Crunching numbers on my home energy use

Written by erin


During an attempt to bring order to our household files, my husband and I discovered that we saved enough utility bills to reconstruct a nearly complete record of our household energy and water use since we moved in in 2000. Unable to let such a treasure trove sit idle, I ordered the pile and input everything into a spreadsheet.

I hoped, indeed expected, the resulting data to show clear if subtle downward trends across all three resource types (electricity, natural gas, and water). Our household size has not changed over this time, and we have replaced several major energy-using appurtenances and made numerous small changes over time.

Unfortunately, the data is not so clear. After viewing the first graph:

…I found myself spinning up several kinds of analysis trying to find one which told the story I was looking for. I know that isn’t an objective way to study data but I was not ready to conclude that all our diligence was for naught.

First, I compiled wintertime data, as our natural gas use is dominated by our house’s radiant boiler:

If I let Excel generate trend lines over these graphs, they do point ever so slightly downward. But I don’t think it’s compelling. I don’t know if what I’m looking at is all weather-related. We replaced our boiler before the 2008 winter so I would have expected gas use to drop there, and it does a tiny bit. But it doesn’t take us down to our 2003 levels. Meanwhile, electricity use has dropped more markedly and I don’t know why that is, either. We did use some space heaters before we replaced our boiler; would that account for the large drop?

Then, I examined summer data:

Notice that I’ve used a multiplier on the natural gas use to make it visible. There’s a nice drop in 2009 which derives, I am fairly sure, from installation of our solar water heating system last year. I wish its magnitude in actual energy were greater. But look at the electricity — what’s going on here? I can’t think of anything that can account for peaks in 2004 and again this year. We don’t have air conditioning. Indeed I’m not sure why the electricity use shows the strong seasonality that it does (winter is about 1.5 x summer).

So I’m puzzling over this. I’m feeling empathetic toward real scientists trying to isolate causes and effects in the complex system of climate. I have a much smaller system but it’s affected by many of the same variables and my data isn’t quite up to the task. But I’m not going to let that stop me from taking some meaningful action. Something is going on with our electricity use and I’m going to find out what.

In the meantime, I am comforting myself with the one graph I can be proud of.

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  1. Maurice

    Regarding your winter-time gas use: our gas bill notes the number of heating degree-days in the month, a proxy for the weather. Factoring in the heating degree-days will help level out the variability in the weather for any given winter.

  2. Marty

    Another resource to consider is Weather Underground ( they allow you to search historical data from Airport as well as Personal Weather Stations so that you can see the averages for the months. You can even do a custom period so that you can tie it to the billing period. The Almanac allows you to see the heating and cooling days for cities.

  3. Eric

    I too am analyzing my utility usage, electric and water (we have no gas) and although we only have 2 1/2 years of data so far we are seeing some results but it is inconclusive. I just did a search and found this site with a free energy spreadsheet. I haven’t tried it yet but I wanted to share because it looks pretty good.

  4. Don

    Our public utility tracks our monthly electric usage over a sliding 2-year window, and makes it available online in Excel-compatible format. I’ve used that excellent information to track my energy usage as I’ve made various improvements to the house. My climate is cooling-dominated; by far the most electricity is used in the summer. I’ve dropped my daily average usage by more than 50% since 2006. You can learn what I learned and experience my travails in my blog “Energy Efficiency Waits For No Man!”

  5. trent

    I have put up solar attic fans over the garage and over the attic which has cut my cooling costs. It lowers my attic temps by 20 degrees so my ac starts up later in the afternoon or not run at all. My gas has dropped too in summer. I insulated the water heater,wash with cold water and dry on the line when weather permits.

  6. Andre

    Seasonal variations in electricity consumption, particularly in non heating energy costs can be explained by a number of non-weather factors too. We generally wear more clothes in winter and these don’t air dry as quickly – so washing and drying appliance frequency tends to increase in winter. This is why in Austrlia at least, washing machine manufacturers tend to increase their marketing in the winter months.
    Another significant factor is decreased sunlight hours – in response you’ll tend to use internal and perhaps external lighting more frequently and for longer.
    Lastly, hot water use for showering may be greater in winter (longer showers to keep warm and hotter water to overcome cooler ambient air) and this can undermine the benefits of solar hot water as the electric or gas boost will need to be used more frequently to compensate for shorter days where the sun is at a less efficient angle to the hot water system’s solar collector.

  7. Greg

    I have a similar plot for my electric bill (all-electric home, including well water) for a half dozen years. Sadly, like you I see no clear trend, despite significant insulation additions through the years.
    I think my winter peak usage depends strongly on how my travel schedule happens to correlate with winter weather, since I turn the thermostat down to 50 when I’m gone. That erratic effect may mask the insulation’s effect.
    A plot of miles traveled in my car for 10 years (I’ve kept a record of mileage and month for every oil change) shows a steady downward trend, of which I am proud.

  8. Dave

    Good job capturing the data for use in the future as you may continue reducing your eco footprints. Knowledge is power!
    You wrote, “I

  9. Jason

    Here’s a neat trick for the hands-on solar savvy homeowner: install a “Summer bypass”. In most climates, and with enough storage (80 to 120 Gallons) you can run on 100% solar for all of your hot water, all summer long.
    A “Summer bypass” consists of a set of valves that allow you to bypass your gas-fired water heater and feed water directly from your solar storage tank into your house hot-water plumbing. By doing this, you can turn off the gas to your back-up water heater and you won’t see those 5-10 therms of gas use show up on your bill.
    Here in Colorado, I know many solar friends that have successfully turned off their back-up (i.e. gas-fired) water heater between May and September. And this is often done on older “legacy” systems that are over 20 years old!
    “Summer bypass” used to be a standard feature of many “Carter-era” SHW systems, but has all but dissappeared as a feature due to over-cautious installation companies that don’t want to burden the homeowner with moving a pair of valves twice a year.
    Note: this trick only applies to two-tank active-indirect systems. I am assuming from your utility bills that this is your system type.

  10. Tom Harrison

    Dave —
    All what you say is right on the mark. I have done year-over-year comparisons of my household electrical and other energy usage (very visible in the excel chart I did here: — as you say, the number of daylight hours is a big culprit in seasonality, as well as AC.
    For you or anyone who wants to get even a layer or two deeper into exactly where all that electricity goes, beyond what you can infer from your utility bill, I strongly recommend a PowerCost Monitor, or the new TED 5000 — both do real-time metering so you can see what you use when you use it. It’s really pretty incredible how knowing what you use affects behavior. I think the TerraPass folks sell the PowerCost Monitor — Google for TED 5000 to get one of those.

  11. Mark Schaffer

    I just wanted to mention that my wife and I do not, as a rule, use outdoor lights at all. They only blind you if you need to get up and look outside, interfere with the functioning of natural systems, and allow people to buy into the media hype about crime and violence. With all the other improvements we have done such as solar PV, CFL’s, energy star, clotheslines, change of habits we have kept our utility bills low.
    In Las Vegas, NV that means a high July bill of $86 for electricity, ~$13 for gas, and around the same for water. Don’t forget that pumping water is expensive from a electrical standpoint so don’t discount the effect of saving water on electrical use.

  12. Peter

    We live in Seattle. I’m happy to report that we actually did experience a sharp drop (48% – 50%) in overall gas consumption after replacing our 28-year-old forced-air furnace with a new, multi-stage furnace. After much analysis, I was persuaded that the new unit’s improved combustion and heat exchanger (among other features) would conservatively deliver 50% savings.
    FYI, we installed it in ~2000. It cost just over $5,000, a somewhat stunning number that was slightly offset by energy-efficiency incentives of about $500. I tracked the results for a couple years vs. baseline and they held up.
    Our only other gas appliance is our stove. I suspect our stove use has actually increased a bit due to family expansion (from zero kids to two kids).

  13. Eric H

    I too think it’s great you’re doing this. It’s way more information and knowledge than most people take the time to look at about their homes.
    This really highlights how challenging it is to understand the very coarse feedback on our energy use we get from the utilities on a monthly (or so) basis. Instantaneous feedback devices (as suggested in the comments above) can help you identify the big energy users when they turn on/off. They also help get a better understanding of how you use electricity in the house, leading possibly to better behavioral patterns. Some of the smart metering that has been part of the smart grid discussions will help present data in better granularity than monthly statements, but it still requires folks like you to pay attention to it.
    These things are hard. Building scientists use expensive datalogging systems and get paid to sift through big piles of data. Much of the time, the dynamics in houses are too complicated to really tell much from the efforts as well. One must do a lot of measuring to disaggregate the end uses and make the numbers meaningful. The variability in weather, daylight, home occupancy, occupant behavior, and the fact that things that use energy are being brought into and out of the houses over time all make this subject very challenging.
    Understandably, you’re frustrated as to why you aren’t seeing greater savings from the efficiency upgrades you’ve made. I recommend a feedback device. I’ve not used the Power Cost Monitor, but it seems pretty useful for it’s cost. The old TED devices were useful, but somewhat limited (that’s what I have in my house right now). The new 5000 series shows a lot of promise, especially with the computer/software interface.
    Good luck.

  14. Tom Harrison

    Mark —
    I did a little research a while back on how wasteful outdoor lighting is, as it rarely actually achieves the assumed result. Quite often, it does the opposite.
    Highway lighting often creates a greater danger than on unlit roads, due to the need for eyes to adjust. External lights on houses create a great pool of blackness where the house it not lit where people can hide (a fact well understood by the occasional neighborhood games of hide and seek played by the kids). Excess lighting messes with our natural rhythms and sleep patterns. I always enjoy our trips to Maine in the summer — they have actual stars up there; here in Boston, we have a huge bowl of light pollution.
    I also wanted to thank you for pointing out the cost of water use — as a city dweller, we get our water through a mostly gravity-fed system … although the subsequent sewerage treatment is far from free.
    We use resources willy-nilly because energy has seemed to be, effectively, free. I have an electricity bill like yours and have a way to go before my gas bill is as low. But the dollar cost, compared to everything else, is pretty small — heck, I now pay more for cable/phone/Internet than I do for electricity. Bizarre, indeed.

  15. Mark Schaffer

    Hi Tom,
    Nice to know others are paying attention to this issue but now how do we get DOE to also pay attention and push for light pollution reduction as a efficiency measure?

  16. Stacy

    My pwr bill goes from $150 in the heat of summer w/ house on 76 degrees to $365 in winter w/ house on 71 degrees and heating 50% of the time with wood stove. I have tried my best to figure what is going on and no one can tell me. I have a New Trane i19 energy eff a/c unit. Every thing in my house is electric.
    House was built in 1999. 1800 sq ft, 1 story.
    Only thing I can figue is washer and dryer as I have to wash every day, atleast 2-3 hours a night. Plus we are on a well pump.
    Any advice would be greatly appreciated.
    My pwr meter is digital so I cant just go turn on dryer and see the dial turn more.
    Oh yea, all bubls have been replaced with new energy eff bulbs.

  17. Adam Stein

    Hi Stacy,
    My guess is that electric heating is the problem. I’m not sure what else would account for the big swing between summer and winter. Maybe there’s some way for you to use a Kill-A-Watt to test the power draw of your heaters. Or you can get an energy audit done.
    – Adam

  18. Tom Harrison

    Stacy —
    You can read a digital meter just as easily as an old dial meter: just note the time, note the reading on the display, then wait a few minutes and repeat both to get W/hour.
    Given the significant difference in your usage, it’s probably worth spending a little money to buy an electricity monitor — BlueLine’s PowerCost monitor, the TED Energy Detective, and several new ones are available to read your power consumption all the time. If you’re using that much electricity, it probably makes sense to start understanding what’s soaking it up. TerraPass has some monitors, and (full disclosure: I work there) has a good roundup of monitors that range in price and capability.
    Not sure I agree with Adam on the use of the Kill-a-Watt (if you have one) — my heaters are wired into my electric box, and the Kill-a-watt is only useful for stuff you plug in. I recommend an energy monitor — I have been able to systematically reduce my energy use by seeing how I use electricity and finding little ways to cut down.
    In the last few weeks, for example, I realized that when we do two loads of wash, we can dry them both in one load in the dryer, and it only takes about 10% more to do both than it does to do each load separately. This is a significant weekly change for us. I have learned many other things as well over the last several years of monitoring electricity. It’s really pretty cool.
    And if you feel like you have learned all there is to learn, you can give or sell your monitor to a friend and pay it forward!