Energy Use Over the Last Year In the ADU

In earlier posts, I shared how living in smaller spaces reduces per capita energy consumption, which was part of my motivation for living more compactly. Furthermore, after constructing an energy efficient ADU, Energy Trust of Oregon even gave me a $1,750 rebate for meeting Northwest ENERGY STAR standards and achieving a modeled Energy Performance Score of 35. 

Now that I've lived in the ADU for over a year, I wanted to share the actual energy bills, and compare them to the energy bills of the main house. Comparing the utility bills of the two dwellings will allow me to draw a few conclusions regarding the nuanced personal energy consumption virtues of living more compactly.

Both homes are heated by natural gas. The ADU is heated via a tankless water heater and in floor radiant hydronic heat. The main house, built in 1906 and recently weatherized, is heated through a conventional forced air heating system.

Below are the natural gas bills for the ADU and the main house, respectively. The ADU consumed 277.5 therms, and the main house consumed 930.4 therms. A "therm" is a unit of heat equal to 100,000 British thermal units.

The total therms used in the last year in the ADU

The total therms used in the last year in the main house

At only 277 therms, the ADU used only 29% as much energy as was used by the main house for heating!

That's very cool, but that's not the real story. 

The real story must include the total energy use divided by the number of occupants. The designed occupancy level is calculated by taking the number of bedrooms in the house, and adding one additional occupant. This designed occupancy level assumes that two adults are living in a 'master bedroom' together.

The ADU is a one bedroom dwelling designed for two people. And, in fact, that is exactly what happened in the ADU;  two people lived in the ADU for the full year. 

The main house is a four bedroom house, with a designed occupancy level of five people. However, in reality, the actual occupancy rates of the main house were lower than the designed occupancy level; the main house maintained an average of three occupants throughout the year. 

Interestingly, census data shows that the average national occupancy rate for homes larger than mine at 1,700 sq ft, is surprisingly low. The average occupancy rate for homes of 2,500 sq ft is actually only 2.59 occupants.

So, let's look at the therms used per capita for both the designed occupancy and the actual occupancy.

If the main house were occupied at the design occupancy rate, each ADU occupant would have used 75% as much energy as the average resident in the main house. Living in the Northwest ENERGY STAR certified ADU would have been 25% more energy efficient than living in the main house.

But, in reality, with only three occupants in the main house, the actual therms used per capita for heating and cooling in the ADU was still 138.75, but the actual therms used in the main house was 310.13. This means that by residing in the ADU, I used only 44% the amount of energy that the average occupant used in the main house. Living in the ADU has been, in reality, 56% more energy efficient than living in the main house.

These are significant data points. Here are the stories that these data points tell us:

#1) Building the new structure to Northwest ENERGY STAR standards resulted in building a very efficient building envelope and in choosing to use efficient heating systems.

#2) By living at the designed occupancy in the ADU, my partner and I each lived more energy efficiently. Building a smallish dwelling alone did not make the dwelling energy efficient. It was dwelling in a smaller footprint per capita that had the most substantial energy efficiency impact.

Said another way, if I lived alone in the ADU, and the main house was fully occupied at the design occupancy of five people, I would have actually used 50% more energy than the average resident in the main house.

#3) In smaller dwelling spaces like my 800 sq ft ADU, I was prone to live at the designed occupancy level of two. Conversely and representatively, the main house was prone not to be fully occupied (according to US Census data).

These last two points are a thinking person's fodder for a housing revolution.

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I'm also including my electrical bills from the last year for reference. Since neither the ADU nor the main house used electricity as the primary heat source (where the bulk of a home's energy is used), these data points are less relevant.

That said, indeed, the ADU was more efficient than the main house in terms of electric power consumption due to the types of appliances and light fixtures that were installed. But, one will still draw a similar conclusions that I have drawn above regarding designed and actual occupancy

Under actual occupancy rates, living in the ADU used 30% less electricity per capita than living in the main house.        Under designed occupancy rates, living in the ADU would've used 18% more electricity per capita than in the main house.

In terms of electricity, the ADU was more efficient under actual occupancy rates. But, if the main house was occupied by five people, the electricity use per capita in the main house would have actually been lower than in the ADU.

Electricity used in the last year in the ADU

Electricity used in the last year in the main house

Note the July spike in electricity use in the main house and the lack of a spike in July for the ADU.

The main house is set in direct sun, has R12 wall insulation, and has an AC unit.

The ADU is kept cool through deciduous tree shading, R33 wall insulation, and a ceiling fan. :)

The Builder’s Contract

After much back and forth about how to best build this structure, I decided that it would be in my best interest to hire a builder to construct the waterproof shell. The construction process will be done in two phases. Phase I will be bulk of the structural work, and Phase II will consist of the finishing touches. 

The Phase I builder will be responsible for the foundation, framing, siding, roof, windows, insulation, and drywall. He is also going to provide construction oversight for the electrical, plumbing, and other subcontractors that will be working on this project. I haven’t yet determined how I’ll do phase II—I hope to do much of that work myself, if I capable of it. 

In signing the contract with the Phase I builder, we used a Cost + Fixed Fee with Guaranteed Maximum Price ($50,000) and Bonus Contract. This is a very loaded term that means there is a $50,000 cap for the cost of this project; I will not have to pay more than that for the project even if the builder’s costs are greater than this amount. However, leading up to the final stage of the project, we’re going to operate the contract as a time plus materials contract. He’s going to track the material expenses and the hours for himself and his subcontractors. 

For any amount that he completes the project in under his bid price, he’ll get to keep 60% of that amount and I’ll keep 40% of that amount. For example, if he completes the project for $40,000 ($10,000 less than his original bid), he’ll get a $6,000 bonus. If he completes it for $46,000, he gets a $2,400 bonus. There’s a few nuances to this financial structure that I won’t bother to mention here. 

But, needless to say, we both share the goal of completing this project on a low budget, and this payment structure gives the builder a financial incentive to complete the project under budget. In tandem with quality standards, this payment structure makes sense to me. The other key aspect of our contract worth mentioning is the 3^rd party certifications. 

His work must not only pass building code inspection, he will also be held to meeting the standards of the Northwest Energy Star program. 3^rd party inspectors will checking in on the project during construction using the Thermal Bypass Inspection Checklist as the standard against which to measure the quality of the work. All the Energy Star details for which he is responsible are described on their website, which use clear illustrations and even have Spanish versions. This checklist holds him accountable to a relatively strict benchmark for his construction quality. 

I have to pay $700 for the third party certifications necessary to complete this Energy Star program, but I will receive cash incentives from the Energy Trust of Oregon for successfully building to these standards once the building is complete. These cash incentives will in turn, totally offset the cost of the 3^rd party verifications, so it is a no-brainer to participate in this program, as it will inevitably result in a better process and product for both of us.  

Because we’ll be building a certifiably green building, the builder will be able to use this as an additional marketing piece. This project is not going to make the builder rich, but it is a desirable project for his portfolio, and he hopes that this project will lead to more projects like it down the road. I hope it does too.