Archive for January, 2013

335 days and 6 slices of data

Composite heatmap, generation, usage, ashp, dishwasher, hdd

The image represents 11 months of daily data from February through December 2012. There are 6 vertical slices in order from left to right; Generation, Usage, Air Source Heat Pump, Dishwasher, All other and Heating degree days. Gray boxes indicate no data for that day.

Estimating heat energy for 2012

April 7 Update: Using a different method I estimated  that we used 903 kWh for heat energy in 2012, that’s 16% of our total energy use. That would have cost us about $117 (using $0.13 per kWh).

We don’t have a full 12 months of data for our heat, but using heating degree days (HDD) and performance thus far we can estimate that we used 591 kWh (+/-20%) for heat in 2012. That’s 11% of our total energy use for the year.

Here’s the math…

Our estimate relies on heating degree day measurements. This discussion assumes you are comfortable with HDD. If not, we recommend this excellent article.

First we had to determine the optimal base temperature for our HDD. After some trial and error using a quick prototype, we determined that 50 degrees (F) most accurately predicted heat energy usage. This results in the following formula to calculate kWh.

kWh = 0.2261 * HDD + 0.756

To start, we know the ASHP used 283 kWh during the time period March 16-May, and September-December. We just need to estimate January through mid-March.

We started recording temperatures in February so let’s start there. There were 522 HDD (base 50F) in February.

119 kWh = 0.2261 * 522 HDD February + 0.756

For comparison, there were 518 HDD (base 50F) in December. We used 108 kWh in December for heat. That’s about 208 Wh/HDD. We could assume that we would have used about the same amount of energy in February, although we generated twice the amount of electricity in February, which means we would have used a lot less energy for heat because we were getting heat from the sun. But let’s use the more conservative estimate for now, 119 kWh.

We don’t have our own temperature data for January but we can use to find the HDD at any base temperature for January in our area. Albany International Airport has the closest matching temperatures. Using a base temperature of 50F degreedays tells us there were 654 HDD (base 50).

149 kWh = 0.2261 * 654 HDD January + 0.756

For comparison we could multiply the 208 Wh/HDD measure from December above times 654 HDD to get 138 kWh. This number is lower but since the temperatures were cooler in January than December we know the ASHP would have to work harder to make heat, so the higher number makes sense. January was about 23% cooler than December. If we add 23% to 138 kWh, we get 170 kWh. Not very scientific but if gives a sense of margin of error.

As for March, we only have the second half of the month’s data for the ASHP, which used 19 kWh. There were 90 HDD from March 16 through the end of the month. That’s 211 Wh/HDD. As a check we can plug 90 HDD into the formula to get the estimated kWh.

21 kWh = 0.2261 * 90 HDD + 0.756

It’s pretty close.

There were 174 HDD from March 1 to March 15.

40 kWh = 0.2261 * 174 HDD + 0.756

Again for comparison, 211 Wh/HDD (from above) * 174 HDD = 37 kWh. Pretty close, it was twice as cold and about twice the amount of energy use.

40 (3/1-3/15) + 19 (3/16-31) = 59 kWh total for March.

Now we can estimate our total heat energy for 2012.

283 (3/16-12/31) + 149 (Jan) + 119 (Feb) + 40 (for the missing part of March) = 591 kWh for 2012. That works out to $77 for heat, not counting delivery charges. 591 kWh represents 11% of our total energy use for the year. That seems about right since the ASHP represents 9% of our total energy use for the time period we have circuit level data.

There is at least a 20% margin of error for the estimate and that’s not even taking into account space heat energy contributed by the sun.

Chart showing HDD vs ASHP kWh use.

Polynomial curve fit analysis

I’ve also been looking at a polynomial fitted curve to better estimate kWh based on HDD. This makes some sense because heat pumps are more efficient at higher temperatures and less efficient at lower temperatures. A linear regression analysis would not be able to capture that type of operating behavior.

* Note: Electricity supply cost calculated using the last bill we payed for electricity, May 2011.

We’re net positive for 2012!

Pie chart showing circuit usage YTD

Bar chart showing circuit usage per month

Year Summary

We’re net positive for 2012! We used 5,601 kWh and generated 8,856 kWh for a net surplus of 3,256 kWh. We used an average of 15.3 kWh per day.

We’re within 6% of our projected energy use of 5,950 kWh for the year. It also looks like we used about 80% of the estimated 2,440 kWh for hot water energy. It’s too early to tell how we’re doing against our heat energy estimate since we missed recording the primary heating season. We should know better by mid-March.

But we can calculate the current BTU/SF/HDD, which is a common measure to compare the efficiency of houses of different sizes and climates. First we need to know the total heating degree days (HDD) and heat energy usage for the period we have data. We recorded 5,885 HDD (base 65F) for 2012. 3,434.800 HDD were recorded when we had circuit level data for the ASHP, which used 277.331 kWh for heating in that period.

Now we convert kWh to BTUs,  277.331 kWh * 3412.14163 (kWh to BTU conversion factor) then divide by 1408 SF and divide again by 3,434.800 HDD (March 16-Dec 31, 2012) = 0.196. This is not a completely accurate number as it does not include Jan 1 through March 15, but soon we will have a full year of circuit level data and a more representative number.

We used an average of 56 gallons of water per day. 34% of our total water usage was hot water.  For the time period we have circuit level data that works out to 258 watts per gallon of hot water. I’ve found some estimates on the web that say we should be using about half that wattage to heat our water, so we’re going to do a bit more investigation.

December Summary

In December, we used 668 kWh and generated 302 kWh for a net deficit of 366 kWh. We had nine consecutive surplus months. This is the first month since February that we used more than we generated.

We used 152 kWh for heat, a 227% increase over November. Dividing 152 kWh by 979 HDD we get 155 watts per HDD. That’s a 182% increase from November. Not only are we using the heat more, but the heat pump has to work harder because it’s colder outside. Heating accounts for 22% of the month’s total usage. Hot water accounted for 32% of the energy for the month. We used 65.9 gallons of water per day, up 7% from the previous month.

The largest energy increase in the last few months has been in the All Other category, followed by the water heater, ASHP and the stove. Most of this makes sense. We’re spending a lot more time indoors. The holidays bring visitors, more showers, more laundry and lots and lots of cooking!

We generated 302 kWh, roughly half of November’s production. It was down 24% from the estimated December value of 399 kWh. A very cloudy and snowy month. In fact we only generated 116 watts the last 5 days of the year due to snow blanketing the collectors.

Usage Solar PV Net usage or (surplus) Avg. daily usage HDD3,4
Total 5,601 8,856 (3,256) 15.3 5,885
Jan1 873 369 504 28.2 1,1255
Feb2 666 597 69 23.0 957
Mar6 515 860 (345) 16.6 619
Apr7 925 378 (538) 12.9 535
May 309 867 (558) 10.0 170
Jun 325 939 (614) 10.8 97
Jul 319 970 (651) 10.3 17
Aug 315 1,018 (703) 10.2 31
Sep 292 883 (591) 9.7 168
Oct 433 511 (78) 14.0 357
Nov 498 615 (117) 16.5 830
Dec 668 302 366 21.6 979
All values in kWh (except HDD).
1 January values based on meter reads.
2 February values based on TED data.
3 Heating Degree Days, a measure of how many outside degrees in a day it is below a base target inside temperature, 65F.
4 Calculated from our HOBO outdoor weather monitor hourly data, unless otherwise noted.
5 January HDD data downloaded from, Station ID: KALB (Albany International Airport).
6 March values based on meter reads. (TED died March 1st, eMonitor installed March 16, 2012)
7 Values starting in April are based on eMonitor data.

Next month we’ll start including information gathered from our two additional temperature/humidity sensors. We will now be able to see how temperatures differ from floor to floor.

You can see heat maps and detailed charts of temperature and electrical usage at View solar, usage, net usage, temperatures and HDD for all of February and circuit-level data for 16 days in March and the full months of April through December.

Happy 2013 everyone!

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