How to Choose a Commercial Solar EPC: Part 1

Most commercial solar procurement starts with three quotes, a price-per-watt comparison, and a handshake. It ends, sometimes years later, with a system that generates 15% less power than projected, an EPC that stopped returning calls after commissioning, and an ROI model that no longer holds.

The gap between a solar project that performs and one that disappoints is almost never the panels. Panels are commoditized. The gap is the EPC contractor: how they engineer, how they build, how they stand behind the system for the next 25 years. For manufacturers running high-load facilities and property portfolio owners managing energy costs across dozens of rooftops, that gap is the difference between a deflationary asset and a capital write-down.

This article—the first of a two-part series—lays out how to evaluate an EPC’s true price and engineering capabilities before signing a contract.

Why does the price-per-watt comparison fail for commercial solar?

Price per watt tells you what the hardware costs. It tells you nothing about whether the system will produce what it promised in year seven, whether the contractor will answer the phone when it does not, or whether the company will still exist to honor its warranty.

The reason is scope. Two EPC proposals for the same facility can look comparable on price while covering fundamentally different ground. One includes structural engineering, full utility interconnection, commissioning documentation, and a five-year workmanship warranty. The other covers panels, inverters, and installation labor, with everything else listed as “client scope” in the fine print. The cheaper quote is often the one that excluded the most.

A manufacturer evaluating a 500 kW rooftop system should normalize every proposal to the same scope before comparing price: engineering coverage, structural assessment, grid interface work, monitoring, commissioning documentation, defects period, and O&M provision. Without that normalization, you are comparing line items that were never meant to be compared.

Portfolio owners face the same problem at scale. When you are evaluating EPC partners across multiple buildings, a $0.05/W difference in headline price can mask a $100,000 difference in total project cost once excluded scope items surface mid-construction. The financial discipline you apply to any other capital allocation applies here: compare total cost of ownership, not installation price.

What does an EPC’s engineering capability actually tell you?

Engineering is the part of the EPC evaluation most buyers skip, and it is the part that determines whether your ROI model survives contact with reality.

A yield model with no visible assumptions is a marketing number. You should be able to see the irradiance data source, the losses applied and why, the soiling assumption for your specific site and climate, the temperature model, and the degradation rate over the modelled period. If the contractor cannot produce the model, or produces one with default assumptions for a facility in a dusty industrial zone, that tells you how the rest of the work will be done.

For manufacturers with large-footprint facilities, structural assessment is where engineering either protects you or exposes you. A rooftop array that overloads an older industrial roof structure is a serious and under-discussed risk. The question to ask is who assessed the roof and whether that assessment carries an independent professional stamp.

Electrical design matters equally. The single line diagram, protection coordination, earthing, cable sizing calculations with voltage drop, and the interface arrangement with your existing installation should all be documented. For a facility that operates alongside backup generators or an unstable grid, the control philosophy matters as much as the hardware.

Otter Energy maintains in-house engineering teams that handle design, PV modeling, and electrical engineering, while partnering with specialized third-party structural engineering firms like Compass to independently validate roof integrity before a single panel arrives on site. This hybrid model gives you the speed of in-house design alongside objective, third-party structural sign-off.

How do you verify an EPC’s track record beyond the capability statement?

Every EPC has a capability statement with photographs. Photographs prove that somebody installed panels somewhere. They prove nothing else.

The most informative document in the entire selection process is one comparatively few buyers ask for: a full year of monthly generation data from a reference site, measured against the original prediction. A contractor whose systems consistently perform close to conservative predictions will be pleased to share this. One whose systems fall short will find reasons the data is unavailable.

Contact references directly and ask the questions that are hard to answer well. Did the system meet its predicted output in its first full year? What happened when something failed? How long did the contractor take to respond? Were there cost variations, and what caused them?

For manufacturers, ask specifically about experience with commercial and industrial facility types. Residential roof experience does not translate to a 1 MW commercial installation—the structural ballasting, high-voltage cabling, inverter topologies, and grid interfaces are entirely different disciplines. Contractors who primarily build residential systems often simply don’t know what they don’t know when scaling up to commercial roofs, introducing massive operational risk to your building.

Otter Energy’s track record includes deployments for global logistics operators, industrial manufacturers, and commercial property portfolios across Eastern Canada. With 200+ MW installed and 500,000+ panels commissioned, the reference list runs deep enough to find a comparable project and verify the numbers directly.

In Part 2 of this series, we look closely at production forecasts, delivery structures, tax credit compliance, and how to structure your EPC contract to protect long-term asset value.