Electrical Use Geekery

Given my profession, my geekery in measuring my electrical use is probably no surprise to my friends. However, I realized it might be good to spread some of my knowledge to my readers, who are both geeky enough that they might want to know how much power they're using, as well as environmentally conscious enough to do something about it. If you have friends who would benefit, please feel free to forward this link along. And if anyone hasn't seen it yet, I wrote up a primer on house energy efficiency a few years ago.

"To measure is to know."

"If you can not measure it, you can not improve it."

-Sir William Thompson, Lord Kelvin (1824-1907)

I have been logging my electrical use (via the power bills) for a long time--the graph below covers my time in Cambridge (1998-2004), before I moved up to Canada. The yellow bars are the monthly use (in kWh; electricity was about 11-12 cents per kWh); the blue dots & lines are cooling degree days (CDD)--an indicator of how hot each month was in total. As you can see, there is some summertime spiking when my roommate ran an air conditioner. These bills are for a third-story two-bedroom apartment in a triple-decker that was ~1200 sf; occupancy was two people.


(click on image to expand)

A few interesting patterns can be noted. The big drop from ~500 to ~300 kWh/month (late 2001) occurred when I changed roommates: the old one had several desktop computers running 24/7. Also, he worked from home, so the AC ran for good parts of the summer. The other noticeable drop (from ~300 to ~200 kWh/month) matched up to my replacing the old fridge with a new Energy Star rated model--see this Home Energy Magazine article on the economic benefits of replacing an old inefficient fridge before the end of its service life. The fridge power use dropped to ~300 kWh/year, from ~1200 kWh/year.

FYI, the electrical use in my new place (two people in a duplex) is in the 200-250 kWh/month range. I have not yet worked on doing full-bore CFL replacements yet.

[Edit]: As an additional data point, my electrical use in my apartment in Canada was ~400 kWh/month; that was just me living by myself. However, it is important to note that the hot water tank there was electrical, as well as an electric stove, and an electric clothes dryer--all of those would add major amounts of load to that total (actually, I'm surprised it wasn't higher). Space heating was electrical baseboard radiators, so that further mixed all of the end-uses together.

This leads into the next topic--toys to measure electrical use. I've worked with some research/utility submetering grade logging equipment for electrical consumption, but there are lots of consumer-level products that are really useful.

One is the Kill-a-Watt (about $30)--I own one, and have loaned it out on occasion. You plug it into the wall outlet, and attach a load to it. It measures immediate wattage, as well as kWh summed over time (it records the amount of time since it was plugged in).


I had some fun using this to measure computer power consumption. My laptop machine ran about 30 watts, on average--after all, battery life/power consumption is a huge design constraint. In comparison, a desktop machine that I measured (CPU box only) consumed ~75 watts. And that doesn't include a monitor: a CRT would run 150 watts (awake)/30 watts (asleep), or an LCD at 30–65 watts awake (numbers from this Google Answers page). That's part of the reason why one of my colleagues who designed an ultra-low energy building pushed the use of laptop machines to his clients.

Another electrical concern is "vampire" or "phantom" loads--electronics such as VCRs and cable boxes consume power in standby mode, and unfortunately, there is no incentive for manufacturers to minimize this number. As a result, the amount of standby power can vary a lot--minimal in a well-designed product, and substantial in other ones. So that's an item to measure.

Another device we have been working with is T.E.D. ("The Energy Detective") ($140). You connect it up inside the circuit breaker panel (either on the power mains, or an individual circuit that you want to measure--e.g., the air conditioner).


Then, you plug in the display unit anywhere in the house, and it will receive a signal from the unit in the panel over the electrical lines, and display power use. It also has some simple logging capacity, and it can set off alarms for various criteria ("Danger danger! Monthly power bill going over $200!")


We were checking out our instantaneous usage at the office--it was pretty fun for us energy geeks. We were normally running around 600 watts (for five of us, all running laptops, LCD monitors, etc.), and you could see the effect when other loads kicked in--such as the printer (~1000 watts) or the toaster oven. And when the AC kicks in--up to 2200 watts it goes.

Incidentally, if there are any locals who want to install a TED, but don't feel comfortable mucking around inside the electrical panel, let me know if you're going to get one, and I'll see if I can swing by to help pop it in. Also, the folks at TED put up a few helpful power savings tips on their website--they appear to better than the usual run of the mill items.

There is also a product called Watts Up ($100-200), but I have never played with it.

All told, you might ask why I am doing all this measurement, when we're talking about a $40/month electrical bill. Well, one reason is the difference between "site" and "source" power. Each unit of electricity that is delivered at your house ("site") requires about three times that amount of energy at the power plant ("source")--the process of burning coal/oil/etc. to make juice is about 30% efficient. This is partially reflected in the $/energy unit price. Also, 50% of electricity in the US comes from coal-burning power plants (see this Energy Information Agency table). As noted in another EIA document, "coal combustion emits almost twice as much carbon dioxide per unit of energy as does the combustion of natural gas, whereas the amount from crude oil combustion falls between coal and natural gas." I don't have good numbers on just how much of a typical person's carbon footprint is due to electrical use, and I know that airplane flights are totally overwhelming [note], but electricity is probably not insignificant.

[note]: I sometimes wonder if we might be the generation that saw cheap air travel and massive international mobility arise, and then go away when the energy costs get too high. I also wonder if we might be the last generation to have sushi-grade tuna before the fisheries collapse, but that's a whole other topic.