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“Off-Grid” to Net Zero: Building Off-Grid with a Hybrid Inverter, Whole House Battery and Ultra-Tight SIPs

“Off-Grid” to Net Zero: Building Off-Grid with a Hybrid Inverter, Whole House Battery and Ultra-Tight SIPs

by | Nov 5, 2026 | Building Science

Prelude:

Author and Legend Homes' client Brent Hartwig, owner of Neptune Beach House, is an professional electrician and chose to do all the electrical and solar installation for their home. His solar installation is different from Legend Homes' standard installation and very compelling and innovative. So, Legend Homes asked if he would share the details of his system in this blog for others to learn from.

This solar installation is especially great for off-grid living and places that have very frequent power outages.

When we started out constructing a modern, high-performance home, one of the most frustrating logistical bottlenecks, electrically speaking, is waiting on utility interconnection schedules, net metering approvals, and final grid hookups. Months can pass where a fully built structural shell with operational solar sit unused while relying on temporary construction power—or worse, relying on loud, inefficient gas generators.

When our rooftop solar photovoltaic array went live in December, we took a different path. Rather than letting the array sit idle while waiting for utility permission to backfeed the grid, we configured our system to operate in a self-contained "island" mode. From December through final project completion and net metering approval in June, our energy infrastructure ran almost completely off-grid, supplying clean power for heavy construction tools, mechanical testing, and finish work.

Residential Solar Systems

As of 2026 there are basically two major options for solar build outs that are grid tied on a residential structure. Either you generate in DC and convert to AC at the panel using micro inverters, or you generate in DC and send all the power down to a hybrid inverter and do all of your conversion to AC there. There are pros and cons to each system with the micro inverters being more cost effective but unable to be utilized in a power outage or off-grid scenario and hybrid inverters being more expensive but offering greater control and freedom with your generated power system. We went with a Hybrid setup for this house, as I didn’t want to be reliant on a standby gas generator during an outage and I liked the control for time of use metering and charging and usage of my own battery system.

Technical Architecture: The Sol-Ark 15k & SunGold Powerhouse 16.1kWh

At the heart of this power system is a balance between heavy-duty inverter throughput, long-duration energy storage, and ultra-efficient generation.

Heavy Inrush Capacity & Inverter Robustness

The Sol-Ark 15k is built to bridge the gap between complete off-grid independence and grid-tied net metering. Unlike standard residential string inverters that shut down during a grid power loss, the Sol-Ark acts as a high-capacity microgrid hub. Operating with dual MPPT trackers and an internal 200A bypass switch, it provides a continuous 12,000W of power off-grid, surging up to 24,000W for 10 seconds if needed.

During construction, this surge buffer was critical. High-inrush loads—such as table saws, large air compressors, and induction pumps—create massive initial current draws that easily trip lower-tier inverters. The Sol-Ark absorbed these inductive spikes seamlessly without voltage sag or breaker trips.

Assuming you’re powering a Tesla Model 3, which uses 250 watt hours per mile, and factoring for 15% loss, then you only need around 3.7kW (10 panels at 400w) of rooftop solar to charge your car. That’s also assuming you drive around 15,000 miles per year. If you drive less then you, of course, could get away with even less solar.

Battery Chemistry & Closed-Loop Intelligence

Paired with the inverter is the SunGold Powerhouse 16.1kWh outdoor rated wall-mounted battery bank. Built with Lithium Iron Phosphate (LiFePO) cells, the unit prioritizes thermal stability and operational lifespan over weight reduction.

The critical factor in this pairing is closed-loop BMS communication. Via a direct CAN bus connection, the SunGold battery continuously transmits real-time cell voltages, temperatures, and state-of-charge (SOC) metrics to the Sol-Ark 15k. The inverter dynamically adjusts its charge curves and current delivery based on live battery diagnostics, preventing overcharging, over-discharging, or cell imbalance without requiring manual intervention.

Navigating Winter Off-Grid Construction (December – March)

Installing solar panels in December introduces an obvious hurdle: 8-hour daylight windows, low solar elevation, and frequent cloud cover. Operating mostly off-grid with a temp power hookup as a backup required strategic power management:

  • Zero-Export CT Safeguard: Because backfeeding energy into the grid prior to official utility sign-off is illegal and hazardous to utility line workers, the system ran in Zero-Export / Limited Power to Home Dual Current Transformers (CTs) installed at the main panel monitored power flow, dynamically throttling solar output to match exact site demand and battery charging—ensuring zero current leaked back into the utility lines.
  • Bifacial Production Advantage: The ground-reflected light (albedo) captured by the undersides of the Heliene 530W panels delivered extra energy yields during overcast winter afternoons.
  • Overnight Base Load Management: The 16.1kWh storage capacity easily bridged the overnight gap, maintaining power for site security, battery tool chargers, and low-draw construction fans.
Interconnection & Transition to Net Metering (April – June)

By spring, solar generation expanded rapidly. The system easily sustained heavy finish-carpentry tools, paint sprayers, heaters and dehumidifiers for drywall and the initial commissioning of major mechanical systems—including the hydronic heat pump and smart home control hardware.

When local utility inspectors granted formal Net Metering interconnection approval in Late May, transitioning the home from an isolated off-grid microgrid to a grid-tied power station took less than five minutes. No rewiring was needed; we simply toggled the Sol-Ark’s software interface from Zero-Export Off-Grid to Grid-Tied Net Metering. Peak summer generation began exporting excess clean energy to the grid for utility bill credits under PSE’s net metering agreement, while the SunGold battery shifted to act as an instantaneous, seamless UPS backup during grid outages.

Annual Energy Balance: Modeling Production vs. Consumption

Because the home features an ultra-tight building envelope—constructed with Structural Insulated Panels (SIPs), achieving an airtightness rating of 1.0 ACH50 and a HERS index of -10—its annual net-energy profile is exceptional.

A HERS rating of -10 signifies that the building produces 10% more energy than a standard Net Zero Energy building consumes on an annual basis.

                    ANNUAL ENERGY PRODUCTION & CONSUMPTION              

 Estimated PV Solar Generation (10.6 kW Array in Ferndale WA):     ~11,500 - 13,000 kWh

 Estimated Whole-House Total Energy Load:                                         ~7,500 -  9,000 kWh

 PROJECTED ANNUAL NET ENERGY SURPLUS:                                        +3,000 -  4,500 kWh

1. Predicted Annual Household Consumption (~7,500 – 9,000 kWh/yr)

With an envelope rated at 1.0 ACH50, space heating and cooling thermal loads are reduced by up to 50–60% compared to code-minimum construction.

  • HVAC / Thermal Control: Radiating floor loops powered by an air-to-water heat pump paired with a high-efficiency HRV (Heat Recovery Ventilator) require minimal compressor output to maintain setpoint temperatures inside an airtight SIP shell.
  • Baseload & Domestic Hot Water: Efficient LED lighting, heat pump water heating, and modern electronics form the majority of the remaining load, resulting in a low, consistent baseline demand throughout the year.
2. Predicted Annual PV Generation (~11,500 – 13,000 kWh/yr)

A residential array paired with a Sol-Ark 15k will experience significant seasonal variation between winter and summer:

  • Summer Peak (May – Aug): Monthly production averages 1,400 to 1,800 kWh/month. Peak daily generation frequently exceeds 50–60 kWh/day, easily filling the household demand and exporting substantial credit back to the grid.
  • Winter Trough (Nov – Feb): Monthly production drops to 300 to 600 kWh/month. The tight SIPs thermal shell minimizes heating demand during this period, keeping grid demand low.
Final Takeaways

Building off-grid during the construction phase acts as the ultimate real-world stress test for a home’s energy infrastructure. Pairing a heavy-duty inverter like the Sol-Ark 15k with closed-loop LFP battery storage proves that you don't need to rely on fossil-fuel generators or wait on slow utility schedules to build efficiently. Combined with an ultra-tight SIPs building shell, the setup transforms from a temporary construction power plant into an operational Net-Negative energy engine. In the future, with additions of EV’s, sauna and a 4-season greenhouse we are hoping to end the March to March balance sheet with PSE at a very nice 0.

If you’re interested in adding solar to your home and have questions about this array or solar in general, give us a shout.

If you’re interested in building or designing a home, we would love to connect with you! Give us a call at (360) 872-6814 or contact us here.

DOE Efficient New Homes Program

DOE Efficient New Homes Program

by | Jun 26, 2026 | Building Science

The US Department of Energy has a new certification for newly constructed homes, focusing on promoting energy efficiency.

Every US Department of Energy Efficient New Home meets rigorous efficiency and performance criteria found in the DOE ENH National Program Requirements. Most types of new homes in the U.S. are eligible to participate in the program, and the homes are verified by a qualified third-party as part of the certification process.

Designed to deliver truly best-in-class performance, homes that meet the Efficient New Homes program requirements allow buyers to immediately recognize when a home has been certified through the rigorous efficiency and performance third-party verification process. The outcomes of that process mean homes include:

          • Energy-efficient heating, cooling, appliances, plumbing, windows, and lighting, which leads to lower bills and better air quality
          • Advanced insulation, air sealing, quiet window technology, and high-performance appliances, fans, and lighting that create a quiet, comfortable environment
          • Advanced building materials and products throughout the house that are proven to last.

    Peace of Mind

    Every Efficient New Home is fully inspected by multiple third-party, certified professionals. This means your home will meet and exceed the highest levels of performance—for everything from insulation to non-toxic paints.

    Better Living

    What makes an Efficient New Home different?

          • Air sealing to block dust, pollen, and pests. 
          • Construction materials with low or no chemical content. 
          • Water protection from roof to foundation minimizes moisture-related health problems and mold. 
          • Better indoor air quality
          • …and the list goes on and on.

    Best in Class Technology

    DOE Efficient New Homes exceed current building codes across every element—and some technologies are decades ahead.

    From high performance windows, to #energyefficient appliances, to future -ready infrastructure, you and your wallet will notice the difference with a DOE Efficient New Home.

  • To learn more about the Efficient New Homes Program go to the ENH website for more information.
Home Energy Rating System

Home Energy Rating System

by | Dec 12, 2025 | Building Science

Home Energy Rating System

Written by: Sarah DesRosiers

How energy efficient is YOUR home?  There are many factors that contribute to a home’s energy efficiency.  These factors include the home’s building structure such as insulation, windows, and doors.  It also includes the efficiency of appliances and lighting as well as the occupant’s living habits.  For example, does the occupant turn down the thermostat when leaving the home or unplug appliances when not being used?  In addition to the building structure of a home, the building orientation or how the home is positioned, on site, plays a role in how the home performs energy wise. How We Use Site Orientation to Maximize Energy Efficiency

According to the US Department of Energy US DOE, homes and buildings in the US consume 40% of the energy in our country.  On average, Americans spend around $2000.00 per year on energy.  Not only does energy efficiency matter because of the expense, but when it’s considered in your building practices, it improves the quality of living in the space; making it more comfortable

What is HERS?  HERS stands for Home Energy Rating System.  It is a way to measure your home’s energy efficiency by testing the Heat Recovery System.  How does it work?  A HERS Index rating is determined by a certified RESNET HERS Rater.  The steps to determine a home’s HERS rating is a multi-step review of a home’s energy usage/performance.  The first step involves a pre-construction or pre-inspection plan review where the HERS Rater inputs floor plans, building materials, HVAC equipment, windows/doors and insulation plans into an approved energy software, which predicts a HERS Index prior to the build. Next, the certified HERS rater inspects a project at a couple different stages of the project.  An inspection is performed at the pre-drywall phase and then again at the home’s completion to test the buildings performance. The final diagnostic testing is the way in which the final HERS rating is determined.  During this final inspection, several diagnostic tests are performed.  First, a Blower Door Test is done where a calibrated fan is placed in an exterior doorway.  The home is then depressurized.  The fan can then measure how much air leaks through the exterior envelope of the home. Secondly, the certified Rater will perform a Duct Leakage Test where a duct blaster is used to pressurize the ducts and measure the leakage to the outside.  Thirdly, the Rater inspects the insulation installation to determine air barrier continuity.  The fourth inspection includes the Mechanical System Verification where the Rater considers the HVAC type and its efficiency, the water heater efficiency, the ventilation systems, and the efficiency of appliances and lighting.  With all the above testing, the Rater determines a final calculation of the HERS index.  The final HERS Index score reports the home’s energy use breakdown, projected cost of utilities, efficiency recommendations, and test result of duct leakage. 

What does the HERS Score actually mean?  According to RESNET HERS INDEX, the lower the HERS score, the more efficient the home.  A HERS rating of 100 is equivalent to a standard home built to code.   A score of 50-60 is a build that performs above average. Every point under 100 is 1% more efficient.  A HERS 0 is a Zero Energy home, which means the home requires no additional power.  Essentially, it’s a home that produces enough electricity to offset what it uses and a HERS <0 is a Net-Positive energy home meaning that the home produces more power than it uses. Imagine charging your electric car AND still having a $0.00 power bill at the end of the month; this is a Net-Positive Home. 

At Legend Homes, we build award winning Net Zero and Net Positive Energy Homes.  A HERS rating is important in determining just how energy efficient a build is upon completion.  Our most recent spec home has a HERS rating of -4, which means that this home is Net-Positive. The image depicts the HERS rating score for this recent build.  

 

A HERS rating is essentially the gas mileage rating of a vehicle, but for a home. It is the best gauge in determining the quality of the invisible or internal workings of a home.  At Legend Homes, we use EPS foam in SIP’S panel walls and roof systems to create a super insulated envelope. FAQ's about SIPS  We use insulated concrete forms or ICF’s to complete the envelope in the ground around the footprint.  Additionally, we use high efficiency triple pained windows as well as the most efficient electric heat pumps available in North America. We orient our homes to the sun to capitalize on solar exposure for both solar panels and solar heat gain. We look for High R-values and air seal our homes. We use a process called aero barrier, which is a process that involves pressurizing a home and misting latex particles into the air that collect and seal any unseen cracks.  We use HRV’s (heat recovery ventilation system) that harvest warm conditioned air from the inside of the house and transfer it to fresh air being brought into the house. This allows for high air quality in combination with energy conservation.  How We Build for Energy Efficiency

Building an energy efficient home requires so much more than simply adding solar panels to a roof.  There’s a lot of buzz around Green Building and we want consumers to be aware of misleading advertising often referred to as “Greenwashing” where products are advertised as environmentally friendly or Green when they really are not.  Solar power is an important piece, but only one factor in achieving a true Green Built Home.   Our award-winning Green Built Homes are designed in a manner where every part of the design is geared toward achieving a Zero Energy/Net Positive outcome.

At Legend Homes, we build houses that are not only comfortable to live in, beautiful to the eye, but also inexpensive to run and better for the planet!  Why Sustainable Housing Is Important

 

Case Study & 2024 HIA Winner: The Lynden Powerhouse

Case Study & 2024 HIA Winner: The Lynden Powerhouse

by | Jul 18, 2024 | Building Science

 The Lynden Powerhouse Case Study, written by Talia Dreicer, was featured in this season's Master Builders Association Publication!

"The Lynden Powerhouse is a highly efficient, net-positive energy home that cultivates meaningful aging-in-place living and community connections in Lynden, Washington. Designed by Powerhouse Designs and built by TC Legend Homes, this 5-bedroom, 4.5-bath home features three distinct living areas and seamlessly merges innovative design techniques, a high-efficiency HVAC system, and automation technology to create a beautiful and comfortable home. Receiving DOE Zero Energy Ready Home, ENERGY STAR, and EPA Indoor airPLUS certifications, along with a Built Green 5-star certification, this home is a shining example of
high-performance housing.

In the years leading up to their retirement, Baby Boomer couple Lee Anne Caylor and Bob Edmiston saw two potential paths. According to the couple, the choice was “a decision between lives of generativity or stagnation.” Caylor and Edmiston chose the path of generativity, focusing on creating a future where they could share time and knowledge with family, friends, and community. They started envisioning their future home, the Lynden Powerhouse, as an accessible, comfortable, and healthy living environment that would
enable meaningful intergenerational living and support human connection. The COVID-19 pandemic only furthered their interest in positively impacting the lives of their family, community, and the planet, helping them refocus from their professional goals towards providing solicitude for both younger and older family members alike. The couple also saw the Lynden Powerhouse as a chance to push the high-performance housing envelope. Edmiston, a self-described “mad scientist” with a technical background and keen mind for
understanding how things work, wanted to incorporate automation technology and add resiliency measures to optimize resources and reduce future financial burdens. Seeking a team that could deliver on their project goals, Caylor and Edmiston approached Powerhouse Designs and TC Legend Homes to design and build the Lynden Powerhouse."

Read the full case study here!

Additionally, we were excited to hear this week that Lynden Powerhouse is one of the 2024 DOE Housing Innovation Award winners! We don't know what category the home has won yet, but we are beyond ecstatic and grateful to be receiving yet another win.

This was truly possible because of the collaboration with our incredible clients who were willing to push the envelope with us. We have the deepest appreciation for them and the innovative smart home features they created!

6 Frequently Asked Questions About SIPs

6 Frequently Asked Questions About SIPs

by | Mar 14, 2024 | Building Science

SIPs FAQA photo of a man applying a liquid mastic to the edge of a SIP during the installation process.

There seems to be a lot of confusion around Structural Insulated Panels (SIPs) in home building. In this article, we will run through the most common questions we’ve seen!

Are SIPs panels non-combustible? / Are SIPs panels fire resistant?

While SIPs can still catch fire, they are less combustible than traditional stick-framed homes. That’s, in part, due to less wood being used and because of the flame retardants used on the foam. In combination with the gypsum drywall, the whole assembly has a 1-hour fire rating. SIPs construction also does not require an attic space, which can be a fire hazard, thereby eliminating a major threat.

How are SIPs panels made?

SIPs are manufactured off-site to the exact specs of the house. Each panel is made up of two OSB panels sandwiching an EPS or GPS foam core. During the manufacturing process, workers cut out each window and door opening and cut all of the pieces perfectly to size, so that when they are installed on site, they will fit together perfectly like Legos. This also means that the panel off-cuts can be recycled and reused at the manufacturing plant, reducing the overall waste!

Are SIPs panels load bearing? / Are SIPs panels structural?

Yes! SIPs can be used for the entire exterior shell of a house. According to Insulspan, “SIPs have structural properties similar to that of a steel I-beam.” In fact, SIPs are actually considered more structurally sound than traditional stick-framed homes.

Are SIPs panels environmentally friendly?

Yes! SIPs are generally considered to be more environmentally friendly than traditional stick-framed homes. That being said, SIPs can use either EPS or GPS foam cores and EPS is not a very eco-friendly option. So, it’s best to use GPS SIPs in order to lessen the home’s overall carbon footprint. Also, since SIPs are manufactured off-site, they are made to the specific dimensions of a home which reduces the waste. Any scraps produced during manufacturing are also reused. Furthermore, when shipped, the panels are tightly flat-packed together reducing fuel needed for transportation.

Can SIPs panels get wet?

Yes! SIPs are built to dry out and can withstand exposure to the elements for many months. The foam core doesn’t absorb water unless it’s physically submerged or constantly wet for a long time. In all the years we’ve built using SIPs in the rainy Pacific Northwest, we have never had an issue with the SIPs compromising due to moisture.

How long will a SIPs house last?

SIPs are a super sturdy and hardy building material. Assuming you are properly maintaining your house (i.e. replacing your roof every 30-50 years, fixing any broken siding or trim, fixing leaks as soon as they happen, etc.), then a SIPs house will last 100+ years.

Still have questions? Shoot us an email! We’d be more than happy to answer any questions you have!

If you’re interested in learning more about our net zero energy homes or want to get started designing your very own net zero energy home, contact us today! We also offer pre-made net zero energy home plans!

Why Sustainable Housing is Important

Why Sustainable Housing is Important

by | Feb 23, 2024 | Building Science

To answer why sustainable housing is important, we first have to add context to the question. Each year Washington State adds roughly 35,000 to 43,000 new housing units to the market. However, to keep up with housing demands, we need to be building at least 50,000 new units each year in the state alone. Nationwide, that equates to needing upwards of 3 million new housing units each year. However, the construction industry accounts for a huge portion of the greenhouse gas emissions (GHGs) nationwide and globally accounts for roughly 37% of the CO2 emitted each year. Imagine if we made just a portion of those new homes more sustainable. The amount of CO2 we could save from being emitted into the atmosphere could have a profound impact on the future health of our planet.

Why sustainable housing is important A pie chart titled "Global Greenhouse Gas Emissions by Economic Sector." Each pie sections is as follows from greatest to least, 25% Electricity and Heat Production; 24% Agriculture, Forestry and Other Land Use; 21% Industry; 14% Transportation; 10% Other Energy; 6% Buildings.

Source: IPCC (2014)

Delving even deeper, there are many different ways the construction industry impacts the overall GHG emissions. As the above chart shows, there’s a sector just for “Buildings,” which consists of the GHG emissions caused by the daily use of buildings such as cooking, heating, waste management, and providing electricity to a building. In construction, we also need to consider the GHG emissions that are produced during the process of building the homes and making all of the materials.  That means that, during construction, we are also emitting GHGs from the “Industry,” “Electricity and Heat Production,” “Forestry and Land Use,” and “Transportation” sectors. Altogether, this makes up over a third of all GHG emissions.

In addition to GHG emissions, the production of every material, the deforestation to clear a jobsite, the excavation of a site – all of it has an impact on the environment as well. Deforestation for materials or land removes habitat from the ecosystem, and contributes to erosion which can impact our waterways, as well as impacts the albedo of that plot of land. Producing different materials may require mining out precious metals, minerals, or rocks which has a whole slew of negative impacts on the environment from habitat destruction to the leeching of pollutants into our waterways (think about Flint, MI). Beyond mining, the actual production and delivery of the material takes a lot of energy, produces waste, and can produce a multitude of different pollutants, not just CO2.

If you think about it, every item you choose for your home will have some level of impact on the environment. So, when we talk about sustainable housing, we are thinking about each of these materials and what choices we can make that will lessen our overall impact.

Finally, once a construction project is complete, the building itself will continue to impact the environment.  Remember the “building” portion of the pie chart above? When we are building sustainably, we are also thinking about post-construction and how the home will perform sustainably. We want to make the home as energy efficient and water efficient as possible, to lessen those respective environmental impacts after move-in. Of course, the less energy the home is using while occupied, the less GHG will be emitted in order to create that energy. If renewable energy is used to power a home, then that also helps reduce the GHG emitted, but we still want to aim for decreasing the overall energy use since every energy source we use will emit GHGs in some way, some just naturally less than others. We also want to reduce the water consumption of a home while occupied, as clean water is a finite resource, and we are rapidly draining our valuable aquifers and the glaciers that supply our rivers are melting and not returning. Therefore, it’s critical that we reduce our overall water usage.

So, to answer why sustainable housing is important, we have to look at the cumulative impact of the construction industry. Simply speaking, sustainable housing helps us minimize negative environmental impacts from pre- to post-construction, not just to reduce electricity consumption or because of climate change.

At TC Legend Homes and Powerhouse Designs, we are committed to building only net zero energy homes or net zero energy ready homes and are actively working to reduce our home’s overall carbon footprint for a carbon neutral future!

 

If you’re interested in learning more about our net zero energy homes or want to get started designing your very own net zero energy home, contact us today! We also offer pre-made net zero energy home plans!