The Spectre Of Phantom Loads
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If you start reading about off-grid electricity or RV boondocking, you’ll come across mention of phantom loads pretty quickly. A phantom load is something that draws power but isn’t doing anything useful for you directly (and might be keeping itself secret, just to piss you off). Reading about these, they sound like a major obstacle to off-grid living, but our personal experience has shown that these loads are manageable once you understand them.
Phantom loads can be just about anything, but there are two common types that most of us will have in our homes (or RVs).
The first common one is extra “features” on plug-in appliances, such as clocks, nightlights, thermostats, and programmed operation. These "feature loads" are conveniences, but they’re not the primary function of the appliance (making coffee or toasting bread). Because you don’t think of them as part of the “appliance”, their energy use is “phantom”.
The second common type of phantom loads are devices which uses a “wall wart” to get power. If the power cord for a device has a heavy box on it, that’s a wall wart. They are usually built into the plug, but sometimes are in the middle of the cord. Just about all consumer electronics are like this, especially charging cords for mobile electronics. The little cube on the end of the iPhone cord – that’s a wall wart (albeit an unusually cute one). That “wart” is a small AC-to-DC converter, which is changing the 120V AC power at the receptacle into 12V DC power for the device to use. If you’re doing this in your RV, you’re inverting battery power to AC, only to have it converted back to DC! Both of those conversions incur losses, so this is a pretty annoying waste to deal with.
The Impact of phantom loads
Many RVers, and certainly a lot of full-timers, don’t have much space in their rigs. This is doubly true about counter-space. We keep appliances stored and only bring them out and plug them in when needed. Let’s say you do want to leave things plugged in though, and take a look at the numbers:
The electric space heater we use is one we owned before hitting the road. It’s a good heater but it has a digital thermostat and temperature readout, which means that it is a phantom load – it always consumes a little energy so it knows when to turn on and maintain the room temperature, and so we can glance at the room temperature at our leisure.
I used our Kill-a-Watt meter to measure the heater’s energy consumption while turned off, and came up with 1.4W. That means even if it’s toasty inside, the inverter is running all night long at 1.4W. Because that is such a small amount of power compared to the inverter’s 2800W capacity, such a tiny load is basically the same as “idling”. So the power usage isn’t only that 1.4W, it’s actually the “idle” power consumption of the inverter – 30W. Eight hours of idling would therefore consume 240Wh of electricity, without actually doing anything. This works out to 20Ah, about 2.5% of our usable battery capacity. Right now, in the sunny desert, this is not a problem for us. But when we try to spend our winters further north, we will definitely not want to waste that much power.
What’s interesting here is that it’s not the load itself that’s the problem – 1.4W is nothing – but the fact that it forces the inverter to run, which wastes electricity.
I have read many places online that wall warts draw power even when their device is unplugged. Writers point out how they are warm to the touch and that’s evidence that they are drawing power. This made sense to me but I wanted to quantify it. I tested our iPhone and MacBook chargers with the Kill-a-Watt, however, and it didn’t register any power consumption from those power adapters when the phone or laptop were unplugged. I also checked the chargers for my point-and-shoot, Ching’s DSLR, our external hard drive (spinning type, not SSD), and our Doxie portable scanner. None of them register any Watts when their devices or batteries are disconnected. I finally tried plugging in 3 at once, and saw 0.5W on the Kill-a-Watt. So I guess they do all draw some power even when disconnected; it’s just so small that the Kill-a-Watt doesn’t accurately measure it. Somewhere around 0.16W for each wart.
It should be noted that we’re not very heavy users of electronics. The devices listed above are everything I could find in our RV to test. Other types of products may draw larger phantom loads.
Here’s where wall warts get interesting (did I really just say that?): We use “search” mode on our inverter, which means it stays off until it sees something trying to use power, then it turns itself on. This saves a lot of energy, because otherwise the inverter would be on all the time, even though it’s not needed. In search mode it only consumes 7W (vs. the 30W it would consume while idling). So search mode lets us use devices on-demand without making a trip to the control panel to turn on the inverter each time. But I have noticed that wall warts fool the inverter into dropping out of search mode, even if they are disconnected from their devices. This might have to do with the inductance of the coils inside them...but honestly that's just me guessing. I have no idea why. We have “search” set to 5W – I like that setting because the light still comes on in the fridge when we open the door (ah, luxury). I thought this would be enough to prevent the wall warts from turning on the inverter, but in practice, I was wrong. The inverter hums away with nothing but a Macbook charger plugged in. To keep the inverter off while only one wall wart is plugged in I have to set search mode over 25W (determined through trial-and-error). Trouble is, this also prevents the inverter from turning on even if it’s connected to a computer! So for us, it’s still important to not leave those warts plugged in. The warts themselves aren’t much of a phantom load, but they cause the inverter to become a phantom load. Sinister!
So do we really need to worry?
Worry, no — but pay attention. We are fortunate that we can mostly ignore our phantom loads and just accept the background losses. Whether we have only one wall wart plugged in or twenty, the inverter will still waste about the same 30W regardless, because it’s their mere presence in the circuit, not how much power they use, which makes them a problem. Most days of the year, there’s enough sun that we don’t need to care. But in the winter, every amp-hour counts. So it’s wise to have a handle on this, and be accustomed to managing it. Or at least know how, so you can modify your behavior when you need to.
The Kill-a-Watt is a really useful tool that I have used for the last 9 months to get a better handle on how things work. It’s not a high-end Fluke meter, and even though it displays to the tenth of a Watt, I don’t really believe it’s accurate to that resolution. But it’s accurate enough to help us understand our usage better.
Anybody who lives off-grid (as in dry camping) needs to be informed and aware of their power usage. RVers already do this anyway; being fully electric and free of both generator and propane only means we have to do it a bit more than most boondockers. But it’s the exact same skill-set. Managing power quickly becomes second nature; you can’t live with something without coming to understand it at least a little. Before we hit the road, I read other blogs where people said this same thing. And I thought, “What a pain in the ass, I don’t want to have to get used to new concerns, especially not electricity stuff!” So I tried to make the Toaster as brainless to use as possible. It helped, but didn't fully relieve us of the need to pay attention. Well-designed systems do help, but it’s just not practical to make an off-grid anything that doesn’t require someone to be paying a little attention. Our brains are pretty awesome, and they learn quickly, especially when important things like heat and cooked food are on the line. If you’re skeptical about this, give yourself the benefit of the doubt.
If you’re living with RV solar now and have figured out any neat tricks for minding your power consumption or spotting unwanted loads, please share them in the comments! We’re always eager to learn new power management techniques.