This modern Tudor Detached Accessory Dwelling Unit (DADU) demonstrates a way to add density to an existing neighborhood while honoring the historical design and character of its traditional Seattle community. Designed by CAST architecture, and built by Align Builders, the cottage is packed with plenty of Built Green features that integrate into its blending of modern and historical design details.

The interior celebrates values of beauty and shelter by the timbered and vaulted living area and comfortable flow of spaces through the kitchen and bedrooms downstairs. The Seattle code limits DADUs to 1000 sf, but the cottage gives the feeling of a spacious and comfortable 2-bedroom home.

Energy Efficiency

• Wall insulation is the standard code R21, but the builder focused on executing the install to the HERS Grade 1 standards. This requires third-party inspection and batts that are evenly fluffed and filled with extra care to eliminate gaps and compressions.
• The vaulted ceilings have R38 fiberglass batts.
• Windows are low-e argon from Andersen.
• Panasonic Intellibalance HRV installed to provide filtered fresh air directly to bedrooms and exhausts stale air while recovering about 80% of the heat energy.
• The flush-mounted ceiling cassettes of the multi-head ductless heat pump system provides energy efficient space heating with unobtrusive design.
• Low-flow toilet, sink, and shower fixtures.
• ENERGY STAR appliances and 100% LED lighting.

Low-Impact Development and Rainwater Infiltration

The DADU occupies the existing home’s former back garden. The site included street access to both sides of the property, allowing for separate private entrances for both homes, and instantly integrating this DADU into the existing streetscape of the neighborhood. The bay window and arched front entry make it a familiar-looking addition for those walking around through the neighborhood.

There is no garage, but a side parking driveway is done with permeable paving stones. Also roof drainage is directed to infiltration trenches and pits so that it can be treated and infiltrated onsite. Landscaping is drought tolerant with only a small shared grass area that connects to the existing parts of the backyard.

Author: Nina Olivier, Built Green Coordinator

Installing a solar battery storage solution provides the greatest benefit to homeowners who live in areas that experience frequent power outages, where full-retail net metering isn’t offered, or where battery incentives are available.

When solar panels are paired with a home battery, the excess electricity goes into the battery instead of going to the grid. As a result, when the sun goes down and your panels aren’t producing electricity anymore, you can use the energy you have stored in your battery—instead of paying for electricity from the utility.

Investment Opportunities in Battery Systems

Given local proposed rate increases for utilities, solar PV systems and battery storage are becoming a more economically viable option. The style of battery greatly affects how the batteries are used and how long they last. Many factors contribute to variability in battery life including temperature, proper discharge and recharge, and optimal application usage.

Average household batteries cost anywhere from $4000 for a small 4kWh battery up to $15,000 or more for a large 13kWh battery depending on the type of battery, installation location, backup power requirements, and type of inverter used. To give some perspective, an average 3-bedroom home uses around 20kWh of electricity per day. However, a lot of this energy is used during daylight hours and could be directly powered by solar alone. Therefore, a moderately sized 10kWh battery would generally be large enough to cover the evening and overnight periods. However, every household is different, and usage patterns can vary greatly. For those that are more energy conscious and use efficient appliances, a smaller 6kWh battery may be suitable.

The payback period or return on investment (ROI) for most battery systems is around 7–10 years. It’s generally more cost-effective to install rooftop solar and run efficient appliances or heat hot water during the day rather than store excess energy in a battery. However, for some people, the value and security of having a reliable, sustainable power supply easily outweighs the cost.

The high cost of clearing trees, creating access roads, and building power poles can be prohibitive to getting power in remote locations. After recent grid events in Texas and California, the value of battery backup is harder to ignore. In states like Hawaii, residents are even required to store energy because there are restrictions in place that do not allow for the export of excess solar power to the grid.

Incentives and Financial Assistance for Washington State

Congress extended the federal solar tax credit (known as the ITC) for an additional two years. This means that you’ll receive a tax credit of 26% of your system’s cost in 2021 and 2022 and 22% in 2023. If the full value is not used within one year, the remainder will roll into the next year. Federal tax incentives can also be extended to home battery storage systems.

The 100% Clean Electricity Act passed in 2019 by the Washington State Legislature removes the sales tax for most typical residential solar energy systems and offers some sales tax relief for larger commercial scale systems. For the average customer, that means you save around 10 percent right off the bat.

In addition, the Rural Energy for America Program (REAP) provides grants and loans to farmers and rural businesses to help them purchase renewable energy systems and make energy efficiency improvements by paying up to 25 percent of the project.  To see all available renewable energy incentives available in your zip code please visit the Database of State Incentives for Renewables & Efficiency (DSIRE).

Leading Solar Energy Storage Batteries

Solar energy storage options have been rapidly evolving and improving over the last decade and are no longer limited to large, inefficient, and unattractive nickel-iron storage systems. Now sleek, sophisticated, high performing, and long-lasting storage options are readily available. Examples are listed below with typical prices (2021 retail prices).

Tesla

Since 2015, Tesla has been at the front of the pack in the production of dependable residential solar batteries. Their flagship model, the Tesla Powerwall, is pegged by critics to be the best in the range. The Powerwall is a stackable solar battery that has a capacity of 13.5 kWh and is enough for the average home’s energy needs. Tesla has developed its lithium-ion battery in partnership with Panasonic, and the resulting product is believed to be of very high quality. The cost of one battery unit is around $10,100.

LG

LG is one of Tesla’s main rivals in the solar battery market. It has a battery model called the LG Chem solar battery system. At 9.3 kWh, the LG Chem is a decent solar battery that should be able to power a home that does not have massive power-consuming machines. It is best suited for homes that are connected to a power utility grid. Including installation charges, the LG solar battery can cost between $11,000 and $13,000.

Sonnen

Sonnen is a fast-growing German company founded in 2010. It has a share in the European and American markets through a network of agents and installers. Sonnen’s Eco battery core unit has a small capacity of 4 kWh but can be stacked to 16 kWh. Smart software within the battery system stores data relating to your home’s energy needs. It then uses the information to provide you with a genuinely futuristic solar experience. With pre-installation costs of $9000, the Sonnen Eco is an expensive solar battery. However, each battery comes with a warranty of functioning at 70% of optimum levels for ten years, which is higher than that of competitors.

The following table shows other leading solar storage energy batteries currently available on the market:

Credit: Clean Energy Reviews

A&R Solar, a local solar installation company, explains that in addition to achieving energy independence and reducing your carbon footprint, solar is a wise investment with positive financial returns. A&R Solar recommends roof-top mounted grid tied solar coupled with a double Tesla Power Wall system for a typical single-family home.

The following are price estimates for a typical single-family home provided by A&R Solar using their recommended Tesla Power Wall or SolarEdge Energy Hub systems:

Credit: A&R Solar

Achieving Total Energy Independence and Resiliency

Solar PV plus batteries boosts a home’s resiliency to power outages where gird electricity may be off for a few hours or for several days. The battery provides the means to avoid the use of fossil fuel burning generators to power the home in the event of an outage. In addition, unlike generators, solar panels and battery storage systems don’t create noise and air pollution that will bother you or your neighbors. This is a unique benefit and is a great way for anyone who currently has a generator to update their system.

Learn more about how A&R Solar provides solar and storage to build a homes resiliency during an outage:

Batteries can be used to store solar energy and then allow the owner to utilize battery power at night or anytime to reduce the purchase of highest cost of peak power. Utilities often charge the highest per kWh rate for electricity used during the peak time of day or season. Battery power can also be used to keep the building’s total grid energy usage low over time to ensure the customer is paying for the lowest price in a tiered rate structure.

In a remote location, or any location that doesn’t have an electrical grid, the combination of solar PV and batteries can supply reliable ongoing electricity to power typical household functions. Unlike grid-tied solar systems, off-grid systems have no connection to the utility grid and must make all the electricity necessary to power the home.

Credit: Lopez Community Land Trust

The Path to Community Microgrids

To meet climate action goals, many large utilities are looking to generate more electric power through solar and wind. Currently there are challenges with large scale utility storage and energy transmission. Microgrids have been shown to alleviate these challenges through functioning like a smaller version of a larger traditional electrical grid powered by solar with battery storage systems. With the increased cost of traditional grid power generated from fossil fuels, many microgrids are switching to a combination of solar and battery storage to supply electricity that is cost effective and has environmental benefits.

They provide opportunities for optimal solar placement and scalability, as they do not need to be placed on home roofs—they can be installed on parking garages, open fields, etc.—reducing roof penetrations and complexity during roof repairs and replacements. Microgrids also achieve resiliency by islanding from the central grid during an outage. Community microgrids represent a new approach for designing and operating the electric grid that achieves a more sustainable, secure, and cost-effective energy system while providing indefinite, renewables-driven backup power for prioritized loads.

Further Reading:

Backup & Storage. Solar Washington.

Webinar Presentation: Battery Based Systems: Design Considerations for Washington Solar. Solar Washington.

Home Solar Battery Systems - Comparison and Costs. Clean Energy Reviews.

Solar Integration: Solar Energy and Storage Basics. Office of Energy Efficiency & Renewable Energy.

Fact Sheet | Energy Storage (2019). Environmental and Energy Study Institute.

Guest Author: Cybil Tribie, Senior Project Associate, Evergreen Certified

Both the new code and Built Green offered a few surprises with their updates. Code now includes the Fuel Normalization Credits (Table R406.2); and Built Green now offers three different pathways to meet their energy efficiency requirements. Two of those three paths for projects permitted under the residential code are modeling-based utilizing a performance approach, and the last is a prescriptive approach – selecting additional credits from WSEC 2018 Table R406.2 & R406.3. While all three paths are viable ways to meet their requirement, this post will only focus on the prescriptive approach for residential projects.

If you aren’t already familiar with it, the WSEC requires that units (based on size categories) obtain a certain number of credits from Tables R406.2 and R406.3 (the full table can be found here on pages RE-41 – RE-48). Those credit options span from thermal envelope improvements to HVAC upgrades to Energy Star appliance and even reducing overall air leakage. While most of the categories limit you to only choosing one option per category, you can mix and match all the different categories to obtain the minimum credit number required for your unit size.

The table below breaks down how many credits are required per unit size:

*Small units must have less than 300SF of fenestration otherwise they fall into the medium category
**Additions greater than 500SF but less than 1500SF fall into the small category

Earlier I mentioned that the new code threw in a surprise, the Fuel Normalization Credits (Table R406.2). Table R406.2 provides you with additional (or negative) points depending on the primary heating source being used for the units. For example, if you’re planning on using ductless mini-splits and electric resistance heaters (System Type 4 in Table R406.2), you’ll obtain 0.5 credits that is applied towards the total number required in the table above. That means that throughout Table R406.3, you’d only have to claim 2.5 credits, 5.5 credits, and so on depending on the unit size.

So now that we’ve gone through how the new code changed, let’s look at a couple suggestions on how to just meet code depending on your unit size.

The tables above represent two different options for how to meet your energy credit requirements for code. While going all electric with a hybrid heat pump water heater gets you above and beyond the 3.0 credit requirement, it will also help propel you forward with the Built Green prescriptive approach and being eligible for green building incentive programs, like Seattle’s Priority Green. Priority Green – faster permitting with the City of Seattle – requires that your project achieve 2.0 credits in addition the R406 requirements for new dwelling units, the same as Built Green 4-Star certification, so why not double dip and meet your Built Green and Priority Green requirements all in one shot?

Built Green 4-Star certification requires that you get (depending on star level) 3-Star - +1.0pt, 4-Star - +2.0pts, and 5-Star - +3.0pts beyond the minimum required for code based on your size level and fuel type. So, let’s review what that would look like.

*Small units must have less than 300SF of fenestration otherwise they fall into the medium category
**Additions greater than 500SF but less than 1500SF fall into the small category

The following tables provide examples of what a 4-Star Built Green project might want to consider claiming if pursuing the prescriptive approach for Built Green’s energy efficiency requirements. Note that the Priority Green program does not allow for natural gas to be used in the home and projects must select electric-only options to qualify.

Small Units (less than 1,500 SF)

Small units with less than 1,500 SF of conditioned floor area, with less than 300 SF of fenestration, benefit the most from electric ductless mini-split systems for space heating. If space allows in the small unit for a heat pump water heater, it will reach the required prescriptive credits with less action items.

Medium Units (1,500 SF-5,000 SF)

What about a medium sized unit (1,500SF-5,000SF) using different space and water heating options? There are a multitude of mix-and-match options that you can utilize, but below are some examples of how to get there for code and Built Green. It’s important to remember though that when using the Built Green prescriptive approach, less than 50% of your WSEC credits can come from PV.  

Large Units (5,000+SF)

And we can’t forget about the large units (5,000SF+). Below are just a couple options for how to obtain those Built Green points prescriptively.

Multifamily R-2 Occupancies

All the tables above demonstrate a variety of different compliances paths to meeting Energy Code as well as Built Green. Below are two ways to do that for R-2 Occupancies. The important thing to keep in mind is that the points values in Tables R406.2 & 406.3 are different for R-2 Occupancy.

All of the above tables provide options for exceeding the 2018 WSEC-R and meeting the requirements for Built Green certification and Priority Green expedited permitting. Builders will need to evaluate which prescriptive credits work best for the project’s site, design, size, and budget. To do this we recommend discussing options with your energy consultant or Built Green Verifier during the design process. Lastly, by selecting electric-only options builders will also earn additional indoor air quality credits to further their Built Green certification, future code-proof their energy designs, and provide more resilient and desirable homes for their buyers.

This webinar provides a detailed look into the prescriptive energy efficiency code credit options for residential projects that meet the 2018 Residential Washington State Energy Code and Built Green requirements. Speakers include representatives from Evergreen Certified and Built Green. Access webinar HERE.