Why Solar Production Estimates Vary Between Installers

You have received three solar quotes, and each one predicts a different annual production number for roughly the same system size. Installer A says 12,500 kWh. Installer B says 14,200 kWh. Installer C says 16,000 kWh. They cannot all be right, so what is going on? The differences come down to the software tools used, the assumptions entered, and how conservative or optimistic the estimator is.

Design Software Differences

Solar production estimates are generated by specialized design software. The three most commonly used tools in Canada are:

PVWatts (NREL)

PVWatts is a free tool developed by the U.S. National Renewable Energy Laboratory. It is simple, widely used, and considered a reliable baseline. You enter the system size, location, tilt, azimuth, and a few system loss assumptions, and it returns an estimated annual production. PVWatts uses TMY (typical meteorological year) weather data and does not model individual panel placement or site-specific shading. It tends to produce conservative but realistic estimates and is a good benchmark for checking other tools.

Aurora Solar

Aurora is a comprehensive commercial design platform used by many solar companies. It creates a 3D model of your roof using satellite imagery and LIDAR data, then simulates shading throughout the year based on surrounding trees, buildings, and roof features. Aurora can model individual panel placement and account for micro-shading effects. Its production estimates are typically realistic when the 3D model is accurately built, but results depend heavily on the quality of the LIDAR data and how carefully the designer models shading obstructions.

Helioscope

Helioscope is another professional design tool that models panel layout, wiring, and production. It uses a different simulation engine than Aurora and may produce slightly different results for the same roof. Helioscope is well-regarded for its accuracy but, like Aurora, its output depends on the inputs the designer provides.

Input Assumptions That Vary

Even when two installers use the same software, they can arrive at different estimates based on their input assumptions:

Shading

Shading is the single largest variable in production estimates. A designer who carefully models every tree, chimney, vent, and neighbouring structure will produce a more conservative (and accurate) estimate than one who ignores or underestimates shading. Trees grow over time, so a responsible estimate accounts for future growth, not just current conditions.

System Losses

Every solar system has losses from wiring, soiling (dirt and debris), snow, inverter efficiency, temperature effects, and panel mismatch. Standard system loss assumptions range from 10% to 18%. An installer who uses 10% losses will show higher production than one who uses 16%, even with identical equipment and layout. Neither is necessarily wrong, but you should ask what loss factor is being used.

Panel Orientation and Tilt

The angle and direction of your panels significantly affect production. Due south at a tilt matching your latitude (roughly 51 degrees in Calgary) is optimal for annual production, but most roofs do not match this ideal. A designer who models the actual roof pitch and azimuth will give a more accurate estimate than one who uses idealized inputs.

Weather Data

Different software tools use different weather datasets. Some use 10-year averages, others use 30-year averages, and some use satellite-derived data rather than ground station measurements. These differences can cause production estimates to vary by 3% to 8% for the same location.

Degradation

Some estimates show first-year production, while others show average production over 25 years with degradation factored in. Year-1 production will be the highest, so make sure you are comparing estimates on the same basis. Ask whether the number represents year-1 output or a lifetime average.

How to Compare Estimates

When evaluating production estimates from different installers:

1. Ask what software was used: Knowing the tool helps you understand the methodology.
2. Check the shading model: Ask to see the 3D model or shading analysis. A blank roof with no shade trees when you clearly have shade trees is a red flag.
3. Ask about system loss assumptions: Get the specific percentage used and understand what it includes.
4. Verify panel orientation: Confirm the estimate uses your actual roof pitch and direction, not idealized values.
5. Clarify year-1 vs. lifetime: Make sure all estimates are showing the same metric.
6. Cross-check with PVWatts: Run your own PVWatts estimate as a baseline. If an installer's estimate is significantly higher than PVWatts (more than 10% higher for the same system size), ask them to explain why.

What Realistic Looks Like for Alberta

For a well-designed, unshaded solar system in Calgary or southern Alberta, realistic production benchmarks are:

• South-facing, 30-45 degree tilt: 1,300 to 1,400 kWh per installed kW per year
• Southwest or southeast facing: 1,150 to 1,300 kWh per kW per year
• West or east facing: 1,000 to 1,150 kWh per kW per year
• Flat roof (10 degree tilt): 1,200 to 1,300 kWh per kW per year

For a 10 kW system on a good south-facing roof in Calgary, expect approximately 13,000 to 14,000 kWh per year in year one. Estimates significantly above 1,500 kWh per kW for this latitude should be scrutinized.

Why Accuracy Matters

An inflated production estimate makes the financial return look better than it is. If an installer tells you your system will produce 16,000 kWh per year but it actually produces 12,500 kWh, your actual payback period is 28% longer than what you were told. Overpromising and underdelivering is a common complaint in the solar industry, and inflated production estimates are often the root cause.

A conservative estimate from a trustworthy installer is far better than an optimistic estimate that sets unrealistic expectations. The best installers provide realistic estimates and then their systems outperform them.

Key Takeaways

• Production estimates vary because of different software, shading models, and loss assumptions
• PVWatts is a reliable free baseline for cross-checking installer estimates
• Shading is the biggest variable; ask to see the shading analysis
• South-facing systems in Calgary should produce approximately 1,300 to 1,400 kWh per kW per year
• Always compare estimates on the same basis (year-1 vs. lifetime, same loss assumptions)
• Be wary of estimates that seem too good to be true; get a transparent quote from a company that values accuracy