How to Choose a Solar EPC Contractor: 10 Questions to Ask Before You Sign

The EPC contractor you choose for a commercial solar project will be responsible for the system's design, equipment quality, installation workmanship, grid connection, and performance for the next 25 years of warranty coverage. A low quote from an under-qualified contractor costs far more over the system's lifetime than the upfront saving. These 10 questions reveal whether an EPC is genuinely capable — or just cheap.

Why EPC selection matters more than equipment selection

The highest-efficiency modules and most reliable inverters underperform when poorly installed. Shading losses from incorrect string grouping, voltage drop from undersized DC cabling, yield losses from incorrect tilt angle, and premature failures from improper waterproofing — these are contractor failures, not equipment failures. Commercial rooftop system design has dozens of decisions that only a qualified engineer can make correctly. The 10 questions below help you distinguish contractors who know these decisions from those who do not.

Question 1: Can you provide a detailed technical design before contract signing?

A bankable EPC will provide, at minimum, a preliminary single-line diagram, module layout plan, string configuration, shading analysis output, and a P50 yield estimate from recognised software (PVsyst, SAM, or equivalent) before you sign a contract. Contractors who offer a final design only after you pay a deposit are concealing their design capability from scrutiny. Request a design deliverable list as part of the RFQ response.

Question 2: Which specific modules and inverters will you use — and are they on the approved equipment list?

Many grid operators and lenders maintain Approved Equipment Lists (AELs). Ask for the exact model numbers of every major component: modules, inverters, mounting system, and monitoring. Then verify those models appear on the relevant local utility AEL. An EPC who proposes "tier-1 modules, TBD brand" is either planning to substitute at procurement or does not know the local grid code requirements. Equipment should be specified before signing, not after.

For reference, Sungrow inverters (SG250HX, SH250CX, and the full commercial range available from Econo Solar) are certified for IEC 62109, CE marking, and most national grid codes — confirm with the EPC that certification applies in your specific jurisdiction.

Question 3: How many completed projects of similar size can you reference — and may we visit one?

References are easy to claim; site visits reveal the truth. Ask for three completed projects of at least 60% of your project's size in a similar climate and grid environment. Request to visit at least one, with access to the monitoring dashboard showing actual vs. predicted yield. A project performing within 5% of P50 after two years of operation is a strong quality signal.

Question 4: Who handles the grid connection application — and what is your track record?

Grid connection is the most common cause of project delays for commercial solar. A capable EPC handles the full DNO/DSO (distribution network operator) application process in-house, with a named engineer responsible for the submission. Ask specifically: How many grid connection applications have you submitted in this region? What is your average approval timeline? Have any been rejected, and why?

Question 5: What does the performance guarantee cover, and how is it enforced?

A performance guarantee (PG) should specify a minimum annual yield in MWh (not a percentage of "designed output" — that's circular) for at least the first 5 years, with a clear remedy: typically, the EPC pays for lost revenue or corrects underperformance at their cost. Scrutinise the exclusions: most PGs exclude losses from grid curtailment, soiling, and irradiance below P90 levels. These are reasonable exclusions — flag any that also exclude equipment faults or shade from the contractor's own design errors.

Question 6: What is your structural load analysis process for roof-mounted systems?

Every commercial rooftop project requires a structural assessment to confirm the roof can carry the additional dead load (modules + racking) and wind uplift loads. The EPC should commission a structural engineer's report, not self-certify. Ask: Do you use a licensed structural engineer? Is the mounting system wind-load tested to the relevant standard (EN 1991 in Europe, ASCE 7 in the US, local equivalent elsewhere)? Will you provide the structural engineer's signed report as part of project handover documentation?

Question 7: How is your installation workforce qualified?

Ask whether the installation crew are direct employees or subcontractors. For subcontracted crews, ask what qualification standard is required — relevant certifications include IEC/NABCEP (US), MCS (UK), and national electrician licences. High subcontractor turnover, or unwillingness to name the electrical subcontractor in advance, are warning signs. The commissioning engineer should be a named individual with specific inverter manufacturer training — Sungrow-certified commissioning engineers, for example, are required for some warranty provisions.

Question 8: What monitoring system is included, and who owns the data?

Commercial systems should include string-level or at minimum inverter-level monitoring with remote fault alerting. Ask: Is monitoring included for the full warranty period, or just year one? Who owns the monitoring data — the EPC, the platform vendor, or me? If the EPC goes out of business, can I access historical yield data? iSolarCloud (Sungrow's platform) retains data in the owner's account — confirm equivalent data portability for any third-party monitoring system proposed.

Question 9: How do you handle equipment manufacturer warranty claims during the O&M period?

Module and inverter warranties are manufacturer obligations, but the practical process of claiming them requires the EPC or O&M provider to log faults, submit RMAs, handle shipping, and arrange reinstallation. Ask: Do you have a direct authorised service relationship with the inverter manufacturer? What is your typical inverter swap-out time for an in-warranty fault? Our Sungrow warranty guide covers the claims process in detail — a capable EPC should know it.

Question 10: What is your financial stability, and do you carry professional indemnity insurance?

An EPC that goes out of business 18 months after commissioning leaves you without a workmanship warranty enforcer. Request three years of audited accounts (or equivalent financial evidence), confirmation of professional indemnity and public liability insurance, and ask whether they offer a retention or bond arrangement for the first 12 months post-commissioning. Small EPCs without these protections are high-risk for long-duration projects.

The EPC evaluation scorecard

If you are sourcing equipment independently from an EPC-supplied package, contact Econo Solar for direct-supply pricing on Sungrow inverters, BESS, modules, and mounting systems. Supplying bankable, certified equipment directly to EPCs and project owners is our core business — with FOB pricing and 24-hour RFQ response.