The journey to net-zero buildings and high-offset solar PV system projects begins with a compelling story. PV models forecast annual production that meets or exceeds building energy needs, charts align and stakeholders confidently move forward. It’s a powerful goal, but one you need an experienced team to achieve.
Modeling tools are incredibly valuable for understanding potential performance. They help visualize what’s possible and set initial targets. The key is to remember that these models provide a representative value within a distribution, not a single, absolute prediction.
After a couple of years of operation, the actual utility bills and monitoring data for the building might not quite match the opening chapters. When energy use is higher or generation is lower than anticipated, the “net-zero” label can begin to feel less secure.
At a recent Illinois Green Alliance education event, a group of engineers, construction professionals and service technicians from Windfree Solar talked about why this happens. The main idea that emerged is simple but uncomfortable: Most projects live in a “gray zone” between modeled performance and reality — and that gray zone usually works against the
owner.
So, what can contractors, EPCs and developers do to shrink it?
Two models, two different biases
Most net-zero or high-offset projects are built on two core pieces of analysis:
- A PV production model, often using tools like PVWatts or more advanced software
- A building energy model that estimates annual consumption
Each of these models brings some uncertainty. More importantly, each tends to be biased in a different direction. On the PV side, tools and users often lean toward optimistic assumptions. Tilt, soiling, mismatch and loss factors are entered as “typical” or even “best-case.” In practice, the output number many teams use for decision-making often sits near the 75th or 80th percentile of what the system will produce over the long-term.
On the building side, concept energy models usually represent the most efficient version of the building. Schedules and setpoints are clean. Systems are perfectly commissioned. Plug loads are modest. The model often describes an ideal building that is rarely what happens in actual operation.
When you place an optimistic PV curve next to an optimistic building-load curve, they appear to overlap in a comfortable way. The PV model shows enough production to cover the modeled demand. On paper, the project is net-zero.
In reality, both curves are wider than that and the real outcome tends to fall somewhere in the middle. There is a region where lower-than-expected PV production and higher-than-expected building use overlap. That is the gray zone where projects that were modeled as net-zero end up as “close, but not quite.”
Real-world forces that pull PV performance down
Even when the system matches the design drawings, reality sets in to quietly pull actual PV production toward the low side of the expected range.
Some examples that came up during the Illinois Green Alliance discussion:
- Air quality and smoke events. During recent Canadian wildfire smoke episodes in the Midwest and Northeast, many solar PV systems saw 5 to 10% drops in production during peak summer weeks. Those kinds of events are not built into typical data.
- Soiling and shading changes. Vegetation that grows higher than expected, slight differences in rooftop equipment location and local dust sources can all reduce production compared with clean, unshaded model assumptions.
- Design evolution during the project. As a project moves from concept to detailed design, structural limits, roof loads, parapet changes, or site grading may force modules to move or be removed. The PV model is not always updated each time that happens.
- Module degradation over time. Standard systems usually carry warranties that guarantee performance at around 80 to 85% after 25 years. If the original model assumes little or no degradation, then real production will be lower than forecast almost immediately and the gap will grow over time.
None of these factors are unusual. Taken together, they make it far more likely that long-term
PV production lands in the middle or lower part of the modeled range, not right at the optimistic value many teams use for early decisions.
Why buildings usually use more than the model suggests
The same drift happens on the building side, but in the opposite direction.
- Envelope and air leakage. Small differences in door installations, window details, air barriers and long-term seal durability lower building efficiency.
- Operations and maintenance. Controls do not always stay as tight as originally specified. Schedules can be extended. Economizers may not work as intended. Commissioning findings may not all be acted on. Over time, these realities increase energy use.
- Plug and process loads. Tenants and staff bring more equipment than the design team imagined. Coffee machines, under-desk heaters, network hardware, lab or shop tools and other loads add up quickly.
- Occupant behavior. Doors are propped open. Setpoints go up or down to address comfort complaints. Blinds are used differently than assumed. These behaviors compound over years of operation. The building model is often treated as a one-time design tool. Once permits are issued, many projects never revisit it, even when changes in programming, equipment or controls clearly affect energy use.
Acknowledging the gray zone
If we treat both PV production and building use as ranges instead of fixed numbers, the existence of the gray zone is obvious. There is always some chance that:
- PV production will be lower than the central forecast
- Building use will be higher than the central forecast
When those two things happen at the same time, the project misses its net-zero or offset target.
The question for contractors and developers is not whether uncertainty exists — it is how to reduce the chance that their project lands in that overlap region.
Practical steps to shrink the gray zone
The panel at the Illinois Green Alliance event pointed to several practical habits that can be built into everyday work.
- Use more conservative PV assumptions.
Instead of defaulting to best-case like good weather, consider applying slightly higher losses for soiling, wiring and downtime. If you have enough data, think in terms of P50 and P90 values instead of a single annual kWh figure. Being candid about these assumptions with building owners early on helps manage expectations. - Push for realistic building models.
Ask energy modelers to use more realistic schedules, plug loads and occupancy patterns. Where similar buildings already exist, compare model outputs with actual metered data as a reality check. If the model looks much better than real buildings of the same type, that is a warning sign. - Update both models when designs change.
When roof layouts shift, modules are removed, or mechanical systems change. It needs to become standard practice to request updated PV and building model runs, even if they are quick. Make it usual to ask, “What did this do to our production margin and our load?” - Design for a downside case, not just the middle of the road case.
When sizing PV for a net-zero goal, test a simple scenario: PV production is 5 to 10% lower than modeled and building use is 5 to 10% higher. If the project can still meet its target, then the performance claim is more robust.
Making net-zero claims that last
Most contractors and developers want their net-zero and high-offset projects to perform as promised. The goal is not simply to pass design review, but to deliver buildings that continue to meet their targets long after the ribbon cutting.
The first step is to acknowledge that both PV and building energy models are just that — models — and they aren’t hard and fast production expectations. Also, their natural biases often work against each other. The next step is to treat performance as a range and to make deliberate choices that narrow the gap between “low PV” and “high load” outcomes.
That process does not require gatekept knowledge or exotic tools. It mostly requires clearer communication, more frequent updates to models as projects evolve and a willingness to use conservative assumptions where they matter. If the industry can make those habits routine, more net-zero labels will match what the meters show and solar contractors and developers will strengthen both their reputations and the broader credibility of net-zero claims.
Joseph Clair is Vice President, Commercial at Windfree Solar. With a background in zero-energy building design, solar PV integration, and energy modeling, he helps align realistic PV production with building energy use on commercial and institutional projects. As Marketing Director, Brittany Brilliant translates Windfree’s technical expertise into clear articles and educational resources for contractors, architects, and other key stakeholders in net-zero building projects. Jack Johannesson, Director of Sales & Business Development, draws on his foundation in energy finance and project management to guide commercial clients through the economic and practical benefits of going solar.
