Saturday, August 15, 2009

Kill A Watt and Electricity Usage

Last month, our electricity bill skyrocketed due to greatly increased air conditioning usage. As you all know, the marginal cost of the electricity increases the more you use. Now that I think about it, it seems a bit strange because in almost any other case, it's the other way around; the more you buy the lower the unit cost. Anyway, I digress.

This is how PG&E prices power in our area (for a 32 day billing cycle):

Baseline Usage 387.20000 kWh @ $0.11531
101-130% of Baseline @ $0.13109
131-200% of Baseline @ $0.25974
201-300% of Baseline @ $0.37866
Over 300% of Baseline @ $0.44098

As you can see once you get over 200% of the rather low baseline, you really get hurt with rates that are 3-4x the initial baseline rate.

Well, we ran a few experiments where we didn't use any AC and checked the meters, and we learned that we were still consuming a lot of power. For example, we consumed 10 kWh overnight (11pm to 9am) with no AC running. This figure is roughly the 12.1 kWh designated as daily baseline usage. In other words, we were being pretty dumb about our energy use, and it probably wasn't limited just to stuff running passively.

I went out and picked up Kill A Watt, which was only $15 at Fry's Electronics. This little device is pretty neat. You pretty much plug stuff into it, and it tells you how much electricity is being consumed. It also keeps running track of kWh over time, which is useful for those devices/appliances that have variable power consumption.

Some interesting things that I noticed and don't really understand...

1) Our mini-fridge (aka the beer fridge) uses 0 when it isn't actively trying to cool down. During its cool-down phase, it draws 75 W. This phase only happened 4 times over a 2 hour period or so. A total of 0.06 kWh were used after 2 hours in the evening. It probably uses more during the day as it's hotter.

2) A cell phone charger without charging anything draws 3 W. Computer charger without a computer plugged in draws around 4-5 W. A sub-woofer that is not in use draws 13 W.

3) Computers (not laptops/notebooks) use a crapload when not in standby. My computer setup with an external hard drive, LCD monitor, cable modem, router, and speakers draws 200-250 during normal use. It is only when the hard drives really start going that the 250 peak is reached.

When in standby mode with the monitor going into energy saving mode, the power draw drops to under 30 W, if I turn off the external hard drive and the speakers.

4) Halogen lamps are evil. I always knew that they used up a lot of power, but I have always used one in the office because of how much light they produced. Not any more. The halogen lamp draws a whopping 300 W. We swapped it out for a different torchiere lamp that uses only 25 W. It's not as bright, but there's no way it makes sense for us to use the halogen, especially since the office is used a lot with JC working from home.

5) Fish tank filter draws 11 W, and the fish tank light uses 17 W. This means that if you keep your light on all day (which I used to do a lot, because I'd forget to shut it off at night), then over 5% of the daily baseline is being consumed by the fish tank. So, now I'm trying to remember to turn off the fish tank light at night, which is probably better for them anyway.

I've got a lot more stuff to test, but at least now I feel that I've gotten a better handle of what devices and appliances are costing us what.

And, in case it's not obvious, I really do recommend the Kill A Watt product. It really does work as advertised. My only complaint is that it's a bit large so sometimes you'll have to unplug other things in order to test something.


Unknown said...

A couple of suggestions, from someone who has done a similar energy audit using a Kill-A-Watt:

Get a "smart powerstrip" for the subwoofer. You plug the stereo into the control outlet, and plug the sub in the switched outlets. It will cut power to the sub when the stereo is off.

For the fish tank light, get a cheap lamp timer from IKEA so you don't have to remember to turn it off.

A friend and I built a site for sharing Kill-A-Watt readings. We don't have a lot of data yet, but I did measure several subwoofers, which you may find interesting:

Good luck on your energy reduction!


gaamblor said...

did you figure out how much the ac is using per hour?

have you thought about putting solar on the roof?

Brute Force said...

It's pretty tough to figure out the AC's consumption accurately, mostly because I think how hard it works is strongly correlated with the temperature. So, as that fluctuates, the energy use does as well.

However, we did do some quick experiments where we looked at the actual meter for two hour time periods when AC was on and also when it was off. The AC was a huge energy suck (I think it was close to doubling the energy draw), but that might be a bit unfair if we were to look at longer periods, since the AC had to work extra hard since it's starting from scratch (i.e. the house was warm at the start of the test).

stephenmorphey said...
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stephenmorphey said...

An energy audit is an inspection, survey and analysis of energy flows in a building, process or system with the objective of understanding the energy dynamics of the system under study. Typically an energy audit is conducted to seek opportunities to reduce the amount of energy input into the system without negatively affecting the output(s). When the object of study is an occupied building then reducing energy consumption while maintaining or improving human comfort, health and safety are of primary concern. Beyond simply identifying the sources of energy use, an energy audit seeks to prioritize the energy uses according to the greatest to least cost effective opportunities for energy savings.

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