energy hook plots

The graph above shows monthly averaged temperature on the horizontal access and the BTUs per square foot on the vertical access for our house. I'm calling this a "hook plot" because, well, it looks like a hook! The BTUs include energy from gas and electricity. The graph is hooked because we have, interestingly enough, more energy consumption during the cooler, winter months than the warmer, summer months. The low at about 60 to 70 degrees are the shoulder seasons--spring and fall when we don't heat or cool the house.

The above plot includes only the months since we installed and activated the photovoltaics. Without the PV, the warmer side of the graph rises about 500 BTUs per square foot.

Now here's the hook plot for our old house, the 1880s shotgun in central Austin:

Yikes! The maximum BTUs per square foot of our new house are in the bottom range of the old house. That tells you what single pane glass and uninsulated walls get you. To be fair, the new house is about twice as large as the old house, but even there, the new house is kicking the old house's butt.

Anyway, I made these plots wondering if there was a solid relationship between temperature and energy consumption, which, of course, there is.

tracking power

The nudge for getting the wifi extenders up and running was to get a signal from the wifi network out to the garage such that we could hook the energy monitoring from the outbox to the interwebs. After a visit from one of Pecan Street's electricians (a nice dude that was in Crust as well as a number of other local bands I know about [as well as a friend of a friend]), we got the system all hooked up. Fortunately, the data systems Pecan Street uses stores its data, so although we were down for about six months, all the data was there for inspection. 

One cool thing about the system is the ability to see the contribution from our solar energy system (see graph at top where red is total energy consumption and green is energy generated from solar). You can pick out the annual cycle (summer energy use peaks), weekly cycles (we use more energy when we are home during the weekends), and daily cycles (more energy when we are home in the evenings). 

We can also see what our energy consumption and generation look like at various time scales as well as real-time (the bar on the right which shows, at that moment, more energy being generated than being used). 

The graph immediately above shows energy consumption from the dinner party we hosted Saturday night (to remember a friend who passed away 10 years ago) for about a dozen or so folks (lower temp setting on the thermostat, doors opening and closing [and sometimes not closing...], fridge opening and closing, lights galore, music). You can also tell when the party ended (about 1 AM). 

You can also see a pattern of higher energy consumption in the evening when we get home, and you can see how the bride lowers the thermostat right when we go to bed (she needs it COLD to sleep). The HVAC kicks on at a higher level for about an hour at to attain the target temperature, runs for an hour or so at a lower level, and then cycles on and off all night. The power consumption pattern of the HVAC has me wondering about the variable-cycle compressor we're supposed to have. This looks, by my eyes, like how a two-cycle compressor would work rather than a variable-cycle compressor. This has me wondering about the installation/programming of the thermostat...

Another neat thing hidden in the energy monitoring system is being able to track individual circuits to see how certain appliances are doing. It doesn't look like I posted about this earlier, but back in December the refrigerator went out for about two months (Whirlpool should be ashamed of itself, since it was selling defective merchandise in the hopes it would last through the one-year warranty). It was something of a slow death that took FOREVER to recover from with two sets of technicians employed before the problem was figured out. 

Here's the energy consumption history of the fridge:

You can see that something clearly started going wrong toward the end of November when the energy consumption doubled from about 70 watts to more than 150 watts. After the thing was fixed, it had a new baseline of about 80 to 85 watts. Pecan Street's electrician said this suggested that something was still wrong with the fridge (it's been noisier than it was before, that's for sure). You can also see that slowly, over time, it's consumption has increased to 100 watts, suggested it's on a slow road to death. Sigh... So much for having an energy-efficient (and quiet) fridge.

The novelty of this stuffs wears off fast. I hope at some point to investigate energy consumption about the house to try and optimize (and minimize) our consumption. This circuit-by-circuit stuff will be helpful in this effort. I also now know that when something electric is acting wonky to check out its energy consumption for clues on what might be happening. Pecan Street is using the data to monitor energy consumption and generation at a number of houses across town and the country. Once Tesla's batteries become available (the bride wants one bad...), I'm sure they'll be looking at those (and perhaps Pecan Street will have a deal on them for a study!). This also makes me want to get more solar cells. With the neighbor's tree down, we could get more solar and place it on the roof... 

monitoring power with the pecan street project

Lilly helping with the installation.

We are now part of the Pecan Street Research Institute’s project to better understand how people use electricity. The Streeters sent over a friendly electrician to wire up our circuits to a wifi interface that collects and sends data back to their central facility for analysis. They measure nearly every circuit in the house by placing small clamp-on ampmeters around each wire heading out of the electric box and into the house. These devices connect into a collection/transmission device which then shoots the info through the interwebs to Pecan Central. We can then log in to see what's going on with each of our circuits.

The data is collected in real-time, so if someone turns on a light, I see it show up on a graph like one of the ones below. Once we get solar installed, we'll also be able to see how much electricity we're generating and then take a look at the net. Pretty neat! I hope to wander about the house and see what phantom power we have being used about the house.

Hopefully in the near future they will start doing something similar for water!

Whole house energy usage for past month.

Here's what the refrigerator looks like. Spikes are for defrosting.

Guest bedroom; increased activity due to bride sleeping upstairs because of my surgery and new kittens. This will allow the Pecaners to see (1) when we have company, (2) when we have kittens, and (3) when we are having marital difficulties.

different housing:different power usage

Check out this graph. It shows BTUs per square-foot of conditioned space per month for our living situation over the past 10 years. Yep, that's right: I've been inputting our monthly energy bills into a spreadsheet since 2004 just for this moment. This plot encompasses three different living arrangements:

old haus: An old house built in 1886 and 1910 with no insulation in the walls and (mostly) single pane windows. Note that the biggest BTU spikes are for the winter when we use a lot of gas to keep the house warm.

apartment: A green built apartment in a high-rise; BTUs are artificially low because of cooling provided via a service that didn't allow us to include the energy costs.

new haus: The place we are in now!

Not surprisingly our new place is much more energy efficient, and perhaps maybe on par with the apartment? It will be interesting to see how this summer and winter go now that we have the HVAC working better...

howz that energy efficiency coming along?

A reader recently asked how the house is doing energy-wise. Good question! I still think it's premature to pass full judgement on how we did (I feel like we really need a year's worth of data and, given some issues we've had with the HVAC system, perhaps at least two years), but what the heck, let's see how we're doing so far...

conversions

Since we use multiple sources of energy, the first thing we need to do is combine the energy from electricity and natural gas into one number. That requires (in the U S of A, at least) converting energy use into BTUs (British Thermal Units):

1 cubic foot of natural gas equals 1,020 BTUs
1 kilowatt hour of electricity equals 3,412 BTUs

Note that natural gas may be reported as CCF: 1 CCF of natural gas equals 100 cubic feet of natural gas which in turn equals 102,000 BTUs.

our house

We currently have data for a seven month period from July of last year through January of this year. Using the above conversions, calculating an average monthly energy consumption per square foot (based on a 2,281 square-foot house) and then multiplying that by 12 to get a yearly number, we come up with a projected annual energy consumption of 28,876 BTUs per square foot.

how we compare

The plot below from U.S. World News shows millions of BTUs per occupant per year (the blue bars) and thousands of BTUs per square foot per year (red bar) and trends over time in the vintage of the housing stock. Not surprisingly, the trend is down as houses get more efficient. Since a metric based on the square-footage is a better metric of how a house is performing, that's what we'll focus on in this post.

(click on that sucker if you want to see it BIG)

For the most recent home construction (2000 to 2009), homes have had an energy consumption of about 37,000 BTUs per square-foot per year. Our place comes in at about 29,000, about 80 percent of the energy consumption of an average home built last decade. Nothing spectacular, but not bad.

According to the Department of Energy's 2010 Buildings Energy Data Book, an average American home (all home stock) used 94.5 million BTUs in 2005 (Table 2.1.10 of that report). Ours is currently at 66.1 million BTUs, about 70 percent of the average American home. Focusing only on the West South Central part of the country, the average household use was 82.4 million BTUs in 2005 with 56,600 BTUs per square foot. Compared to the average home in our part of the country, we use about half the energy per square foot.

Looking back to our old house built in the 1880s (and doubled in size to 1,100 square feet in the 1910s), it used about 83,000 BTUs per square foot. That means our current per square foot usage is only 35 percent of what we were using before! Despite having a house more than twice the size of our original place, we use about 30 percent less total energy than the old house.

The downtown apartment? We used about 12,000 BTUs per square foot. (Big giant envious) gulp. However, we had neighbors on four of six sides warming and cooling our sides. Nevertheless, apartment/condo living has a much smaller energy footprint.

Although we toyed with trying to build a passivhaus, we ultimately abandoned that (potential) goal. Nevertheless, how do we compare to that Germanic ideal? Annual energy use in the passivhaus standard for heating and cooling cannot exceed 4,755 BTUs per square foot; we are currently at about 11,000. Looks like we missed that one... According to passivhaus standards, total energy usage should be less than 38,000 13,300 BTUs per square foot. [See notes at end. I wrote Passive House US about the mistake on their site. No response... Seems pretty basic to get that number right, doesn't it?] That kinda shocks me and makes me wonder if the hausers made a conversion error somewhere (otherwise, what the hell are those Germans doing behind those thick walls and tiny closed curtains?!?!). I find this hard to believe because that would mean that half of newly built homes in the United States (assuming a normal distribution...) meets this particular passivhaus standard! And ultimately, who gives a hoot on how much energy you use for heating and cooling: Total energy consumption should govern the standard.

Assuming the hausers know what they are doing, we totally miss the passivhaus standard for heating and cooling but totally kill it on total energy consumption. Passivhaus in spirit?

the future

We've had some issues with the HVAC dumping air upstairs that surely caused us to use more energy than we should have (down to 60 degrees in the summer, up to 98 degrees in the winter!), so we're working to address those issues. That should reduce overall energy use. We're also in the process of replacing our halogen bulbs with LEDs (and yes, Green House Lady, we're installing a timer for our front "porch" lights...). We also plan to employ our energy measurer to see where there may be phantom power issues. Our current baseline power use (that is, power use without heating and cooling) is about 700 kilo-watt hours per month whereas it was about 270 in the apartment and 650 to 900 (growing over time...) in the old house.

a vaguely fascinating side point...

I was surprised at how high the electric consumption was for December. Our HVAC system uses hybrid heat: electricity for the heat pump as well as gas, and decides which to use. I'm not sure what the algorithm is, but I adjusted the thermostat to only use gas for heat in January. Indeed, electrical consumption was down and gas consumption was up for January resulting in a combined energy bill that was 50 bucks less; however, overall energy consumption (expressed as BTUs from electric and gas) was up! That leads me to think that the algorithm is optimized to minimize energy consumption and not cost.

postnote

March 10, 2014:

Here's a screen capture from the Passive House Institute US:

It appears the total energy usage of no more than 38,000 BTUs per square foot per year is accurate. Holy guacamole!

However, Building Science Corp reports a whole house energy consumption goal of 13,300 BTUs per square foot per year, which makes a lot more sense as a passivhaus goal. In that case, we're a wee bit beyond twice the goal.