MattG;

They would not release the bills because of privacy issues, but you could try contacting your power company to see if they would release the previous KWH usage. I know they keep those records because Progress Energy continues to monitor our 2003 Aurora winning home.

Thanks for the suggestion.  For now, I feel like I have adequate base information, I'm just looking for how to best use both the data I have and that data that I will collect within the next year or so.

Posted By cmkavala on 31 Dec 2010 09:56 AM
MattG;

They would not release the bills because of privacy issues, but you could try contacting your power company to see if they would release the previous KWH usage. I know they keep those records because Progress Energy continues to monitor our 2003 Aurora winning home.

I know that our local utility co. does this, so it may be a fairly standard thing.  At one point they were "advertising" that you could get average usage/monthly costs if you were looking to buy a home - find out what it costs to heat & cool before you buy.  The problem with this is that you don't really know how the people before you compared - if it was a family of 4 and you're a single guy, your usage is going to be much different.  There's probably still value in it, but I think you'd have to take it with a grain of salt..

jerkylips;

I think you need to step up to the plate and in the interest of science "take one for the team" ........... go out there and get a wife with 2 kids for a fair analysis

Posted By cmkavala on 05 Jan 2011 10:24 AM
jerkylips;

I think you need to step up to the plate and in the interest of science "take one for the team" ........... go out there and get a wife with 2 kids for a fair analysis

ha!  I do have a wife, and actually, kid#1 will be here in about 7 weeks!!  If anyone was "taking one for the team", it's my wife....

Is there someone here who can answer my initial question? How to track KWH usage as tempered by HDD/CDD data over a period of time?

I'm not interested in doing some long drawn out whole house energy analysis, nor do I care about data from before I owned the house as I have no idea what the previous residents living habits were.

What I'm looking for is someting like: x / y =z now plot z over the period of time the data as been collected thus far.  I can plot it out in Excel once I get some direction....  TIA

The data files are linked to in the first post.

I'd make three changes before I proceeded:

First, I'd use more a more accurate guess than 900 kWH / month as a base. I'd use the number of actual days, so 30kWH * #ofdays, and I might even be tempted to use different numbers for weekdays and weekends.

Also, since you have a heat pump, I assume that it is more efficient when the outdoor temperature is closer to the indoor temperature. So, your efficiency is lower at 40 degrees outside than 55 degrees outside. I'd try to quantify the "nature" of the degree days before I tried to somehow graph the data.

And finally, I'd try to allow for times that you were on vacation and the house was dormant.

Jeff

Your data sheet indicates a rather inconsistent measure of energy (kwH) consumed per HDD. You've flagged the high reading for Jan 2010 as maybe due to leaving the attic door open. I do see consistency between the December rates for 2010 and 2011. If you are satisfied that these are representative of your normal living, you could use 1.6 kwH/HDD as your baseline prior to the insulation upgrade. Continue to fill out the chart going forward and you should see some amount of improvement depending on other factors affecting your home's energy efficiency, e.g. quantity and quality of windows and doors, air infiltration rates, etc. I didn't catch how many square feet of conditioned space you are heating, so it's not clear how energy efficient your house was before the new insulation was installed. If your house was already pretty energy efficient, going from R30 to R50 in the ceiling should produce a measurable improvement. If not, you may not be able to detect a difference.

I can definitely help with your statistical analysis, if you need it.  Here is what I would do:

Regress your kWh onto your hdd and cdd variables.  You will also want to include a variable that indicates before and after insulation was installed.  You can do this by simply adding a column with a 0 for each month that you had R30 and a 1 for each month that you had R50 (starting this month).  You will want to collect at least 1 years worth of data since installing more insulation.

By regressing kWh on hdd and cdd you will get a slope that tells you the relationship between hdd and cdd and kWh.  You can use this slope to predict future kWh usage depending on specified hdd and cdd. 

I can either calculate this for you or attempt to walk you through the calculation, if you don't already know how to do it.

I went ahead and calculated some statistics based on the data you gave in the first post.  The analysis I ran was for kWh usage prior to installation of more insulation.  Furthermore, I used the raw unadjusted kWh usage indicated in column D of the spreadsheet.  I did this to get an idea of the actual percentage of use you heating and cooling is accounting for based on hdd and cdd.  Here is what I found:

Your overall kWh mean was 1768.353.  So, on average you used 1768.35 kWh per month over the course of the 17 months you lived there prior to more insulation being installed.

HDD and cdd accounted for 76.7% of the variance in your kWh usage per month.  This loosely indicates that your heating and cooling are consuming roughly 77% of your kWh in any given month.  The rest of the kWh usage would be accounted for by anything that uses electricity other than your hvac system.

HDD was a slightly better predictor of kWH usage (76%) than was cdd (62%) in any given month. 

Looking at the slope of each variable indicates the following:

hdd slope = 2.986
cdd slope = 4.892

Each value was calculated while holding the other value constant at it's mean.  So, on average, for every 1 point increase in hdd your kWh/per month is going to increase 2.986 units while controlling for cdd that may be occurring in the same month.  For every one point increase in cdd, your kWh usage increased 4.892 units while holding hdd constant in any given month.  These values indicate that you used substantially more energy to cool your house than to heat it.  This potentially makes sense given the way heat pumps work.

We can calculate these differences again next year to see how installing insulation improves your energy consumption (that is assuming everything else is held constant - don't change out a bunch of other things between now and next year).

In the mean time, you might want to calculate the expected energy efficiency improvement based simply on your having upgraded the attic insulation from R30 to R50. There are online heat loss calculators based on simple heat transfer models that can approximate the energy savings you will acheive.  Typically these kinds of tools are used before deciding to upgrade insulation to determine the payback period for a given dollar investment, but you can use them to estimate your cost savings this winter.  The Home Heat Loss Calculator at Build It Solar should do the job.  You can build a heat loss model of your house with R30 insulation in the attic and another with R50, with all other parameters held constant.  The energy savings can later be compared to your actual electrical consumption data gathered over the next several months.

OK - great! This is the kind of feedback I was looking for.

Jeff: I modified my spread sheet using 32KWH/day as a base rather than a monthly number. Re" try to quantify the "nature" of the degree days " I think that may be key but I need to think it out some. Not sure if I want to do anything with the vacation time... we leave the programmable thermostat at the same setting and we have a pet sitter so the house is occupied.

Clark: Re "Your data sheet indicates a rather inconsistent measure of energy (kWh) consumed per HDD." Yes - that is the conundrum. Re the house, it is a 5 YO 2700 sq ft ranch with a bonus room and I'd rate it at above average construction as it has a sealed crawl space (foamed), low-E windows, most of the HVAC ducts are on the conditioned space, and it has radiant barrier roof sheathing. The doors I'd rate as average. The builder built other Energy Star certified homes exactly like this one, although this one is not, which is perhaps because the original home buyer didn't want to pay for that. I'm really not sure if this insulation is going to make much difference or not, although I have about $650 into it so if there is a savings the payback period shouldn't bee too long. OTOH, I may conclude that I really don't have enough (or high enough quality) data to quantify the true effect.

One way to "quantify the nature" of the days would be to divide the actual HDD for each day by the COP (is that the correct term?) of the heat pump at that temperature.

So, for example, if your deltaT for Jan 1 is 20 and your COP for "20 degrees" is 3.0, then your "load" for Jan 1 would be 20/3.0 = 6.66. Then, if on Jan 2 the deltaT is 30, and the COP drops from 3.0 to 2.5, the "load" would rise to 12.0. Those numbers - 6.66 and 12.0 - put the days into approximate correlation as to the heating electricity required. (It would be better to do hours, probably, but that would be a lot more complicated, and then you have solar issues and thermal lag and other issues to consider...)

If you were to do this for an entire month, then the totals of the "loads" for one month should be somewhat closely proportional to the heating (or cooling) electrical expenditure for those months.

Keep in mind that I have no idea what normal COPs are, or what they look like. But any ratio of "energy in" to "energy out" - regardless of the units - should work.

Jeff

McKibbe:

First let me say thank you very much for taking such a thorough look at this and for taking the time to perform the calculations. 

Let me say though that it has been at least 30 years since I've been in a college math course, so I am having a little trouble wrapping my brain around what you have posted.  You did confirm what I first noticed when I started looking at the numbers - cooling is more expensive than heating.   

Regarding heating and cooling in the same month, let me say that our programmable thermostat has an auto setting that automatically switches between heating and cooling.  We never use that though.  Rather sometime in the fall we switch the thermostat from cooling to heating and in the spring, the opposite.  So on days when there is say a low temp of 55°f and a high of 80°f we would not run both the heating and cooling.  I'm sure it was an oversimplification, but therefore if, for example in a given month we had 300 HDD and 100 CDD I "threw out" the CDD. 

I guess the reality of it is that in order to track the actual energy usage of a house it would have to be a strictly controlled environment - ie - people not living there.

RE "don't change out a bunch of other things between now and next year). " no other changes are planned or likely.

Hi Matt,
No Problem.  I am happy that I was able to help.

I understand that the statistics I presented are a bit difficult to grasp.  Furthermore, given your explanation for how you switch the thermostat in the spring and fall they are not entirely accurate, either.  I can update the analysis to reflect when the change occurred if you will indicate in your data file in which months you switched from hdd to cdd and back.

I can also try to explain things a little more clearly.  Please let me know what you picked up and what was less clear.  Also, please let me know what you would like to know from the data.  I originally wrote the post using language that somebody with some statistics background would understand.  If it has been a while since you have encountered statistics I can understand why it might look like Greek!

Finally, you have all that awesome cdd and hdd data.  I assume that you don't have interval kWh data, otherwise I guess you would have posted it.  I think it would be interesting to model the relationship between cdd and hdd over time if somebody has interval level kWh data that they want to share.

Eric

Posted By Clark on 12 Jan 2011 08:42 PM
In the mean time, you might want to calculate the expected energy efficiency improvement based simply on your having upgraded the attic insulation from R30 to R50. There are online heat loss calculators based on simple heat transfer models that can approximate the energy savings you will achieve.  Typically these kinds of tools are used before deciding to upgrade insulation to determine the payback period for a given dollar investment, but you can use them to estimate your cost savings this winter.  The Home Heat Loss Calculator at Build It Solar should do the job.  You can build a heat loss model of your house with R30 insulation in the attic and another with R50, with all other parameters held constant.  The energy savings can later be compared to your actual electrical consumption data gathered over the next several months.

Clark:

Thanks much.  I spent some time with the Home Heat Loss Calculator and was somewhat dismayed although not at all surprised that it set the yearly payback from going from R-30 to R-50 at only ~$25 /yr.  That is only for heating though, plus as I said before the whole reason I did the 30 -> 50 upgrade was because the R-30 was poorly installed.  That, combined with the fact that the that web page only deals with heat loss, and not the cooling (CDD) aspect, I'm optimistically looking at very~$50/year payback.

OTOH, that Home Heat Loss Calculator web page is definitely home grown....  and suspect.  I noticed that it was assigning the same values to "ceiling loss" and "wall loss" for the same number of sq ft/r-value.  I think the first thing I ever learned about insulating a building (maybe 35 years ago) is that something like 60% of the loss is through the ceiling and 40% is through walls, hence, we typically insulate ceilings at ~2x the rate of walls.  When using the Calculator I suspected something was a little funky so I did a little test where I entered 1000 sq ft of walls, 1000, sq ft of ceiling, no windows, and R30 for both walls and ceiling.  Sure enough - heat loss values given were equal for walls and ceiling.

Another thing to think about....  In a green building certification class I attended, during the marketing chapter - which BTW was the last chapter in the book - I think it should have been the first chapter....  "If you can't sell it you can't build it", what the instructor told us was don't sell payback - because more often than not it's difficult if not impossible to make the $ #s clearly support the case for energy/green upgrades.  His postulation was to sell added comfort, quality of life, enviromental responsibility, and simply the satisfaction of owning something better....  and by the way he is the only really successful green builder I personally know locally.  There is a reason people who can afford it buy a Lexus rather than a Toyota - even though they both you get you from point a to point b, or as my instructor put it - the $5k bath tub.  So, just knowing that my home is better than many is satisfying in itself - especially based on the fact that I'm a building professional.

I know... not what people hear want to hear...

Matt:

You might be right about Home Heat Loss Calculator's accuracy, but I'm not sure you should have seen more heat loss through the ceiling in your 1000 sf model if convection losses are ignored. I think the reason more insulation is installed in the ceiling than in the walls is that the temperature difference across the ceiling is often greater than across the walls. The delta T and the R-value determine the conduction losses. In the summer, when the attic can rise to 120F in my area and the delta T goes to 40+ degrees, the conduction heat loss per sf is double that across the walls. In the winter, it's another story. For a ranch with 8' or 9' ceilings, depending on the interior ventilation present (e.g. ceiling fans), the interior air temperature is at worst a couple of degrees higher at the ceiling than at the floor. And, the attic temperature is typically a couple degrees warmer than the ambient outdoor temperature. In this situation, more heat is conducted through the walls. The bigger problem in the winter is the heat loss due to convection where the higher pressure differential across the ceiling means that any little leak will allow heat flow to the attic. Convection heat losses can dominate your home's energy efficiency rating.

I, too, had poorly installed insulation in the ceiling of a former home. After crawling around the attic (ugh!) taping tears in the insulation facing, foaming ceiling fixture junction boxes, and caulking around ceiling penetrations, my heating bill dropped dramatically. It was awful work, but the out of pocket expense was trivial. I didn't upgrade the amount of insulation in the ceiling (it was R38), because the payback period was so long. As for cooling costs, we only cooled that house during the months of July and August. I did improve the attic ventilation by adding some vents at the roof ridges to help hold down cooling costs, but cooling costs in northern Illinois aren't nearly as high as in North Carolina. I think you're going to see payback for upgrading to R50, though. Your savings will come primarily during the long hot NC summers.

BTW, when I built my "green" house, I concentrated on reducing air infiltration in every way I could think of. My 2200 sf ranch house has triple glass windows, R22 walls, R45 ceilings, and full basement with R22 walls and R10 floor. The way I did the ceiling produced an absolutely leak free ceiling. Last month, December 2010, we experienced 1292 HDDs. My natural gas consumption was 120 therms, or 12 million BTUs. My heating energy efficiency came to 2.1 BTU/SF/HDD, and my December natural gas bill including local taxes came to $85. We keep our main level at a constant 72F and the basement at 68F. I maintain the indoor relative humidity at 38% during the winter. I have never lived in such a comfortable house. The extra cost of construction was worth every dollar.

Eric-

When you say "...if somebody has interval level kWh data...," over what interval would it be useful to have data? Hourly? Daily? I do not have such data, but am slowly adding instrumentation to my house, and would be interested in your opinion (and other folks opinions).

Thanks,
Lee
www.ResidentialEnergyLaboratory.com

Hi Lee,
I know very little about the types of instruments that are out on the market that can provide the type of data I mentioned.  That said, I have read about "Smart Meters" that can collect kWh data at various intervals including every minutes on up to once daily.  This type of data paired with the hdd and cdd data that is available on the site Matt listed would allow one to model energy consumption over time as a function of R values in a house.  It would go a long way toward helping individuals understand their energy consumption and provide some hard data showing ways to reduce energy consumption.

I encourage you to research these "Smart Meters" more thoroughly and let everyone know what you find!

Eric