Introduction: L.O.G.Kill a Watt

About: Lazy Old Geek

So this Lazy Old Geek(L.O.G.) loves technical gadgets. When I saw that had a Kill A Watt on sale, I just had to buy one. This is the standard P4400 model.

 By the way, I finally got the play on words: Kill A Watt – KiloWatt, very clever. I am a little dense so it took me a while.

So what can it do? For most electrical devices, it can measure RMS voltage, RMS amperage, Watts, VA, Hz, PF and timed Kilowatt hours. You may ask what are these things? I will explain them in the Technobabble section so normal people can skip it.

So what good is it?

1.You can tell the current state of your AC power, real voltage, Hz. Stuff only a Geek cares about. US power is fairly well regulated at 120 V and allow a range of 114 to 126 Vat 60Hz. So who cares?

2.You can tell the Amps, Watts, VA and PF of anything plugged into it. While somewhat more useful, Amps can often be obtained from the devices literature or is often written on a label. Again who cares?

 3.You can obtain KWH for the device. Now this could be useful. Here’s where I have some problems.

Problem: Yes, it calculates the KWH from the time it was plugged in and the hours and minutes since but it doesn’t calculate the rate per day, week, year.

Solution: Actually, there is an improved model called Kill A Watt EZ (P4460) that does all of these. It’s only about $10 more. You can even enter your own KWH rate. But, alas, I don’t have one and even for a Geek, I shouldn’t spend the money for something I would probably use for a week. So my solution is in the step MySolution.


It is limited to devices that plug in to 120VAC outlets.

It is limited to 15 Amps. Actually, most residential wiring outlets are limited to 15 Amps. So this isn’t a major limitation.

It can only test one device or one power strip of devices at a time.

It takes up both AC sockets in a standard dual socket (see picture).

It cannot test devices not plugged in like lights, furnaces and especially house AC. This is a big one for me.

Step 1: Technobabble


Voltage (Volts) is the electrical potential in an electrical circuit.

Amperage (Amps) is a measure of the electrical flow through a circuit.

Voltage is always there waiting to be used. Amperage requires a complete circuit like a light bulb or a computer. Together they’re what powers your electronics.

 The following is a good explanation of AC RMS volts and amps quoted from:

Root Mean Square (RMS) Values

The value of an AC voltage is continually changing from zero up to the positive peak, through zero to the negative peak and back to zero again. Clearly for most of the time it is less than the peak voltage, so this is not a good measure of its real effect.(See picture)

Instead we use theroot mean square voltage(VRMS) which is 0.7 of thepeak voltage(Vpeak):

VRMS= 0.7 × Vpeak  and  Vpeak= 1.4 × VRMS

These equations also apply tocurrent.
They are only true for sine waves (the most common type of AC) because the 0.7 and 1.4 are different values for other shapes.

TheRMS valueis theeffective valueof a varying voltage or current. It is the equivalent steady DC (constant) value which gives the same effect.

For example a lamp connected to a6V RMS ACsupply will light with the same brightness when connected to asteady 6V DCsupply. However, the lamp will be dimmer if connected to a6V peak ACsupply because the RMS value of this is only 4.2V (it is equivalent to a steady 4.2V DC).

You may find it helps to think of the RMS value as a sort of average, but please remember that it is NOT really the average! In fact the average voltage (or current) of an AC signal is zero because the positive and negative parts exactly cancel out!

Watts: Watts equals volts times amps. Watts=Volts * Amps. As used by Kill-A-Watt, it means the instantaneous voltage times the instantaneous amps in a circuit.

VA: VA= (RMS)Volts * (RMS) Amps

 There is a subtle but important difference between the two and that is the power factor. Mathematical this is: VA·PF = Watts

Thepower factorof anAC electric power system is theratio of thereal powerflowing to the load to theapparent power in the circuit.

Here is my explanation: AC voltage (and current) are as shown in the picture. That means that the voltage goes positive than negative. By the way, in the US, it does this 60 times a second which is why US AC is called 60Hz (Hz means cycles per second). Now current also goes positive and negative at the same rate. In an ideal world (from power companies point of view) the voltage and current would be in sync, e.g., the voltage and current would rise and fall together. This would be true for a strictly resistive load.

In the real world, there is no such thing as a strictly resistive load. Besides resistance there are reactive loads called inductance and capacitance. A resistor is the physical example of resistance, the inductor for inductance and the capacitor for capacitance. Electric motors are highly inductive. However all electrical components have some of each even electrical wiring.

Who Cares! You may be asking yourself, who cares? Well, your power company cares.

PF: is between 0 and 1. If it was 0 then voltage and current would be completely out of sync. If it was 1, then voltage and current would be completely in sync.

 Power companies charge customers by the KWH.

KWH: Kilowatt hour is 1000 watts of power used for one hour.

Now if you had something with PF = 0 (which is impossible) and you plug it into the formula, Watts = VA * PF, then watts would be zero and so would KWH and you would pay nothing.

PF cannot really be zero but if it is very low than you will be paying less for the same amount of electricity (amperage). By the way, that is how those energy saver plugins work. I do not use them or recommend them as they will probably reduce the efficiency of the appliances plug into them (or near them).

It is harder for power companies to generate electricity with low power factor loads. If they find customers with low PF, they will charge them more and/or correct the PF provided to them (at least for commercial customers).

Also newer electric meters actually monitor PF.

Step 2: My Solution

By the way, if you haven’t seen it, I suggest you look at this Instructable:


It is an excellent Instructable and has an example of what the current looks like for a laptop. This is why it’s not simply multiplying volts times amps to get watts.

So here are my thoughts about using my Kill A Watt.

First, it won’t be much good for figuring my total usage. It cannot test my furnace, AC or lights which are major contributors to my electric bill. And it can only test limited devices at a time.

Second, while the KWH setting can be used over a long period of time, I figured it is still just an estimate, e.g., a refrigerator will cycle on and off as needed, but it will be affected by usage, say a family gathering and weather, it probably works harder in the summer. My TV usage is going to vary a lot depending on what else I am doing and how much stuff I like is on.

 So I figure the best usage would be to figure a ball park estimate (rough approximation) of costs for various items and basically all I care about is over a year. This will give me some ideas on how I want to use some of my electronics. So I decided to use a spreadsheet, in my case, Excel. This could also be done with ‘free’ Open Office or Google Docs.

 One thing you need to get is your local power rates in dollars per KWH. You should be able to find this on your electric utility bill or their website or call them. Now some of you may have rates that vary by time of day and season. However, again, I am only shooting for ballpark figures so you can probably just take an average. Mine is just under 10cents a minute or $ 0.109.

 Now most electrical devices basically have two states, on or off. You can decide if there are other states that you are interested in so this can be indicated in the spreadsheet.

 As far as the Kill A Watt goes, all you really need is the Watts reading. Volts, Amps and PF are all used to figure Watts. So Watts is a good representation of how much it will cost you. And I am using an instantaneous reading so I don’t have to leave devices plugged in for hours at a time.

This is the formula I used to calculate Cost per year for a device.
Hint: To write ‘per’ in Algebraic formulas use / (which actually means divide).

 Cost/year = Watts * Hours/Day * 365.25 Days/year * Rate*(1KW/1000Watts)

 On the spreadsheet (see picture), I created columns for Device, Condition, Watts, Hours/day Rate and Cost/Year

From the first row under the Titles

‘Rate’ enter your utility rate per KWH. Mine is about 11 cents.

‘Cost/Year’ copy the cost formula. Mine was this =E8*J8*365.25*K8/1000

For those not familiar with spreadsheets: E8 refers to the ‘E’ column and row 8.

In my spreadsheet, E8 is Watts, J8 is Hours/Day and K8 is Rate of the first row.

 Now all I had to do was highlight those two cells and do a right-click with the mouse and copy them to the other cells under ‘Rate’. When you do this Excel knows that it needs to refer to the cells on the same row and not the original cells.

To use, enter the Device name, its condition, the Kill A Watt, Watt reading and how many hours a day you would guess it will be in that condition. Now with some electronics, the Wattage will fluctuate but again this is just an estimate so take a guess at the average. The cost/year will be calculated.

Using the data:

 So if my Phone charger without a phone cost 34 cents a year to run, do I want to unplug it when not in use? If you are a ‘Green’ person probably yes. If you are a Lazy Old Geek, no.

 There are many factors involved besides cost and being ‘Green’

For cost you may have to consider minimum. Our provider has a $22/month minimum charge.

Convenience, I don’t know of anybody who unplugs their refrigerator when it’s not running. The little light wouldn’t work. However a TV might make a difference. I have a DISH receiver. I would suggest leaving it plugged in as it apparently will update the TV guide in the middle of the night and when it loses power, it also has to reacquire satellite connections.

Rechargeable batteries left in chargers is a whole separate topic I hope to address in another Instructable. Information right now suggests that NiCd and NiMH batteries should not be left in chargers after charging but Lithium Ion batteries should.

Wear and tear on AC plugs and cables.

Lightning and power surges.

You may notice that the LED night light is reading 0.0 Watts even though it is on. This could be correct as the Kill A Watt doesn't display less than 0.1 Watts. The literature for this says it's less that 25 cents a year so this could be less than 0.1 Watts.

I hope this is helpful to anyone buying a Kill A Watt.

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