Archive for the 'Performance' Category

January performance snapshot

Living in a super-insulated house, I get very excited when extreme cold weather arrives. It is the only way to see how well the house performs in low temperatures. This January was the coldest month that we’ve lived in the house.

We recorded 1380 HDD for January, 16% colder than January 2013. Our coldest day was January 3rd, with 69.7 HDD. Temperatures on this day ranged from a high of 2.8°F to a low of -8.5°F. Indoor temps ranged from 60°F at night to 70.5°F during the day.

I was particularly interested in how our air-source heat pump (ASHP) would perform in these long stretches of cold weather. Our unit (Mitsubishi MSZ/MUZ-FE18NA) is rated to keep producing heat down to -15°F. According to the specifications, the unit can produce 10,300 btu/hr at 5°F.

In order to determine how well the house performed thru the month I decided to keep the backup heat (electric resistance) off. This meant we relied only on our ASHP and the sun to heat the house for the month.

Looking at the data for January, it looks like our heat pump starts to fall behind demand when temperatures slip below zero. This appears to happen mainly at night. During the day, a small amount of sunlight can raise indoor temps even in the extreme cold. January 1-3 offer a good example.

I normally turn the heat down at night or off. I did this the night of January 1st. Night temps were not particularly cold, but continued to drop throughout the next day. By 7am Jan 2nd, the inside temp got down to 60°F and the outside temp had reached zero. By the time I turned on the heat again in the morning, the ASHP was not able to make up the difference for most of the day. A little sun helped get the inside temp up to a high of 64°F. On Jan 3rd, I considered turning on the backup heat, but I held out because the forecast said clear skies. By the afternoon inside temps were back up to 70 while outside temps hovered around 2°F. By Jan 4th, outdoor temps were well over the zero mark.

The January 2nd experience and a fortuitous conversation with Mike Duclos that day, convinced me to leave the thermostat set to 68°F when temperatures are forecast to be in single to negative digits overnight. This makes it easier for the ASHP to keep up with the demand and lowers the temperature differential it has to make up.

In total, our ASHP used 529 kWh in January. If we paid for electricity, our heat would have cost us just under $80.

2014-01-02-daychart 2014-01-03-daychart 2014-01-04-daychart

Check out the interactive version of these charts on my other site, Netplusdesign.

Fourth quarter 2013 performance

Q4 2013 summary: 10% cooler, 22% more usage and 10% more sun as compared to Q4 2012.

Now that we are starting to collect our second year of performance data, I’ll be comparing our new data to the same period last year. Year-over-year comparisons should be more illustrative than comparing to previous months. I’m also switching to a quarterly reporting period to look for larger trends, but I will continue to post data monthly at netplusdesign.com

In Q4 last year our total usage was 1,599 kWh. This year is was 1,957 kWh, a 22% increase. Despite snow covering the collectors for a few days and cloudy weather, we still produced 10% more energy in Q4.

Overall, here’s how this quarter and the year compare to 2012.

Chart comparing Q4 2012-2013 energy and temperature values

2013 Usage Solar PV Net usage
or (surplus)
Avg.
daily usage
HDD
Total 1,957 1,576 380 21.2 2,372
Oct
482 674 (192) 15.6 405
Nov
559 639 (81) 18.6 838
Dec
916 263 653 29.5 1,129
2012 Usage Solar PV Net usage
or (surplus)
Avg.
daily usage
HDD
Total 1,599 1,428 170 17.4 2,166
Oct
433 511 (79) 14.0 357
Nov
498 615 (117) 16.6 830
Dec
668 302 366 21.6 979
All values in kWh (except HDD which is base 65°F).

We used 22% more energy (358 kWh) this quarter as compared to 2012. That’s about 3.8 kWh extra per day, or 158 watt hours per day more than last year. The last two quarters we’re been running 4 to 5 kWh higher per day. Last quarter I predicted this quarter usage would also likely be up 4-5 kWh/day from Q4 2012. It looks like we stuck to the lower bound, same as last quarter.

One noticeable difference this quarter was our heat usage. Q4 2012 our air-source heat pump used 205 kWh. This quarter we used 438 kWh, a 114% increase. It was about 10% cooler than Q4 2012, but we also had 10% more sun.  It’s not really clear to me at the moment what is causing the increased usage. I’ll have to do a bit more detective work. Overall for the year we would have paid $180 to heat and cool our house, if we paid for electricity.

Chart of ASHP usage values 2012-2013

Cumulative plot of ASHP usage values for Q4 2012 and Q4 2013

Water-wise our usage is down 5%, and our hot water usage is down 13% from Q4 2012.

Year over year comparison

Looking back over 2012 and 2013, most circuits we track stay about the same over time, excluding heat. The one set of circuits that have changed a lot over the last two years are in the category of ‘all other circuits’, meaning the circuits we do not measure individually. This category counted for 43% of our overall usage in 2013. This includes plug loads, lighting and ventilation. (Ventilation is only used from late fall through mid spring.) Plug loads include the home office, the TV, stereo, refrigerator, freezer, coffee maker, incubators and heat lamps for chickens in the spring, the Prius plugin, barn lights, electric fence, water bucket warmers, etc.

Chart comparison of 'all other circuits' 2012-2013

You can see that this category has steadily grown from 100 kWh/month to peaking in April at 330 kWh/month. I’m hoping we’ve leveled off, averaging around 250 kWh/mo. We will still have a peak in the hatching season, until the chickens take over this work for us.

Happy 2014!

You can see heat maps and detailed charts of temperature and electrical usage at netplusdesign.com. View hourly, daily and monthly values for solar, usage, net usage, temperatures and HDD.

We’re net positive for 2013!

Pie chart of energy usage by circuit 2013 Stacked bar chart of energy usage by month 2013 chart of monthly temperature ranges 2013chart of temperature buckets with ASHP usage overlay 2013chart of water usage 2013

chart of grid vs solar supply mix for 2013

Solar we were able to use directly vs. solar that was sent back to the grid

Year Summary

We’re net positive for 2013! We used 7,206 kWh and generated 8,575 kWh for a net surplus of 1,368 kWh. We used an average of 19.7 kWh per day. We used 29% more energy and generated 3% less than last year. 2013 was 16% cooler than 2012. The coldest day of 2013 was Jan 3 at -7 degrees F, and the coldest day overall was Jan 24 at 60.8 HHD.

We’re 21% over our projected energy use of 5,950 kWh for the year. We used about 85% of the estimated 2,440 kWh for hot water energy. We used 26% of our heat energy estimate for the year.

Our BTU/SF/HDD is 0.425. BTU/SF/HDD is a common measure to compare the efficiency of houses of different sizes and climates. We recorded 6,810 HDD (base 65F) for 2013. The ASHP used 1195.782 kWh for heating in that period. Converting kWh to BTUs, 1195.782 kWh * 3,412.14163 (kWh to BTU conversion factor) then divide by 1408 SF and divide again by 6,810 HDD = 0.425.

We used an average of 66 gallons of water per day. 33% of our total water usage was hot water.  It takes about 264 watts to heat each gallon of hot water and 2.644 watts to pump it from the well.

That’s it for 2013. Happy 2014 everyone!

Our NESEA Zero Net Energy Building Award Submission

Presentation Board

Click to view the presentation board (PDF, 5.4 MB)

We submitted our entry materials for the NESEA Zero Net Energy Building Award yesterday. We didn’t quite have a full year’s worth of data for last year’s competition. So we were excited to apply this year. You can find out more about the requirements for the competition on the NESEA site.

In putting together the data for the competition, I realized I needed to update a few items on our About page.

First, square footage. We had listed the house as 1408 sf. This is the outside dimension of the house (32*22) times two floors. Technically we should include the basement, which would bring the total to 2,112 sf. Total conditioned space remains the same at 1,200 sf.

Second, price per square foot. We had estimated $142 per square foot. When we added up the final tally last year, I divided by the 1,408 number rather than the 2,112 number, so we ended up with an actual of $208/sf. But when I went through all the numbers again I found a few extra expenses and realized I divided by the wrong number. So using the revised numbers, we end up with $142/sf, strangely what we estimated originally. (In hindsight, that had to be total luck.)

That $142/sf does not include significant site work. We didn’t pick a nice easy flat site next to the road to build our house. But if you built this house to similar specs on a nice flat lot near the road and didn’t have to drill your own well or install your own leach field, then this about what what you would pay if you did a lot of the design and labor yourself.

One last thing. The submission requirements included calculating the Energy Use Intensity (EUI). This is measured in kBTU per square foot per year. The fun part is converting kWh to kBTU. This is problematic for an Air Source Heat Pump, because the kWh required to create BTUs increase as the temperatures decrease. But as a general measure I suppose this is helpful. It might also be a trick question to check if the person submitting the entry knows what they are doing. Hmmm, hope I answered correctly.

Here’s how I determined our EUI. Our total energy use for the last 12 months was 6,898 kWh. To convert to BTUs we multiply kWh by 3412.14163312794 BTUs which gives us 23,536,952.9853165 BTUs. Divided by 1,000 to get kBTUs is 23,537 kBTUs. Now divide by square footage. It doesn’t specify whether this is conditioned space or total enclosed space, so I chose conditioned space. That gives us an EUI value of 19.6. If we use the larger enclosed space value then we get 11.1. The lower the value the better. Maybe I should have used the 11.1 value.

The winners are announced at the BuildingEnergy Conference in March. Wish us luck!

P.S. If you have built a net zero house in the larger New England area and have a year’s worth of energy data to prove you’re net zero, then apply soon. The deadline is December 15.  Good luck!

Third quarter 2013 performance

Q3 2013 summary: 36% cooler, 38% more usage and 2% less sun as compared to Q3 2012.

Now that we are starting to collect our second year of performance data, I’ll be comparing our new data to the same period last year. Year-over-year comparisons should be more illustrative than comparing to previous months. I’m also switching to a quarterly reporting period to look for larger trends, but I will continue to post data monthly at netplusdesign.com

In Q3 last year our total usage was 927 kWh. This year is was 1,283 kWh, a 38% increase. Although it feels like it had rained a lot more this year then last, we only produced 2% less energy in Q3.

Overall, here’s how this quarter and the first three quarters compare to 2012.

Q1-3 comparison

2013 Usage Solar PV Net usage
or (surplus)
Avg.
daily usage
HDD
Total 1,283 2,826 (1,544) 14.0 294
Jul
384 929 (546) 12.4 18
Aug
447 961 (514) 14.4 58
Sep
452 936 (484) 15.1 218
2012 Usage Solar PV Net usage
or (surplus)
Avg.
daily usage
HDD
Total 927 2,871 (1,944) 10.1 216
Jul
319 970 (651) 10.3 17
Aug
316 1,018 (702) 10.2 31
Sep
292 883 (591) 9.7 168
All values in kWh (except HDD which is base 65°F).

We used 38% more energy (356 kWh) this quarter as compared to 2012. That’s about  3.9 kWh extra per day, or 161 watt hours per day more than last year. The last two quarters we’re been running about 5 kWh higher. Last quarter I predicted this quarter usage would be up roughly 230 kWh from Q3 2012. It was actually up 356 kWh. I didn’t take into account the fact that we would be buying a plug-in hybrid car.

However, we used a lot less cooling this quarter. We used the ASHP for roughly 5 hours in September for a total of 3.7 kWh, a 93% reduction. Third quarter last year the ASHP used 50.6 kWh. We used the air conditioner a lot last year mainly for dehumidification because the house was still drying out and we didn’t want any buckled floors. The house seemed to cope well with the humidity this year.

That means we used roughly 403 kWh more than last year if you factor out cooling. That’s about 4.4 kWh per day. I’m using about 432 Wh per day for the home office. If the Prius is using about 3.8 kWh per day as predicted, that means there’s another 200 or so watt hours unaccounted in there per day in comparison to last year.

Our water usage is down 13%, and our hot water usage is down 12% from Q3 2012. More rain equals less watering the garden. Less barn building this summer equals less showers.

Predictions for next quarter? Will we use 4 or 5 kWh more per day than least year? Stay tuned!

You can see heat maps and detailed charts of temperature and electrical usage at netplusdesign.com. View hourly, daily and monthly values for solar, usage, net usage, temperatures and HDD.

Second solar anniversary

Last month marked our second solar year. We produced 8,519 kWh and used 6,410 kWh for a net of -2,110 kWh from June 2012 to June 2013.

At $0.04 / kWh that works out to a credit of $86.41 from the electric company.

That’s a 46% decrease from last year when we generated a surplus of 3,650 kWh and a credit of $159.68.

Not only did we produce less this year, we used more.

Note: with the new plug-in hybrid, we’ll be using even more of our surplus, and saving more. We’re spending roughly 13 to 19 cent’s per mile in gas.  We’re saving an estimated $400 to $600 in gas each year by using electricity (13 miles/day * $0.19 = 2.47/day * 250 days) which the electric company would only pay us about $55 (1,400 kWh * $0.04) if we sold it back as surplus.

Second quarter 2013 performance

Q2 2013 summary: 13% colder, 44% more usage and 4% less sun.

Now that we are starting to collect our second year of performance data, I’ll be comparing our new data to the same period last year. Year-over-year comparisons should be more illustrative than comparing to previous months. I’m also switching to a quarterly reporting period to look for larger trends, but I will continue to post data monthly at netplusdesign.com

In Q2 last year our total usage was 1,021 kWh. This year is was 1,468 kWh, a 44% increase. Although it feels like it had rained a lot more this year then last, we only produced 4% less energy in Q2.

Overall, here’s how the first and second quarters compare to 2012.

Q1 and Q2 comparison

2013 Usage Solar PV Net usage
or (surplus)
Avg.
daily usage
HDD
Total 1,468 2,626 (1,158) 16.1 906
Apr
528 920 (392) 17.6 571
May
529 904 (375) 17.1 232
Jun
411 802 (391) 13.7 103
2012 Usage Solar PV Net usage
or (surplus)
Avg.
daily usage
HDD
Total 1,021 2,731 (1,709) 11.2 802
Apr
387 925 (538) 12.9 535
May
309 867 (557) 10.0 170
Jun
325 939 (614) 10.8 97
All values in kWh (except HDD which is base 65°F).

Interesting note, we used 44% more energy this quarter as compared to 2012, but that was 447 kWh, or about the same as last quarter. We’re consistently using about  5 kWh extra per day than last year. This breaks down to 208 extra watts per hour. 100 watts is a heat lamp for the chick brooder, and the other 108 watts is roughly what my 2 laptops and task lighting use on average per hour.

Our water usage is only up 12%, and our hot water usage is down 3% from Q2 2012.

As for heating, we used the ASHP for roughly 14 hours in April for a total of 7.5 kWh.

In summary, Q2 usage is up compared to last year. That increase is attributed partially to the incubator and brooder box heat lamp (for the turkeys, chickens and guinea fowl), and partially to the increased use of the home office (last year I spent most of the summer building a barn, not staring at computers). We should be retiring the brooder box heat lamp this month, so I predict our next quarter usage will be up roughly 230 kWh from Q3 2012. Let’s see if my prediction holds.

You can see heat maps and detailed charts of temperature and electrical usage at netplusdesign.com. View hourly, daily and monthly values for solar, usage, net usage, temperatures and HDD.

72% of our usage is grid supplied, even though we produced more than we used!

Supply Mix

In the last year (April 1, 2012 to March 31, 2013), we used 6,046 kWh of electricity and produced 8,576 kWh for a net surplus of 2,530 kWh. That’s awesome right?

But 72% of the electricity we used was supplied by the grid when solar could not supply enough power to cover the need at that moment, like at night or on a cloudy day*.

Or another way to look at it, although we produce more than we use, most of what we produce we don’t really use directly. We only use 28% of what we produce. The rest goes back to the grid to pay back what we used when the sun wasn’t shining and to build up a surplus for a rainy day.

So even with all that sun, we still draw a lot of power from the grid that requires coal and other nasties to be burnt to serve our electricity needs.

It makes sense, most of our heavy use, hot water for showers, cooking, washing dishes, all occurs early in the morning or evening when the sun is not at it’s brightest or best angle. The more we time our usage to occur when the sun is shining, the less we demand of the grid.

There has to be a common industry term for this? Anyone know? Percentage of power supplied by the grid as compared to total usage when solar or other renewable is in the mix?

To me this seems like a much more important number to track if you have solar and are concerned with your direct carbon producing footprint.

* In order to find how much energy we used from the grid I added up all the usage values on an hourly basis that were greater than what was being produced by solar. For example from 5-6am, total demand was 1000Wh. The sun was just coming up and the system was only producing 200Wh. That means the grid was supplied 800Wh during that time. 80% is grid supplied for this hour. Now do that for every hour in a year. Hint, it helps if all your energy values are stored in a database.

Estimating heat energy for 2012 – Revised

Now that we have January – March 2013 circuit-level usage values, I thought I’d go back and revisit my original 2012 heat estimate using a different method.

I had estimated January – March 2012 heat energy based on a linear regression analysis of our April – December heat values. There are a number of problems with this approach. Mainly that heat pumps use more energy the colder it gets outside, and secondly the amount of passive heat we gain from the sun can significantly reduce the amount of energy required for heat.

This time I used a less formulaic approach to estimate heat energy usage. I simply calculated the kWh/HDD per month for 2012 and 2013, and compared the values.

First lets look at the first 3 months of 2013. We recorded 3,239 HDD, a 20% increase from 2012 to 2013. We used 746 kWh for those 3 months. If we divide 746 kWh by 3,239 HDD we get 0.230 kWh per HDD.

Now let’s try the same for the first 3 months of 2012. We recorded 2,107 HDD and I estimated 327 kWh for heat energy. 327 / 2,107 = 0.121 kWh per HDD. That is a 128% difference from 2013. Something is clearly off.

Since 2013 was colder and less sunny, I would expect our 2013 efficiency to be less because heat pumps become less efficient at lower temperatures. So I manually adjusted the 2012 kWh values so that the kWh/HDD percentage was similar to the 2013 values, then I lowered it a bit to take into account the warmer temperatures and increase sun in 2012. Did I mention this wasn’t very scientific?

What we get is closer to 620 kWh for heat energy for the first 3 months of 2012. This is roughly a 90% increase from my earlier estimate. It also means that a 20% increase in colder weather roughly equals 20% more heat energy usage.

Q1 2012-2013 heat energy comparison

Looking at Q1 performance again, that means out of the 445 kWh increase in 2013, 65% of that increase was due to heat energy, 33% was water heating and everything else was 2%. That sounds a little more realistic.

First quarter 2013 performance

Q1 2013 summary: 20% more cold, 22% more usage and 15% less sun.

Now that we are starting to collect our second year of performance data, I’ll be comparing our new data to the same period last year. Year-over-year comparisons should be more illustrative than comparing to previous months. I’m also switching to a quarterly reporting period to look for larger trends, but I will continue to post data monthly at netplusdesign.com

**See April 7 Update at end of post.

In Q1 last year our net usage was 228 kWh. This year is was 953 kWh, a 300% increase! In March 2012 we actually generated a surplus, but not this year.

Overall, here’s how the first quarter compares to 2012.

q1-comparison

2013 Usage Solar PV Net usage
or (surplus)
Avg.
daily usage
HDD
Total 2,499 1,546 953 28.1 3,238
Jan
881 478 403 28.4 1,189
Feb
812 449 362 29.0 1,067
Mar
806 618 188 26.0 983
2012 Usage Solar PV Net usage
or (surplus)
Avg.
daily usage
HDD
Total 2,054 1,824 228 22.6 2,701
Jan
873 369 504 28.2 1,125
Feb
666 597 69 23.0 957
Mar
515 860 (345) 16.6 619
All values in kWh (except HDD which is base 65°F).

Seeing all the increases, I wanted to understand if our 22% increased usage (445 kWh) was due mainly to the 20% increase in colder weather (HDD), or if other factors were leading to our increased electricity usage.

Broadly speaking, hot water, heat and all other circuits (mainly plugs loads and lighting) each accounted for about 30% of the total energy for the 1st quarter. Out of the three, hot water is the only one we can accurately compare because we still lack circuit-level data for Q1 2012.

For the last three months we’ve averaged 74.8 gallons of water per day, up 46% from first quarter 2012. That’s an extra 600 to 800 gallons of water per month, or nearly 24 extra gallons per day. This includes a 25% increase in hot water usage. Although we don’t have circuit-level data on the hot water heater, we do have monthly hot water usage data. We can use this to estimate the energy required to make hot water in Q1 2012.

Our hot water heater used an average of 275.8 watts to heat one gallon of water in Q1 2013 (water heaters have to work harder in the winter). We used an extra 533 gallons of hot water in Q1 2013. 533 * 275.8 = 147 kWh. That’s 33% of the overall usage increase right there.

As for the cause of the increase? To remain happily married I will only say two words, goats and mother-in-law. I don’t think I should say anything more on that topic.

Now let’s look at heating.

Determining the increase in heat energy is not easy to answer because we only have estimated usage values for heat in Q1 2012. Estimated values for heating have a much higher degree of uncertainty. Using this quarter’s values to check the accuracy of my 2012 estimate shows that my estimate may have been off quite a bit. It’s hard to know how much it may be off because I don’t know an easy way to factor in heat gain directly from the sun shining through the windows. Below are the heat pump electric usage values for Q1 2012 and 2013.

Heat (kWh) 2012* 2013 change
Total 327 746 128%
Jan 149 282 89%
Feb 119 270 127%
Mar 59 194 229%
* 2012 values estimated (see Estimating heat energy for 2012).
All values in kWh.

Based on the values in this table, we used an additional 422 kWh or a 128% increase in energy use for heating in 2013. This would almost completely use up the 445 kWh overall usage increase! So I know this can’t be correct. For example, if I use the HDD formula, 0.2261 x 1900 HDD(base 50) + 0.756, I get 430 kWh, which is far below the actual recorded 746 kWh. Which means my estimated heat values for 2012 are off. But we know that heating has increased some amount due to the colder temperatures and cloudier skies. We just can’t determine how much it increased.

So let’s look at all other circuits. Again we can’t really compare apples-to-apples yet, but there are a number of changes that have occurred this year. As I mentioned, we now have a barn full of goats, chickens and other critters. On the coldest days we used heated water buckets to keep the water from freezing. In March we had an egg incubator running for 23 days and and we’re using grow lamps to start our garden seedlings. As of March 30 we’re also now using a 250W heat lamp in the chic brooder. This will last for a few weeks before we switch to a smaller wattage, pushing our energy usage up 4 to 5 times it’s usual base load for about 5-6 weeks. Soon the incubator will be in use again. We’re going to try hatching and raising some turkeys this year.

In summary, Q1 usage is up compared to last year. Some of the reasons for this are fairly obvious, others remain difficult to tease out of the data. Hopefully next quarter will offer better comparisons and we’ll finally be able to compare apples-to-apples with circuit-level data. It should be easier to see where usage has changed and why. If we understand why, then we have a better chance of improving performance over time.

April 7 Update: Using a different method I estimated  that heat energy most likely did increase 20%. That means out of the 445 kWh increase in 2013, 65% of that increase was due to heat energy, 33% was water heating and everything else was 2%. That sounds a little more realistic.

You can see heat maps and detailed charts of temperature and electrical usage at netplusdesign.com. View hourly, daily and monthly values for solar, usage, net usage, temperatures and HDD.


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