A Tesla Powerwall for Up Hill House?

I received a string of emails from friends today asking what I thought of the new Tesla Energy Powerwall. I think it is awesome of course!

Although we may not be one of the first in line to get one (or a few of them), batteries are definitely in our future. And when I say ‘our’, I mean a lot of us. Even if you don’t buy batteries to power your home directly, there’s a pretty good chance some of the power coming to your home through the grid will come from batteries in the next 5 years.

I’m pretty tempted to get a 10 kWh Powerwall instead of a generator for backup power. It won’t power our heat pump or give us hot water, but it will cover everything else, including the freezer, refrigerator, lights, ventilation and water pump.

My main question is how it works when the power goes out. Today if the power goes out, our solar shuts down too. This is for safety, to prevent back feeding the grid and potentially injuring workers trying to repair the lines. If that continues to be the case, then our panels would not be able to recharge the batteries during an extended outage. This would dramatically limit their appeal for us. If they have a solution for this, then that might be enough to tip the scales and become an early adopter.

With out current net metering arrangement with our utility, there is not a lot of financial incentive to use the batteries to store excess power when the sun is out and use it at night. If the utility decides to change that agreement in the future, as is happening in other states, then all bets are off.

First quarter 2015 performance

Q1 2015 summary: 7% cooler, 2% more usage and 14% less sun as compared to Q1 2014.

This is part of our quarterly home performance reporting. All data is posted at netplusdesign.com

Although it seemed a lot colder this winter, it was really only 7% cooler than first quarter 2014. And considering there were many nights much colder than last year, -16°F versus -8°F last year, we only used 2% more energy. The biggest difference was snow cover, specifically the snow that covered our solar panels. This resulted in 14% less energy generated this quarter compared to last year, which means we netted out using 21% more energy from the power company.

Overall, here’s how this quarter compared to 2014.

Chart showing usage, solar, net and hdd for Q1 2015

2014 Usage Solar PV Net usage
or (surplus)
daily usage
Total 3,157 1,726 1,431 35.1 3,724
Jan 1,103 472 631 35.6 1,380
Feb 994 511 483 35.5 1,174
1,060 743 317 34.2 1,170
2015 Usage Solar PV Net usage
or (surplus)
daily usage
Total 3,214 1,488 1,726 35.7 3,975
Jan 1,163 400 763 37.5 1,419
Feb 1,096 300 796 35.4 1,449
Mar 955 788 167 30.8 1,107
All values in kWh (except HDD which is base 65°F).

We also used the heat pump a bit different this year. In prior years I set the heat lower at night. But with many nights in the single digits this year, I just left the thermostat set at 68°F for the last three months. The result was a 13% increase in usage compared to Q1 2014. In total, the ASHP used 1,528 kWh in Q1, a price of $230. Still not too bad considering what most folks payed to heat their homes in our area the last 3 months.

If we subtract heating from total usage for the quarter, our usage was actually down 124 kWh this quarter. So the main difference in usage this year was heating energy. Usually we’ve turned the heat off by this time of year, but Winter has refused to release it’s icy grip on the region.

Our water usage is down 6% from Q3 2012. The water line to the barn froze in mid-February, so we’ve been carrying buckets of water down from the house. I’m sure that has had an affect on our water usage.

We’re looking forward to Spring!

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, circuit-level usage, temperatures and HDD.

We’re net positive again in 2014!

We used 8,108 kWh and generated 8,348 kWh for a net of 240 kWh for the year. The combination of cold temperatures, less sun and the plug-in Prius brought us close to our net point. Here’s our progress for our first 3 years in the house.

Usage Solar PV Net usage
or (surplus)
daily usage
Total 20,914 25,779 (4,864) 19.1 19,758
2012 5,601 8,856 (3,256) 15.3 5,885
2013 7,206 8,575 (1,368) 19.7 6,810
2014 8,101 8,348 (240) 22.2 7,063

Q4 2014 summary: 7% warmer, 8% less usage and 25% less sun as compared to Q4 2013.

In Q4 last year our total usage was 1,957 kWh. This year is was 1,802 kWh, a 8% decrease. Snow covering the collectors for 8 days and cloudy weather resulted in 24% less energy production in Q4.

Overall, here’s how this quarter and the year compare to 2013.

Charts comparing Q4 usage, solar and HDD

2014 Usage Solar PV Net usage
or (surplus)
daily usage
Total 1,802 1,186 616 19.6 2,213
413 538 (125) 13.3 373
570 400 170 19.0 807
819 248 571 26.7 1,033
2013 Usage Solar PV Net usage
or (surplus)
daily usage
Total 1,957 1,576 380 21.3 2,372
482 674 (192) 15.6 405
559 639 (81) 18.6 838
916 263 653 29.5 1,129
All values in kWh (except HDD which is base 65°F).

Year over year comparison

Chart comparing year over year usage

I like to compare the energy we use each year excluding heat. Our little story chart above, shows our usage and how it has played out over the last 3 years. By far our biggest energy increases have been related to hatching and brooding chickens and turkeys each spring. We managed to raise a year+ worth of chicken and turkeys this year, not to mention all the milk and cheese from the goats. It would of course be cheaper to buy chicken, eggs, milk and cheese, and Thanksgiving turkey at the store, but we like to do things the hard way.

Pie chart - circuit breakdown

We started monitoring an additional 11 circuits in March when we switched to eGuage, but I haven’t had a chance to look at that data in any detail yet. The main changes this year, are increased usage of the ASHP and the ‘everything else’ category which included the Prius plugin for eight and a half months. I’ve also started tracking the vampire loads from things like the induction cooktop & stove, solar inverter and the ASHP. These are things you can’t just unplug. In total these 3 appliances used a minimum of 119 kWh to do nothing. I say minimum because I have only been able to start tracking some of these vampire loads with our new energy monitor starting in March.

Chart showing net zero break even point

This year I started plotting when we net out on energy use versus production. I call it the ‘net zero moment’. This year it happened about 2 months later than last year. Thankfully we didn’t cross over again in December.

Chart showing cumulative kWh used by ASHP Oct-Dec 2012-2014

Last year I noticed that our air-source heat pump (ASHP) used a lot more energy (114% more) in 2013 than in 2012. 2013 was cooler than 2012 but it was also sunnier. I never figured out the increase. This year, the ASHP also used more energy than last year despite being 7% warmer. Both the lack of sun (25% less) and our change in use of the thermostat most likely had the biggest impact. In 2013 I turned the heat off at night. This was generally fine until we had a few days below zero and the heat pump was not able to make up the difference. So this year I tried leaving the unit on and just turning it down to 65F at night. I think this has increased our usage without any real gain in comfort. It seems like it’s really only important to keep the unit on if the next day’s temperatures are known to be below zero, which is where our ASHP has trouble keeping up with demand.

Chart showing kWh used by ASHP in 10 degree buckets

Speaking of ASHP usage, here is our plot showing the amount of energy we used for each 10 degree drop in temperature. If you remember the post from last year, 2013 looks a little different. I realized the ASHP vampire load was throwing off my count of hours. This year I only show hours where the unit was actually doing something, heating or cooling the house. We had substantially more hours below 50 degrees. Most of the ASHP usage has consistently occurred in the 20-30 degree range.

Chart comparing water usage 2012-2014

We seem to be fairly consistent in our hot water usage. Over the year we’re averaging 18.4 gallons/day. That hot water requires 262 watts/gallon to heat. Cold water usage averages at 43.5 gallons/day. In total we used 22,575 gallons on water in 2014. It took 58 kWh to pump that water out of the ground. I’m guessing half of that is water for the various assortment of critters on the farm. They don’t drink a lot, but they do waste a lot.

Chart comparing temperature ranges 2013-2014

And lastly, just because I like this chart, the temperature ranges we experienced this year. In case it is not clear, 2014 was 0.9154149°F warmer on average than 2013.

Happy 2015!

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.

Detailed data finally available again

We changed energy monitors in March and the new data format broke my database scripts, but I finally found time to update my scripts and publish* our hourly data for the period March to November. One thing I noticed immediately was that I had been sloppy in my reporting of energy use for that time period. I used the UI to select dates and didn’t realize I wasn’t getting the most accurate timeframe. As a result, I’m going to post an updated energy record at the end of January for the entire year.

Good news, we’re still on track to produce more energy than we used for 2014, but it is going to be a squeaker. At the end of November we had a revised 829 kWh in the bank. As of today, December 19, at 11:31 am Eastern time, we had used 495 kWh and generated 115 kWh for a net usage of 380 kWh. We have roughly 12 and a half days left and 449 kWh left in the bank.

The solar panels have been covered in snow 8 days day this month and our average usage is 26.7 kWh/day. So even if we had no sun the rest of the month and continued our average usage, we’d use 347 kWh leaving us with just 102 kWh to spare. Thankfully, we’ll get a little sunlight on the panels and the forecast is warmer next week. Fingers crossed.

* Detailed data available at NetPlusDesign.

Sol LeWitt inspired doodles

After seeing the Sol LeWitt Retrospective at Mass MoCA last weekend, I felt the need to do some computational doodling today.



Small fat curves (Holiday Microbe)

Curve receding

More curve

Loopy Doopy (inspired) in color


100 lines, none straight, some touching

I produced all images using Processing.

Apply for the 2015 NESEA Zero Net Energy Building Award

Just saw this post on the NESEA site go up yesterday. Looks like they are now accepting entries for the 2015 award.

If you have built or live in a home that has produced more energy than it uses in the last year, then I encourage you to enter the competition. They have worked to make the application process a bit easier this year and the benefits of winning include not only the prize money, but also a chance to meet some of the leading proponents and practitioners of building net zero structures at the yearly conference when they announce the winner. You also get your building’s picture on the cover of BuildingEnergy magazine.

I highly recommend you apply, it is well worth the effort. Do it quickly, the deadline is December 15.

Good luck!

Designing and building your own net zero home

We wrote this article in June for the Fall 2014 issue of BuildingEnergy Magazine. Edited by Mark J. McCourt.

Designing and building your own home can be a huge challenge. It can be even more daunting if you set your sights on a high performance home that will produce more power each year than it uses. Thankfully, there are a ton of resources online—at times too many—that can help make those challenges more manageable. I am not, however, suggesting that any homeowner can do this without considerable learning or prior design build experience.

Jill and I are both designers. Jill works in graphic design and I work in the technology industry to make online applications easier and more intuitive to use. I studied architecture in school and paid for college in the early years by working for builders and a mason. It wasn’t out of concern for the budget that we did not hire an architect; in fact, an architect experienced in high performance homes would have likely saved us time and money. We simply wanted to do it ourselves because we enjoy the challenge and we had confidence we could do it well. We also knew when to ask for help.

We hired DEAP Group to estimate our peak heating load, specify our ventilation requirements and estimate how much solar we would need to achieve net zero. They also offered a number of recommendations that boosted our efficiency further. Please do not rely on anyone without experience in high performance homes to do this type of work for you; our energy star rater had zero experience with homes of our type. Based on their calculations, the heating load would have been half DEAP’s estimate, and our total usage would have been twice DEAP’s estimate. If we had followed our energy star estimate, we would have needed a 12 kW system to net out. Our first year actuals came very close to DEAP’s estimate.

One of our most important decisions was picking a contractor. Ideally we wanted someone that had experience building a net zero home, but there was no one in our rural area. We found W.R. Coolidge and Co., an experienced builder next door, that was interested in high performance homes and was eager to learn with us. Most importantly, he was willing to let us do as much of the work as we felt comfortable doing ourselves, and show us how to do it right.

Since we were involved in every decision and I had built detailed 3D models of almost every house system and detail, it was easy for us to coordinate the work of the contractors responsible for the foundation, electrical, plumbing, HVAC, cellulose insulation and solar PV installation. All of this required a great deal of time, research and planning; it is one of the reasons it took us more than 18 months to complete the house.

Throughout the process we posted much of our thinking and analysis in our blog, uphillhouse.com. The technology community places a great deal of value on open-source work. Sharing your work has several benefits: it creates opportunities for others to comment and make suggestions for improvements; it makes it easier for others to follow in your footsteps with less risk and more confidence; and most importantly, the act of sharing requires you to explain what you’re doing and why, in simple and clear terms. This forces you to think things through much more carefully. If your logic or calculation has a flaw, you want to make sure you find it before you tell the world about it.

After we moved in, we installed sensors to track the performance of our home. All our data is posted online at netplusdesign.com, for anyone to view and analyze. We collect and post our data for several reasons. We believe data should be open and accessible so that others can benefit from our experience. We want to know how the house actually performs, as compared to the estimates. We want to prove whether we have met our goals or not; if we have not, then we need to understand why.

We have been extremely lucky. It is a very comfortable home even in the deepest, coldest days and months of an upstate New York winter. In our first two years living in the house, we produced more energy than we used, despite the addition of a barn, dairy goats, chickens, turkeys, other assorted critters and a plug-in hybrid vehicle. This year, winter temperatures were much colder, and we are attempting to raise more of our own food, both of which use more energy. Last year, we passed into net positive territory in early June. This year we were still 1,121 kWh short at the end of May, a 1,000 kWh difference from 2013. We still expect us to generate a surplus in 2014, but it will be close.

So don’t go it alone unless you’re prepared to invest a significant amount of time researching and learning the nuances of high-performance building. If you don’t have 100 percent confidence in what you’re doing and why, it’s too easy for contractors to convince you that it’s too hard or too expensive to do it that way. Your house will suffer from a thousand small decisions that will ultimately undo your original goal: to live more sustainably in a net-zero home.

Larry & Jill

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