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Understanding Solar Production Estimates

by | September 9, 2021

So you’re comparing solar quotes and the production estimates are very different?  This is actually very common.  But it can also be very confusing.  In this post, we give you the tools you need to understand solar production estimates—and to determine which ones to trust.

The Basics

Solar energy production is measured in kilowatt-hours, or kWh.  Kilowatt-hours are also what you buy from your utility.  When your solar panels produce a single kWh, you typically buy one fewer kWh from your utility.  That’s how solar panels save you money.

You may be familiar with the measure of solar capacity.  Solar capacity (also termed “power”) is measured in watts or kilowatts (kW).  For example, you might be considering an 8.64 kW system.  The “8.64” doesn’t tell you very much about how much energy the system will produce; it’s only a measure of the system’s capacity to produce energy.  Whether it produces a lot or a little energy is a function of both capacity and environmental conditions, like the sunniness of your roof.  Silly illustrative example: an 8.64 kW system installed in your basement, or upside down, will produce zero kWh.

So, given that production is a function of (1) kW capacity and (2) environmental conditions, a basic, generally good assumption is that systems of equal capacity in identical environmental conditions will produce approximately the same amounts of energy.*

*Caveat: Higher-end equipment, especially equipment optimized to work together (like SunPower panels, microinverters, and racking), will typically produce more kWh from the same number of watts.  Technical assumptions like wiring losses, inverter efficiency, inverter loading, etc. ensure that not all watts produce equally, even in identical conditions.

Why Production Estimates Vary

If you (1) vary the system’s capacity or (2) vary the environmental conditions, you’ll almost certainly end up with different production estimates.

Let’s start with the easy one first, capacity.  Imagine a single roof plane, entirely unshaded.  You’re considering two quotes, one for 8.64 kW (24 360W panels) and the other for 9.36 kW (26 360W panels).  The environmental conditions for these systems are identical: They’re on the same unshaded roof.  But the capacities are not the same: The second system is 8.33% bigger.  Thus, we would expect the production estimate for the second system to also be 8.33% bigger.  If it’s not (if it’s more or less than an ~8.33% increase), then whoever estimated the production must have used different assumptions for environmental conditions.

A secondary question here, related to capacity, is: Why is one quote for a bigger system (more kW) than the other?  It’s possible that the solar consultant for the first system had a different (lower) estimate of your 12-month energy usage.  Thus, a smaller system made more sense.  Obviously that doesn’t exclude the possibility of upsizing that system to reflect new information.  Read more about sizing solar systems.

Another possibility is that the first consultant didn’t believe more than 22 panels would fit on your roof—or at least that more than 22 wouldn’t be approved for a permit due to restrictions in the building, fire, or electrical codes.

There are some other Illinois-specific considerations that can affect system size.  Wisconsin systems are typically acceptable under 20.00 kW AC.

Now, let’s talk about environmental conditions.  These include:

  • Solar resource (based on geography; Arizona is better than Maine, for example)
  • Pitch (the angle of the roof)
  • Azimuth (the directional orientation of the roof)
  • Shade (from trees or other structures)
  • Weather (snow cover, etc.)

The first three of these (solar resource, pitch, azimuth) are relatively easy to determine; they rarely contribute to differences in the production estimate.  That leaves shade and weather as the likely culprits.  Shade is somewhat difficult to determine because it requires complex 3D modeling of trees, buildings, chimneys, etc.  Two designers might end up with very different shade assumptions—and thus two very different production estimates.  And, two designers may have somewhat different weather assumptions (the snow cover assumption makes a difference in the upper Midwest).  Ultimately, however, there’s only one future as far as shade and weather are concerned; and it obviously doesn’t change if you pick one solar installer or another.

Who To Trust

With so much conflicting information, it can be difficult to know who to trust.  We have some thoughts that may help:

  • Think about incentives.  Solar companies obviously make more money when they sell you more panels.  But do you really need those north-facing panels that hardly produce any energy?  Probably not.  Certasun’s “No Surprises Guarantee” means we won’t recommend a system bigger than what you need.
  • Look for “skin in the game.”  That production estimate is usually just an estimate.  Picking the company with the higher estimate won’t make trees disappear or tall buildings go away.  So, what happens if the system doesn’t produce all that energy?  Certasun takes financial responsibility for your system’s success with our production guarantee and production-based incentive guarantee.  Don’t let someone tell you the panel manufacturer’s “power warranty” does the same thing; it only guarantees capacity (watts or kW), not production (kWh).
  • Audit the design.  Most Certasun designs use LIDAR data (from planes shooting lasers at the ground and measuring their bounce-backs) to simulate, in 3D, the presence of trees and other structures.  Our designers also use local regulations to determine setbacks (distance of panels from edges or ridges of the roof).  For example, in determining setbacks, municipalities may reference various versions of the International Building Code, International Residential Code, International Fire Code, and the National Electrical Code.  Our designers know which codes and which years of the code each municipality has adopted.  And we’ve received permits in over 200 municipalities.
  • Consider track record.  Look at online reviews.  Look at the Better Business Bureau (BBB) for complaints.  A company that’s doing things right will have many good reviews and few complaints; and of course the reverse is true, too.

Certasun Can Help

Obviously we’re in the business of installing solar panels.  So we’re not exactly unbiased when it comes to your choice of solar installer.  But if you need help sorting through conflicting information, you can count on us to give you straightforward, honest answers.  It’s all part of our No Surprises Guarantee.  Please fill out the form below and you’ll be able to schedule time with one of our expert solar consultants.

This post was written by Jesse Feinberg.
Jesse is one of Certasun's founders.

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