You’ve probably heard the advice about all the little parasites you have plugged into the walls at your house – power converters and plugs for this and that piece of electronics, kitchen appliances, clocks, radios, and so forth. These parasites consume power even when the device they’re attached to is turned off. Many of them do, anyway. So the advice is to not just turn them off, but unplug them when you’re not using them.
OK, sounds like a good idea, but watt’s it really worth? Out of curiosity, I used my handy-dandy “Kill-A-Watt” meter to find out. In case you don’t have one, I highly recommend them – they only cost about $20-$25, are available at most home-improvement stores (and, of course, on-line), and you can use them to easily figure out what your devices are costing you. Here’s a picture of my Kill-A-Watt connected to the coffee maker. The buttons on the front let you switch between voltage, current, watts, VA, or Wh (kWh). Wait, what, VA? Watt’s a VA?
We’ll get back to that question in a minute, but here are some of the readings that I got on devices around my house that are “turned off”, but still plugged in.
| Device | Power (W) | VA |
| Mr. Coffee | 2 | 14-16 |
| Food Saver vacuum pump | 0 | 1 |
| Computer monitor on standby | 0 | 1 |
| Notebook computer in sleep mode | 0 | 3 |
| LCD TV | 0 | 3 |
| DVD player | 0 | 0 |
| Mixer/juicer | 0 | 0 |
| Microwave oven | 3 | 5 |
| USB adapters (tested four) | 0 | 0-1 |
| Music docking station | 0 | 1 |
| Clock-radio (clock running only) | 1 | 1 |
| Clock-radio (clock running only) | 0 | 1 |
So when it comes to power consumed when “off”, the worst offender is our microwave oven. I didn’t get into brands much here, but let’s just say it’s an older microwave. With the exception of Mr. Coffee, the devices that consume appreciable power in this list are older. I’m hoping that means product designers are paying more attention to parasitic power these days. And let’s be clear, zero on this table isn’t necessarily really zero – it’s less than one watt, which is the lowest readout Kill-A-Watt will give. Technically, it’s probably less than 0.5 watt, which rounds down to zero. And I don’t know how accurate Kill-A-Watt is in that range. What I really should have for this study is “Kill-A-milliwatt”. I don’t know if they make that yet, and I’m not sure I really care. But let’s just say for argument’s sake that the worst case holds here, and all of the zeros in the table above are actually 0.5 watt. Then if I unplugged everything in the table, I’d be saving 14 watts. Give-or-take a few for roundoff error. Watt’s that worth? Well, at typical US residential electrical rates of about 12 cents per kWh, saving one watt for a year is worth a dollar. That’s a convenient number to remember. So, by unplugging, I could be saving $14 a year.
Now, back to Mr. Coffee. What do I do with that 14 to 16 “VA” reading? Do I really care? The power company only bills me for kWh. And what is a VA anyway? If you’ve bought a generator, you probably noticed that it’s rated in “Volt-Amps” not “Kilowatts”. But, a volt times an amp is a watt, so they have the same units. Aren’t they equivalent? Well, no, the answer is “it depends”. I’m not an electrical engineer, I’m a mechanical engineer, so I’ll try to explain it in a way that a non-engineer can understand. And the EEs out there will probably point out something wrong in what I say!
In DC circuits, things are pretty easy to understand. Especially if the circuits contain purely resistive elements. A resistor is basically a wire. It puts up a “resistance” to the flow of current. The most basic law of EE is Ohm’s Law, which says that the voltage (volts) across a resistor is the product of the current (amps) and the resistance (ohms). V=iR. That is true in AC circuits, too. At any instant in time, the voltage is current times resistance. But since the current in the AC circuit is “alternating”, meaning it’s constantly switching between positive and negative, the voltage is doing the same thing. And voltage and current are “in phase” with each other, meaning when current goes up, voltage goes up, and vice-versa, as in this graph:
But, throw a non-resistor into the AC circuit, and then things get all weird. A non-resistor is mainly either a capacitor or an inductor (coil). I’ll leave all the other squirrely electronics like diodes and transistors for another lesson, another time (and another teacher). What a capacitor or an inductor will do is cause the current and voltage to get out-of-phase with each other. For example, in the graph below, the current “lags” the voltage by a little bit. When voltage reaches its peak, current still hasn’t gotten to its peak – it gets there a couple of milliseconds later. This will be the case when an inductor is in the circuit; when a capacitor is inserted, the current leads the voltage. When the circuit is a mix of capacitors, resistors, and inductors, it can go either way … or current and voltage could be in-phase again.
This causes the actual power consumed in the circuit to be something less than the product of the average voltage and the average current. That’s because the peaks don’t line up. This introduces something called a “Power Factor” (PF) into AC power systems. And it’s why we need to look at Volt-Amps (the product of average voltage and average current) as well as actual power. Sometimes Volt-Amps is called “apparent power”, or “PA”. So, the actual power (kilowatts), P, is
P = PA * PF
For Mr. Coffee, actual power (P) is 2 watts, and apparent power (PA) is about 15 Volt-Amps. Even though the units of PA are the same as P (watts), the convention is to call them “Volt-Amps” or “VA” so that it’s clear you’re talking about apparent power and not actual power. Mr. Coffee (when in standby mode) has a power factor of 2/15 = 0.133. We’re looking at almost a pure capacitor or a pure inductor connected to the power socket when Mr. Coffee is “turned off”. That’s not necessarily a bad thing; it’s still not consuming a lot of power. But if you had a whole office building full of Mr. Coffees that only were operating for a half-hour in the morning, the rest of the day they’d be sitting on the grid doing nothing useful but driving the building’s power factor away from the “ideal” value of 1.0. For a homeowner, that’s not a problem, but for the office building, it could be a problem. Some power companies (Duke Energy, for instance), will charge a business more for their actual power if they operate at a power factor less than some reasonable value (typically 0.85). That’s because they have to spend some money to keep the whole system in line (in phase, or close to it). They have to inject something called “reactive power” or “Volt-Amps Reactive” (VAR), to correct the power factor back to a reasonable and safe level. How they do that I don’t really know, but it might be by hooking up some really big capacitors or inductors.
What did I learn from this?
- I can unplug Mr. Coffee when I’m done making a pot of coffee in the morning. That would be an annoyance if I used the clock to automatically start the coffee, but I don’t usually do that, so I’m OK unplugging it.
- I’m not quite as worried about my USB adapters as I was before … but now that I’m in the habit of unplugging them, I will keep doing so.
- I want more Kill-A-Watts. And I don’t care that much if they come up with a Kill-A-Milliwatt.
- Now I understand a little better why I should (or shouldn’t) care about that “VA” reading.
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