What’s in a watt?

airstream light switchesWhen you turn on a light in your house, the light goes on. That’s simple. But when you turn on a light in your Airstream while on battery power, it may seem like you’re suddenly expected to have an advanced degree in electrical engineering.

That’s because managing power is one of the biggest challenges new owners face when they start roaming away from reliable campground electricity. Volts, amps, watts, and strange jargon quickly become part of daily conversation.

As Airstreamer said to me the other day: “I don’t know what any of it means, and yet I’m expected to know just to operate my trailer.”

Discussing electrical systems in an RV can be endlessly complicated, but let’s just keep it simple for now.

AC and DC—Your Airstream has two electrical systems, one for 120 volt AC power (just like the power in your house) and one for 12 volt DC power (from the battery).

Think of volts as a measure of pressure, like water pressure in a pipe. The higher the volts, the more pressure. North America uses 120 volts for ordinary outlets, and Europe uses 240 volts.

The 120 volt system in your Airstream is functional when the trailer or motorhome is plugged in. It powers the air conditioner, microwave, television, standard electrical outlets, and the refrigerator (when running in electric mode). It also goes to the power converter, which turns some of that 120 volt AC power into 12 volt DC power. This is used to recharge the battery.

When the Airstream isn’t plugged in, those circuits are off—in most cases. Why “most cases”? Because some Airstreams have generators which can produce 120 volts, and some Airstreams have inverters.

An inverter takes 12 volt power from the batteries and turns it into 120 volt power (the exact opposite of what the converter does). Because this will drain the batteries pretty quickly, inverters are usually wired only to a few things like TV and microwave, just so you can use them while you’re boondocking. You can’t run the air conditioner through an inverter because it draws too much power.

So how do we know much power something consumes? That’s where “amps” and “watts” come in. Every household appliance has a label printed on it somewhere that shows how much power it uses. For example, take a look at the power adapter for your laptop converter, or the charger for your tablet computer. In very fine print it will say something like this: “Input 100-240v~ 1.5A     50-60Hz Output 20v 4.25A max.”

That means this device can accept a range of voltage from 100 to 240 volts of alternating current (AC) at frequency of 50 to 60 Hertz. In other words, it can be used on both North American 120-volt and European 240-volt electricity. That’s nice to know in case we decide to use it in Europe, but we are really interested in the amount of that power it consumes.

This part tells us the rest: “~1.5A”. That means the laptop power adapter requires up to 1.5 amps. Let’s go back to the water pipe analogy. If 120 volts is the pressure, 1.5 amps is analogous to the diameter of the pipe. More amps means a bigger pipe, which of course can carry more water.

The total amount of energy consumed by this device is the product of the pressure and the diameter of the imaginary pipe. This device consumes 120 volts X 1.5 amps, which comes out to 180 watts. That’s the number we were looking for.

(By the way, if you’re wondering about the “Output” numbers, you can do the same math.  This adapter puts out 20 volt power for your laptop at a maximum of 4.25 amps, which is 85 watts.  The difference between 180 watts input and 85 watts output is lost mostly as heat.  That’s why the adapter gets warm when you’re using it.)

From light bulbs you know that a higher wattage bulb is usually brighter. That’s because it’s using more power. Watts tell us the total electrical consumption of anything, and we can use watts to compare different devices.

So why do we talk about 30-amp or 50-amp power cords? That’s the maximum your power cord is rated to carry on a continuous basis. A 30-amp power cord is really a 3,600 watt cord (120 volts X 30 amps) and we could refer to it that way but it’s standard to talk about it in terms of amps.

adapter

“Aha,” you’re thinking, “so a 50-amp power cord is really a 6,000 watt (120 X 50) cord?” No, because there’s a trick. 50-amp power in North America is supplied at 240 volts. So it’s really a 12,000 watt cord. I know, it’s not fair, but that’s the way it is. Now you know why the 50-amp cord is so much heavier: there’s a lot of copper in it to carry all that power. A smaller cord would melt!

That means an Airstream with a 50-amp connection has more than three times the power available to it compared to a 30-amp connection. Even if you’ve got dual air conditioners, you’ve got plenty of extra juice. Those air conditioners will pull up to about 4,800 watts at full tilt, leaving you with 7,200 watts for everything else. Our example laptop at a mere 180 watts is hardly even noticeable in the overall scheme of things.

Once in a while you may need to plug the Airstream into a regular household outlet, which generally provides just 15 amps at 120 volts (which is 1,800 watts). This is fine for keeping the Airstream charged and powering low-wattage AC appliances, like portable fans and laptop computers. But it’s a very bad idea to run the air conditioner on 15-amp power. It might seem to work, but long term it’s likely to damage the air conditioner’s compressor or cause overheating or even melting at the plug.

Now let’s look at the other power system in your Airstream. The 12 volt DC system is driven by the batteries, and it is responsible for powering everything else in the Airstream, including lights, water pump, furnace, “cigarette lighter” 12 volt outlets and USB outlets, fans, refrigerator (when running in gas mode), propane leak detector, stereo/DVD player, breakaway switch, etc.

By running most things on the 12 volt system, most power consuming appliances in the Airstream can be used anytime, which is convenient for roadside stops and overnights without hookups. Since the 12 volt battery is kept topped up by the 120 volt power converter, it will never run out of power as long as the trailer is plugged in. Only when the trailer is unplugged will appliances be working solely on battery power, and with a little conservation, battery power can last for days.

batteryYou might be wondering why we don’t have 120 volt batteries so that everything can use the same voltage. The reason is simple: 120 volt batteries would be extremely heavy and expensive. It’s much more practical to use a 12 volt battery, charger, and appliances—much like your car does. It’s also easier to design solar panels and generators that produce 12 volt power.

In the end, it doesn’t make much difference to most appliances. A light, whether running on 12 volts or 120 volts, will produce about the same amount of illumination for a given wattage. In other words, a 12 volt light that consumes 1 amp is about the same as a 120 volt light that consumes 0.1 amp. Both consume a total of 12 watts and both will be about the same brightness.

So a watt is a watt, whether that power is supplied at 120 volts, 240 volts or 12 volts. If you ever get confused about comparing power from your solar panels, generator, or the needs of various appliances, figure the watts (remember, volts X amps = watts) and you’ll have a fair comparison.

Rich Luhr

Rich Luhr is the author of Airstream Life’s (Nearly) Complete Guide to Airstream Maintenance,  available at Amazon.com and the Airstream Life Store. There’s a lot more about Airstream electrical systems, including maintenance tips, in this 220-page book.