow a fuse or burn meter out. Many electronic accessories have an input capacitor. That means during power-up the capacitor will demand a large inrush current. This inrush current could blow the meter.
An easy fix to this potentially dangerous situation is to short out the ampmeter with a wire jumper before making your battery connection. After making the connection, wait 10 seconds before removing the jumper wire and then make your measurement.
You got the right attitude. Measure away! That's the only way to understand your rig.