I have a touch lamp. I want to know what it's costing me to run.

April 20, 2007 5:13 PM

I have a touch lamp, which I'm finding incredibly useful to turn off when I'm half asleep. However, I'm also a bit of an eco-warrior. I've been looking online, and found that a small amount of energy is supplied to the lamp at all times. Surely that must be costing me something?

Using my Kill-a-watt, I've found that while the lamp is "off" (as in unlit), the only energy being registered is 1.0 PF. Nothing else is active on the display. When the lamp is lit at 20w, the PF is 0.70, when lit at 35w the PF fluctuates between 0.86 and 0.94 and when the lamp is fully lit (40w) the PF is 0.99.

What is a PF?
How much is it costing me (per week/month/year/whatever is easiest) to leave the lamp switched on, but unlit? I'm in the UK, on a 240v supply. Better still tell me how I can work it out myself.

I am an electrical-term-n00b, so please keep your answers simple. Please let me know if you need more info. Bonus points if you can point me at a site that explains this stuff in a very basic way, so I can get my head round it.

posted by Solomon to technology (16 comments total)

I believe that all plugged in appliances have that electrical "drag" that you described (sucking some amount of energy even when not in use - but still plugged in).
posted by Sassyfras at 5:25 PM on April 20, 2007

You're measuring the wrong thing with the kill-a-watt. I imagine it has different modes. PF is "power factor" and is related to the type of load. That's not so important as a power (watts) or current (amps) reading. Maybe you could get back to us with that. If by "nothing else is active" you mean power and/or current read 0.00, that means the power being wasted is so small the kill-a-watt can't register it.
posted by putril at 5:27 PM on April 20, 2007

"Wasted electricity" is usually in the form of heat energy, so it's not costing you anything extra in the cold months when you are using other devices to heat your home. Wearing a sweater would save you a thousand times more energy than this. So your calculation should be only for the warm months.
posted by weapons-grade pandemonium at 5:31 PM on April 20, 2007

You probably used more energy posting this on the Internet than that switch uses in a month. If it really bugs you, switch the lamp to CF bulbs (if they aren't already) and you'll more than make up the difference.
posted by DU at 5:32 PM on April 20, 2007

...it's not costing you anything extra in the cold months when you are using other devices to heat your home.

Only if s/he has electric heat. In a cold climate like the UK, I would hope that's not the case.
posted by DU at 5:33 PM on April 20, 2007

DU is actually off by orders of magnitude. In the time it took you to make your post, your display (regardless of whether it's LCD or CRT) used more energy than your lamp will use over its entire lifetime in its "standby" mode.
posted by dmd at 6:04 PM on April 20, 2007 [2 favorites]

Do not put a compact flourescent bulb in a dimmer circuit. It is a hazard... as in fire. (Read the instructions on the CF packaging and it will emphasize this.)

On the power consumption, the 'off' requirements for a touch controller are going to be negligible compared to the power consumption when on... as in milliWatts (.001 W) versus Watts. IF you reliably make sure you turn it off when you are not using it, you'll be a good ecowarrior! Don't fret about a few milliWatts.

I agree with putril, "PF" is meaningless in this circumstance. It's related to reactive loads (i.e., motors, mostly), not incandescent light bulbs, which are mostly resistive (and have a power factor of 1). You should be measuring current into the lamp to get a useful measurement, but even if you did, it would be negligible, as above.

(IAAEE)
posted by FauxScot at 6:07 PM on April 20, 2007

I've got a Kill-A-Watt, and it does have a setting for Watts (why it's not the default when you plufg the thing in, I don't know.) It's the middle button, labelled "Watt". I would imagine the energy used by your lamp in standby is less than 1.

"Only if s/he has electric heat. In a cold climate like the UK, I would hope that's not the case."

Actually, it makes no difference what the main heat source for the home is. You could light a trash can fire in the living room and the thermostat would still detect the temperature increase and turn the furnace on less often.
posted by contraption at 6:09 PM on April 20, 2007

Do not put a compact flourescent bulb in a dimmer circuit.

Oh, I assumed it was like my grandparents' touch light and had 3 separate bulbs (which is how the non-electrical poster knew the wattages). If it's a dimmer, definitely don't use a CF.
posted by DU at 6:18 PM on April 20, 2007

Just looking at PF doesn't tell you if your lamp is using any energy -- look at the watts reading for that (my guess is that when "off," your lamp has an incredibly low draw, if any.)

PF = "Power Factor" (complicated Wikipedia article here.) To use flagrantly unscientific language, it's a measure of how much trouble the power company has to go through to deliver one watt to your lamp. 1.0 is the best it can be in any circuit. 0.7 is pretty bad. The bad power factor at the low setting is a function of its design -- that's just how SCR dimmers work -- and you can't do anything about it. The good news is the low power factor doesn't cost you anything directly since the power company bills you on watts used. The other good news is these dimmers are much more efficient than previous technology, which threw away a lot of power.

The bad news for the environment is that low power factors require more resources to deliver that wattage, and more energy is lost to resistance in the transmission network. If that bothers you, consider a setup that doesn't require dimmers (e.g., one that can switch discrete lighting elements in/out as opposed to varying supply voltage.)
posted by Opposite George at 6:37 PM on April 20, 2007

Er, the bad power factor at the dimmer's low setting is a function of its design.
posted by Opposite George at 6:56 PM on April 20, 2007

DU is actually off by orders of magnitude. In the time it took you to make your post, your display (regardless of whether it's LCD or CRT) used more energy than your lamp will use over its entire lifetime in its "standby" mode.

This seems like a really inaccurate statement.

According to this list, most displays take somewhere between 20 and 100 watts. Let's call it on the high side, 100 watts. The post might have taken 15 minutes to write.

Assuming the touch-lamp has a relatively short life (10 years), that's 350400 15-minute periods. For your statement to be true, the standby power usage would have to be 0.000285 watts.

As such, I think it's safe to assume that you are off by orders of magnitude.
posted by Tacos Are Pretty Great at 10:37 PM on April 20, 2007 [1 favorite]

How much is it costing me (per week/month/year/whatever is easiest) to leave the lamp switched on, but unlit?

If it's using less than a watt, it's lower than your equipment can measure, and therefore impossible to calculate.

If it was just a hair under 1 watt (0.9999999999), then you could round it to one Watt, multiply by the number of hours of standby in a year, divide by 1000, and then you'd have the number of kilowatt hours used.

Multiply that by your cost per kilowatt hour, and you'd have the cost of the item.

Realistically, it's probably costing you a few small coins per year.
posted by Tacos Are Pretty Great at 10:43 PM on April 20, 2007

For your statement to be true, the standby power usage would have to be 0.000285 watts.

As such, I think it's safe to assume that you are off by orders of magnitude.

Actually, I have off-the-shelf touch switch components that are spec'ed as low as 0.00005 watts.
I'm out of town and can't take a multimeter to my own touch lamps, but I think I could design a circuit that takes wall mains as the input, has a touch switch, and operates a relay, all for a grand total power draw of less than 0.000285 watts when waiting to detect touch. I don't know if the lamp in question does it, but it means it might. (Unless I over-estimate my circuit building skillz :-)

Regardless, use the kill-a-watt to measure the watts drawn. And if it's sensitive enough to gives you a reading other than zero (and it's likely that the draw of the lamp is smaller than the devices margin of error), you can work it out. To make things simple when working out the cost, I round the cost of power to 10c per hour at a thousand watts. You probably actually pay less than that.

At 10c per hour at a thousand watts, I'm confident your time and effort are going to be far better spent saving power elsewhere.
posted by -harlequin- at 12:48 AM on April 21, 2007

harlequin: lots of devices have surprisingly inefficient standby power supplies, to the tune of 5 or 15 watts. (More if they're keeping a CRT filament warm.) If the manufacturer can save 2 cents per unit by using a power supply design that wastes an extra watt when "off", they will.
posted by hattifattener at 1:42 AM on April 21, 2007

"... If the manufacturer can save 2 cents per unit by using a power supply design that wastes an extra watt when "off", they will."
posted by hattifattener at 4:42 AM on April 21

An "instant on" circuit in a CRT isn't "wasting" power, it's using a small amount of power to provide quick picture, making it possible to turn the set/display off (or blank it) for short periods when it's not being used. It's a feature designed to encourage lower overall power use.

And in this discussion of touch control lamps, it's kind of off topic to lump such devices in with the circuits under discussion here. The typical capacitive touch control circuit is an IC with a power MOSFET output. Leakage currents are likely in the microamp range, and I doubt a device as inaccurate as a Kill-A-Watt, with it's 0.2% of full scale accuracy claim, could even correctly register the leakage current of the control circuit device. And I'd seriously doubt the ability of an inexpensive device like a Kill-A-Watt to correctly integrate the non-sinusodial wave forms created by the phase switching circuit of the lamp in dimmer modes, so I wouldn't place any stock in the Power Factor figures it is reporting, either. There just isn't anything in the lamp circuit that is reactive at partial brightness to the extent necessary to drop the PF to that of highly inductive loads like a motor, that the Kill-A-Watt is indicating with its 0.7 PF reading.

A Kill-A-Watt is, at best, a gross tool for verifying the current consumption of medium sized household appliances. It's believable for single phase fractional horsepower motors, like small fans, or perhaps even up to motors sized for refrigerators, and it is probably useful in checking pure resistive loads like standard lamps and electric appliances like toasters and electric blankets. Once you move away from simple loads like this, to electronically switched loads, it's just not the tool for the job. And even for non-steady state loads controlled by conventional switches or relays, you have to take into account duty cycle, and reflected mechanical loads over a complete appliance working cycle, to make use of data collected with a Kill-A-Watt properly.

If the lamp control is working properly (going to full off when shut off), and not developing shocking potential when next touched, any leakage currents through the MOSFET control are in the microamp range, and are thus of inconsequential concern over the life of the lamp. The bigger concern is whether the user can actually be sure such lamps are fully "off," if using large wattage bulbs in them, at low brightness settings. Most such lamps have 2 or 3 "brightness" steps, and if a higher wattage bulb than the recommended maximum is used, the lowest setting may not heat the filament to brightness great enough to detect in partially lit rooms. So, it's important to not exceed the recommended maximum bulb wattage, and in fact, a good idea to use lower wattage bulbs in such lamps, for operational and safety reasons. And, for reasons cited above, not to use CFL type lamps in such devices.
posted by paulsc at 4:05 AM on April 21, 2007