Update on Power Usage – One-Year of Detailed Data

As of December 7th, the eGuage energy monitor has been installed for a full year, giving me the ability to know how much energy it took to heat our house for a full one-year period.  The data was actually a bit better than I anticipated:

Total kWh used to heat the house for the period December 7, 2014 through December 6, 2015:  2,549.6534

At 15 cents per kWh (winter rate) that resulted in a total cost of $382.45 (This includes all charges on our electric bill).

Knowing that natural gas is generally much less expensive than electricity, I thought it might be interesting to compare the two.  In other words, if we heated the house with natural gas, what would the cost have been?  Here are my calculations.  Feel free to let me know if you think my numbers are incorrect:

One kWh of electricity will generate approximately 3.14 kBtus of heat energy.  Therefore, our house used 8,007 kBtus of energy to heat the house during the one-year period.

8,007 kBtus equals 8,007,000 btus.

A “therm” of natural gas produces approximately 100,000 btus and costs approximately 79 cents (of course, this varies by location).

Therefore, if our house had a 100% efficient natural gas furnace, it would have used slightly more than 80 therms of natural gas for heat (8,007,000/100,000) and that would have resulted in a cost of $63.25 for the year.

Now, of course, we can’t use natural gas for several reasons; natural gas is not available where we live, they don’t make a natural gas furnace small enough, and a Passive House is so tight that burning any fuel in it is not advisable.  But I thought the comparison would help put the energy efficiency of a Passive House in perspective for those who use something other than electricity to heat their home.

Some other highlights from the one-year data:

The water heater used 715.516 kWh; about 31 cents per day.

The clothes dryer used 634.7463 kWh; about 28 cents per day on average.

The ERV used 369.02674 kBtus; about 17 cents per day.  However, it should be noted that the ERV was off for an estimated 50% of the three summer months because we had the windows open.

First Year Electrical Usage and Charges

Since we’ve been living in our home for just over a year, I thought I’d post another perspective on our use of electricity; this one based on the billings from the electric company.  It doesn’t contain the detail that is achieved through the use of the eGauge tracker; only the gross kilowatt hours used each month. And unlike the eGauge tracker, the electric company totals do include the electricity that I used in my shop to build cabinetry, etc. over the past year, which skew the results slightly.  At any rate, here are the results for the first year:

Download (XLS, 39KB)

Raw Data on Electric Usage and Temperature/Humidity

In this post, I’m including links to files that contain the historical electrical and temperature/humidity data for our house from February 1, 2015 through the date of this post, October 23, 2015.  The data begins on February 1st because although I started recording in early December 2014, the heat pumps were not working correctly until late January.  My intent is to offer this data to anyone who desires to use it for analytical purposes.

The electrical data, which was recorded by an eGuage energy monitor, contains daily numbers for total electrical use, the first floor heat pump, the second floor heat pump, the combined total of the two heat pumps, the dryer, the water heater, the ERV, the well pump, and the family room, which basically includes the lights and TV in that room.

The temperature and humidity data was recorded by Hobo data loggers that were placed on each floor (basement, first floor and second floor), and outside on the north side of the house under the porch roof.

eGauge Data 2-1-15 thru 7-31-15






I will update this data on February 1st of next year.

In early December (once I have a complete year of eGuage data), I will post the yearly total electrical usage for the appliances and mechanicals that are listed above.  As I indicated, the heat pump data won’t be totally “accurate” (because they were “short cycling” until the end of January).  But the information should still provide a reasonable indication of how efficiently the house is operating.

Energy Usage Update #2

Now that our first heating season is essentially over, I thought I’d provide an update on the cost of heating the house.  As mentioned in a prior post, I wasn’t able to track the energy used by the heat pumps until December 7th, when I installed the eGauge energy monitor. For that reason, I had to estimate the heat pump usage prior to that date.  To do that, I first determined our average non-heating electrical usage during December through April 22nd (the end date of our most recent electric bill) and compared that with the total electric usage in October and November, as reported to us by the electric company.  The difference is approximately the amount of electricity that was used for heat.  It’s not exact, but I’m confident that it is reasonably close to actual.  Here’s the actual summary from eGauge:

Screen Shot 2015-04-23 at 10.55.26 AM

So using the above data, and estimating the usage for the period before December 7th as described above, at our current rate of 15 cents per kilowatt hour this heating season cost us approximately $390.

As previously discussed, I put that in perspective by viewing it relative to our prior house, which is located about four miles away.  That house was larger (2800 square feet vs. 2000 square feet), had two-by-six walls with three-quarters of an inch of exterior foam and was heated with propane.  It took and average of 1,080 gallons of propane to heat that house each winter, at a cost of $2,430 per heating season (based on our most recent cost of $2.25 per gallon for propane).

So from a comparative perspective, given the facts discussed above, when adjusted for the difference in the size of the two houses, our current house costs us about 22% of the amount it cost to heat our prior house.

One other factor can’t be easily be quantified in dollars and cents.  Specifically, we had a programmed thermostat in our prior house, and kept the temperature at 69 in the evenings when we were home, and 61 when we were at work and through the night.  In our current house, we kept the temperature at 69 to 70 degrees the entire winter.

Energy Usage Update

As I said in the last post, I installed an eGauge energy monitor on December 7th.  That gives me the ability to isolate the energy used by the heat pumps. The two heat pumps have used a total of 899 kWh during the 53 days that have since passed.

Broken down further, the first floor unit used 682 kWh during the period, and the 2nd floor unit used 217 kWh.   I anticipate that this ratio will change significantly in the future because the 2nd floor unit idles most of the time, rather than cycling on and off every three to five minute as it did before the remote thermostats were installed.

So overall, that’s an average of just under 17 kWh/day.  At 17 cents/kWh, that amounts to $2.88/day.

During the period, the thermostat for the first floor has been set at 70 degrees, and the second floor thermostat has been set at 68 degrees for the most part.   According to the electric company, the average daily temperature was 35 degrees for the 35 days ending January 22nd.  The average temperature for the month prior to that was 41 degrees.

One side note…I spoke with the electric company after receiving the January bill, and asked if they had a special rate for homes with electric heat.  They do, and will be reducing my rate to 15 cents/kWh on my next bill.

First Full Month Heating and Energy Usage

We’ve now been in the house for about two months. But our electric bill runs (roughly) from the 21st to the 21st.  So last week we received our first “full month” electric bill.  Given that it’s the first full month, the information gleaned from the bill is, of course, very preliminary.  But I thought I’d share it anyway.  The important numbers were as follows:

Average Outdoor Temperature During the Period: 53 degrees

Total Electric Usage: 551 KWH

Total Cost for Electricity $90.94

Cost per KWH: 17 cents (all fees and charges included)

The last week of this billing cycle, it got fairly cold.  Nightly lows were as low as 19 degrees.  Daily highs were in the high 20s to mid 30s.  But again, the average temp for the month was a relatively mild 53 degrees

The house is two floors plus a basement.  Square footage is 1,000 per floor.

All lights are LED; 40 and 60 watt equivilants.  Most are 60s.  Only a few are 40s.

The house is completely electric.  Oven is convection.  Cook-top is induction.  Water heater is heat-pump.

We have two Mitsubishi mini-split heat pumps.  Both are 12,000 BTUs.  Each has one cassette.  One cassette is on the first floor.  The other is on the second floor at the top of the stairs.

For all but the last two days of the month, we used only the first floor heat pump.  Some days I didn’t use it at all.  Other days I left it on.  Mostly I was trying to get a feel for how it operated.  I was surprised to find out that the fan on the indoor cassette unit always runs (albeit at a very low rate), even when it is putting out no heat.

The single heat pump, kept the house “comfortable” down to the 19 degree low.  However, when the outside temp got below freezing, it appeared to put out heat all the time.  This kept the first floor at a pretty consistent 67 or 68 degrees at night and 68 to 70 degrees during the day.  The second floor temperature (which is where the bedrooms are) varied; at night it got down as low at 64 degrees, and during the day, with a little help from the sun, it rose to 68 or 69 degrees.

That’s why I put the word comfortable in quotes.  I found this to be quite comfortable.  After all, for us, almost all of the time spent on the second floor is spent under covers.  But my wife was not quite as content.  So for the last couple of days, I turned on the second floor heat pump.  This quickly brought the second floor up to 68 and significantly reduced the working time for the first floor heat pump.  Both heat pumps were set to 65 degrees. I’m not sure why a 65 degree set point on the heat pumps results in a 68 degree environment.  But it does.

A couple of things seem clear to me.  First, at 68 degrees the house is exceptionally comfortable.  Not a hint of a draft, even when the wind is howling. And second, a clear, sunny day makes a measurable difference in the form of a reduced heat load.

Next week, I will install an eGuage energy monitor on the main feed and 12 circuits.  So beginning then, I will have a much better read on the energy consumption for the heat pumps, water heater, and ERV.  I will also install Hobo temperature/humidity data loggers on all three floors and outside. This will give me the ability to monitor and report on the performance of the house over the long term.

Based on this very preliminary information, I have a few initial thoughts.  First, I was a bit disappointed in the first month’s electric bill.  Both the KWH used and the overall cost were higher than I anticipated.  But I’m thinking that this is due more to unrealistic expectations (and an unexpected 17 cent/KWH overall rate) than house performance.

One way to try to put this data in context is to compare this first full-month electric bill to my energy cost from one year ago, when we lived in our prior 2,800 square foot house, built in 1987, which used propane heat. The average temperature last year was 51 degrees; a couple of degrees cooler.  At that house, I followed our energy usage pretty closely.  Electric usage was easy (just read the bill).  But I had to gauge propane usage by my readings at the end of each billing period.  Each night, we set the thermostat back to 60 degrees and at least three days per week we left it at 60 degrees during the day because we were both at work.  We never raised the thermostat past 69 degrees.  But frankly even I thought that was uncomfortable.  It pretty much always felt colder than either of us would like.  At any rate, my total energy cost for that house during the same period last year was $428.43, and my electric usage was 881 KWH.  But remember, we were heating an additional 800 square feet.

It will be interesting to see how things play out in the long run.  Taking a SWAG based on this admittedly limited data, I’m thinking that when all is said and done, we have a good shot at being under 6,000 KWH total energy usage for the entire year.  That would result in a total energy bill of about $1,000.  And if it looks like that prediction is proving to be fairly accurate, I may start looking for a PV system next spring.

Energy updates will follow, once I get the monitoring systems installed.