# Calculating electricity costs



## Bullseye (Apr 5, 2009)

I'm playing with a Kill-A-Watt (my library lends them free), trying to save some energy on my power bills. I'm not clear about usage, hoping someone can clarify.

If a device uses 10 watts constantly, I think that means that it's using 87.6kwh per year? (10w x 24 hours x 365 days / 1000) Or about $10.51 assuming $.12/kwh cost?

Fo something like a fridge, I think I have to track usage for a day or two, as it cycles on and off. So if it uses 1000 watts over 24 hours, I can know that's about $.12/day?

Thanks!


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## OhGreatGuru (May 24, 2009)

That would be 1000watt-hours for the fridge, not 1000 Watts. But otherwise your calculation is correct. Watts is the instantantaneous power draw. The amount of energy consumed (in Watt-hours or kiloWatt-Hours) is Watts x time 

Is your Kill-a-Watt meter programmable to give you the Kwh consumtion over time?


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## hystat (Jun 18, 2010)

cool the library lends them 


Bullseye said:


> If a device uses 10 watts constantly, I think that means that it's using 87.6kwh per year? (10w x 24 hours x 365 days / 1000) Or about $10.51 assuming $.12/kwh cost?


yep

I have a Kill-A-Watt. They are cool. 

Yes they can tally over time, as well as do a lot of cool things to teach about electricity, outside of the main functions


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## Guigz (Oct 28, 2010)

I have a Kill-a-Watt at home and I really love it.

Consider using 24 hour programable switches (which cost about 5$ at HD) to reduce energy consumption of devices that you don't need functionning all the time.

A few things to think about / check:

- TVs are notorious phantom load consumers. My plasma TV was using 30 watts/hour whenever it was plugged in. By unplugging when not in use, I save 240 KW per year (29$). Multiply by the number of Tvs you have (LCDs are not as bad though).

- I have my dehumidifier on a 24 hour switch where it works 4 hours then takes 4 hours of break and so on. That way, I have a comfortable basement and even better humidity control (humidity extraction is more efficient when humidity is higher; sinusoidal curve). Annual savings : 32$

- Do you need your modem, router, switch, server etc to be working during the night? If not, consider putting them on a timer. I dont turn them off because I need VOIP at all times but could work for you. Annual savings : 21$


- PVRs or cable boxes are also good things to put on timer (say from 2 AM to 10 AM even). Annual savings : 15$


- Be sure to find out how much energy your computer is using idle/load. It might make sense to buy a laptop if you leave your computer on all the time. Otherwise, be sure to use the sleep or hibernate function. Annual savings : 88$ (high performance computer).

- If you have a forced air furnace, be sure to check what kind of motor you have to move the air around the house. Although you wont be able to measure it with the Kill-a-Watt, a regular AC motor can use around 600-800 watts/hour when in use while the much more efficient DC motor can use as little as 100-200 W/h. I received a quote of 700$ for changing my DC motor for an AC one. I havent done it yet but seriously considering it. Payback is around 7 years and the motor is supposed to be quieter. 


I am really excited about the changes I have been implementing at home. Our bi-monthly energy bill is 47$ less than last year (which was around 160$) for the months of June-August despite the fact that it was a hotter summer and that we use air considitionning.


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## brad (May 22, 2009)

The thing about these savings is that individually they look small, but when you add them up and then consider that you'll continue to enjoy those savings every year for the rest of your life assuming you keep up these habits (and the savings will increase as the price of electricity rises over time), you can easily save thousands of dollars.

The computer savings seem high to me (even for a "high performance" computer), since nearly all computers on the market nowadays meet Energy Star guidelines; my desktop computer uses less than $40/year in electricity and my laptop probably uses $20 or less. Laptops in general are much more efficient than desktop machines, and it's easy to find laptops today that are more powerful than high-performance desktops of just a couple of years ago. The main drawback is a slower hard drive, but that's not noticeable in most applications that people use.


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## Guigz (Oct 28, 2010)

I have actually measured the powerdraw from my computer at idle and load. 

But I agree that a normal best buy desktop computer would likely draw way less power. However, my computer is running way out of spec which account for the increased power draw.


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## Bullseye (Apr 5, 2009)

Great, thanks for all the info so far!

I didn't realize programmable kill switches could be had that cheap, have to check it out. 

I'm also going to look at things like the stove and microwave, which are packed with electronics nowadays. I suspect they have a high phantom load as well.


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## pablito (Apr 3, 2009)

Bullseye said:


> I'm also going to look at things like the stove and microwave, which are packed with electronics nowadays. I suspect they have a high phantom load as well.


I'm liking this thread, good info that I'll want to follow up on. I just wish there'd be a way to automatically reset the clocks after the shutdown. Granted, some I wouldn't care much about... but the oven clock would annoy me to no end.


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## Phil-Carney (Aug 11, 2011)

This Kill-a-Watt meter seems interesting. I will have to check them out
on the web. 

Has anybody audited their own energy bills for over billing? I just wondered
if it was worthwhile.


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## Bullseye (Apr 5, 2009)

pablito said:


> I'm liking this thread, good info that I'll want to follow up on. I just wish there'd be a way to automatically reset the clocks after the shutdown. Granted, some I wouldn't care much about... but the oven clock would annoy me to no end.


Black electrical tape over clock solves that problem.


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