Last week, our design team encountered a prime example of why you should always get multiple proposals when exploring solar for your home. A homeowner requested quotes from both us and another company. Both solar bids were sized to offset the same amount of electricity annually and their estimated production was indeed on par with each other, yet our design was about 1 kW larger.
A tale of two solar bids
The customer’s question was completely logical: given our larger system size, why were the production estimates so close to each other? Shouldn’t our numbers be (roughly) proportionally higher?
We had the same question. All things being equal, the two designs shouldn’t have been quite so close in expected output. Both systems were modeled used same solar design software and by default would have utilized the same shade, weather, and solar irradiance data. And though we adjusted our design to reflect the same system specifications as the other proposal, we couldn’t replicate their numbers.
Finally, we thought of one more trick to try, and disabled the onsite LIDAR shading in the model. In layman’s terms, this means to digitally remove the surrounding trees and negate any shade they would have cast upon the roof. When we ran the performance simulation again—bingo—the numbers lined up.
Except…none of those trees were going to be removed.
Factoring shade into your system design
Unless your house is in the middle of a field, you’re likely to have at least some shading, even if it’s just a lone tree down the block that only casts a shadow on your roof for a few weeks every winter. Still, that needs to be factored into your system design, not only for optimal system efficiency and functionality, but in order to meet your goals. If your system was designed to offset 100% of your usage, you’d be pretty disappointed to discover that, in real world conditions, it only covered 85%.
Let’s look at a different example. The most optimal roof planes on the house below face southeast and one is partially shaded by the neighbor’s massive magnolia tree. If the goal is to offset around 14,700 kWh each year, you might see designs similar to the two below.
At a glance, the first design looks like the best bet. It’s the smaller of the two options and yet it manages to generate practically all of the electricity needed. It’s a win-win! Except it’s not, because we know there’s a big tree shading that lower roof. In reality, that system would need a few more panels and some positioning adjustments to meet the production goal.
Modeled without LIDAR shading
System size: 12.88 kW
Annual production: 14,669 kWh – 99.8% offset
Modeled with LIDAR shading
System size: 14.26 kW
Annual production: 14,959 kWh – 101.8% offset
Once you factor in real-world shading on the first design—with zero changes to the layout—that production number starts to look a little different. About 1,379 kWh different. And its pretty obvious that the location of the smaller array is not particularly ideal.
Modeled without LIDAR shading
System size: 12.88 kW
Annual production: 14,669 kWh
Modeled with LIDAR shading
System size: 12.88 kW
Actual annual production: 13,290 kWh – 90.4% offset
Using the right tool for your site
Are there times when it makes sense to model a system without LIDAR shade data? Absolutely! We regularly adjust how onsite shading is modeled due to a variety of scenarios, such as if:
- nearby trees have recently been topped or removed
- the home is newly (or soon-to-be) constructed and located on a cleared lot
- LIDAR data is too old or the quality is too poor to be useful
What’s most important is that your system designer has a conversation with you about how your system was modeled, so you know how they arrived at those numbers. And while a preliminary remote analysis typically gets us 95% of the way there, your designer should always finalize their proposal with a site visit. This ensures that their design assumptions accurately reflect your current roof and shade conditions.
What to expect when comparing solar bids
Typically, systems are sized to offset up to 100% of your annual electricity usage, give or take a percentage point or two. As long as each designer is working toward the same annual goal, you can expect proposals to be similar in overall system size/capacity (kW) and annual production (kWh), even accounting for different equipment.
If you see an outlier, ask how they’re accounting for shade. They should be able to provide you with a report detailing the impact of shading on your roof. All industry-standard solar modeling programs can calculate the TSRF, or total solar resource fraction, of a roof, taking into account its solar access (shading), tilt, and orientation.
Some modeling programs, such as Aurora Solar, also use color to produce a visual representation of how much sun will reach different parts of your roof, with high solar resource indicated by brighter yellows and oranges.
You can also do your own rough ballpark calculation based on average production for our region. The National Renewable Energy Laboratory (NREL) calculates that systems in western Washington produce around 1,100 kWh per kW installed. Multiply your system size (in kW) by 1,100 and that should get you in the ballpark. Of course, this doesn’t account for site-specific shading, roof pitch, or azimuth, so for most systems, you can expect your production estimate to fall below this number.
While the idealists among us would love to design solely for perfectly shade-free roofs, realistically, it’s our job to work with the roof we’re given. That means using all of the tools at our disposal to determine the best location(s) to install on your specific roof. Shade and all.
We hope this information helps you on your solar journey! As always, do your homework, get multiple solar bids, and never be afraid to ask too many questions.
