I have spent a lot of time and thought on how to save electricity, but not as much on how to save natural gas. I got a lot of information right from the bills, but I used a cool measuring device to get to some important details.
I have a gas furnace and water heater, and also a gas stove. Sure, I can see how much gas I use from the bill. But what do I do with that information (other than pay the bill?)
I wonder how our gas and electrical usage compare? They are both in dollars, but how does that translate to energy? To get that I need to read the bills and convert to a single unit of energy. Following the excellent model of WattzOn … sort of — they measure power, in Watts — how much power you are using now, and at every moment (watts measures power, which has the time factor, or rate built in).
But here I am looking at the energy that I use over some period of time, like a day, or a month or an hour. So I have decided to measure energy. And so we can compare, I can convert to a standard measure: kilowatt-hours (think: 10 old-fashioned 100W light-bulbs, all on for one hour). When you’re talking about things that use energy like water heaters, furnaces, lights, refrigerators, and so on it’s more important to think of how much you use them in a given day (or week, or moth, or year). I’ll pick “day”.
Read Your Gas and Oil Bills
According to our utility bills from the most recent billing cycle:
- Electricity: 616 kWh in the 33 day billing cycle, or 18.6 kWh/day
- Gas: 180 therms in the 25 day billing cycle, or 7.2 therms/day, and 1 therm = 29.3 kWh, so 5274 kWh, or 211 kWh/day
Wow! I used more than 11 times more energy in gas than in electricity. (Maybe I should spend more time focusing on that, especially in the winter!). Ok, how about relative price?
- Electricity: 616 kWh cost a total of $137.69, or $0.22/kWh
- Gas: 5,274 kWh cost a total of $291.03, or $0.05/kWh
Yikes! Electricity is 4 times more expensive than gas. (Maybe I should convert my dryer to gas!)
But that’s as much detail as there is to be had from my gas bill. I have put a lot of effort into using less hot water, and we have also put effort into making our house tight and energy efficient. But where I have a Kill-a-Watt and PowerCost Monitor that we can use for real-time electrical use readings, I have nothing like that for gas. I don’t know how much my furnace, water heater and stove each use.
There’s No Problem That Cannot Be Solved With Legos
Legos? I’ll bet you thought I was going to say “coat hanger”, or perhaps “swiss army knife”. Nope! Read on.
I got creative and built a little device to record when my gas burner and water heater were on and off. And now I have a little information to look at that is far more instructive than a monthly bill. Here’s a summary of my findings:
During the gas furnace 24 hour test period, the burner was on for three periods, ranging from 1 hour to 2 hours in duration, for a total of 250 minutes (4 hours 10 minutes). It was an average of 30°F during the time I measured. The gas burner is rated to consume 130,000 BTU/hour, and thus consumed about 542,000 BTUs of gas, or 158.7 kWh (cost: about $7.93).
During the water heater 24 hour test period, the heater was on for seven periods, ranging from 10 minutes to 20 minutes in duration, for a total of 75 minutes, or 1 hour 15 minutes. The water heater is rated to consume 32,000 BTU/hour, and thus consumed 40,000 BTUs of gas, or 11.7 kWh (cost: about $0.59).
Kaazam! I used almost 14 times more gas for heat than for hot water. Said another way, more than 90% of my gas is used to heat the house … in winter.
But there’s more. Because I took readings in 5 minute intervals, I can see that my water heater was on 7 times during the day, for 5 to 10 minutes … except for two times during the day when it was on for 20 minutes. It took me a while to figure this out … then I realized that the test was on the day that we have our house cleaned, and they run a few loads of laundry — it seems like we need to post a note telling the cleaners to use the cold water wash cycle :-). So on a typical day, we probably use even less hot water than measured this day.
The furnace came on three times: once in the afternoon for about an hour (first pre-set of programmable thermostat), once again in the evening for about an hour (temp fell below set-point), and once again in the morning for about 2 hours (second pre-set of the thermostat). I see opportunity in this data — it’s pretty clear that in winter, our biggest opportunity for saving likely comes from heating. I think it’s time for a household energy audit.
The power of measurement and awareness never ceases to amaze me.
Details of My Lego Device
A while back, we bought a Lego Mindstorm robotics kit … for my son and daughter, of course. It has a bunch of sensors (distance, touch, light, sound, etc.), some motors, a little computer called the NXT, and a simple visual programming tool. You can make some really cool robots that move on their own and do various neat things. But you can also build a simple data recorder — all I needed was the light sensor and a little program to write a data file. Every 5 minutes, the program records the light level and saves it to a file on the NXT, and runs for 24 hours. For each test (water heater, furnace) I positioned the light sensor in place where it could “see” the flame of furnace or water heater. After 24 hours, I downloaded the file and read it into Excel and made some pretty graphs.
The main caveat in the data is the granularity of the measurements — it is possible that 1) each period during which there was a measurement, the measurement could have started 5 minutes earlier and ended 5 minutes later, and 2) if the flame was lit for less than 5 minutes, there may have been no reading. The former case is likely, and should be adjusted for the probable error; a typical reading is likely to be under-counted. The latter case, based on observation, does not appear to happen; that is both burners tend to turn on for 5 minutes or more. So I factored this error into my measurements.
Naturally, what I really want is a full-time, real-time device that records and displays data for these two units, as well as for our larger electricity users like the dryer. All on one pretty graph. On the web. It can’t cost more than $150. Personally, I think getting the data is the hardest part :-)
Here are some pretty (not!) pictures.