Quick answer: Compare commercial solar proposals against one written project brief—not just their headline price per watt. Require each bidder to disclose DC and AC capacity, exact equipment, generation-model inputs, included work, exclusions, warranty obligations and acceptance tests. Resolve missing information before ranking bids, and keep purchase prices separate from financing and projected savings.
This guide uses U.S. equipment-certification examples. Confirm the applicable requirements with your local permitting authority and serving utility. Treat the worksheet below as a procurement aid, not a substitute for site-specific engineering or contract review.
1. Give every bidder the same project brief
Start by defining what you want bidders to price. Otherwise, your comparison may become a contest between different projects.
Issue a common information pack containing:
- Site address, serving utility and electricity tariff.
- Electricity bills and available interval-consumption data.
- Roof drawings, condition reports and known replacement plans.
- Electrical drawings and available service-equipment information.
- Areas available for solar and areas reserved for other uses.
- Access restrictions, business operating hours and permitted shutdown windows.
- Your intended ownership structure and required completion milestones.
Ask for a compliant base bid, with alternative designs priced separately. Require bidders to list assumptions, departures from the brief and information still needed.
DOE’s Federal Energy Management Program recommends project validation covering site suitability, electricity contracts, future site plans, access and electrical considerations. Its guidance also carries technical specifications through procurement, design, equipment delivery and commissioning. These are useful organizing principles beyond federal projects. (energy.gov)
For tenants, add a clarification: who can authorize roof use, electrical alterations and long-term equipment access? Do not invite bidders to assume those permissions exist.
2. Normalize capacity before calculating price per watt
Require three separate figures:
- Module capacity, in kW DC: the combined module nameplate rating at standard test conditions.
- Inverter capacity, in kW AC: the combined rated AC output of the inverters.
- Any proposed export limit: a separate operating constraint, not a replacement for either equipment rating.
The DC-to-AC ratio is calculated by dividing module DC capacity by inverter AC capacity. Inverter output can be limited when available array power exceeds the inverter’s output rating; modeling tools account for this behavior. A larger ratio is therefore a design choice to evaluate—not automatically a defect or an advantage. (samrepo.nlr.gov)
Use one denominator throughout your cost comparison:
Installed price per watt DC = comparable installed price ÷ module nameplate watts DC
Check that the numerator uses the same scope, tax treatment and incentive treatment. Ask bidders to show gross project cost separately from financing charges and any assumed incentives.
Common mistake: comparing one bidder’s dollars per watt AC with another’s dollars per watt DC. Another is comparing a gross purchase price with a price presented after assumed incentives.
Keep annual energy production separate. A capacity rating in kW and an energy forecast in kWh answer different questions.
3. Compare exact equipment and certification evidence
“Premium panels” or “commercial-grade inverter” is not a specification. Request a bill of materials identifying:
- Module manufacturer, model, wattage and quantity.
- Inverter manufacturer, model, quantity and AC rating.
- Racking and roof-attachment system.
- Monitoring hardware, meters and communications equipment.
- Relevant datasheets, certification records and warranty documents.
For U.S. module procurement, UL Solutions identifies UL 61730-1 and UL 61730-2 as module safety-qualification standards, while IEC/UL 61215 addresses performance testing. UL 1741 covers inverters and related distributed-energy interconnection equipment. These standards have different purposes; one certificate does not answer every safety, performance or utility-compliance question. (ul.com)
Ask your engineer to confirm the required standards, editions and utility-specific requirements. Require evidence covering the offered model, rather than a manufacturer’s general statement.
Also establish a substitution rule: no equipment change without written approval and updated datasheets, certification evidence, warranties and generation modeling.
Practical check: reconcile module count multiplied by module wattage with the stated DC capacity. Have the technical reviewer similarly reconcile inverter quantities and ratings.
4. Audit generation assumptions—not just annual output
Require the underlying production report, not only a sales-page total. Ask each bidder to disclose:
- Modeling software and version.
- Weather dataset and site location.
- Array layout, tilt, orientation and mounting configuration.
- Shading assessment, including nearby obstructions.
- Inverter sizing and efficiency.
- Soiling, snow, wiring and availability assumptions.
- Bifacial assumptions, where applicable.
- First-year output and the degradation assumptions used for later years.
- Where the forecast energy is measured—for example, inverter output or the agreed delivery meter.
PVWatts exposes inputs such as capacity, orientation, losses and inverter sizing, but explicitly warns that predictions contain assumptions and uncertainties. Site characteristics are represented only to the extent captured by the inputs. An independently run model is a reasonableness check, not measured evidence of future production. (pvwatts.nlr.gov)
If two forecasts differ materially, request a reconciliation. Do not force genuinely different layouts to produce identical results. Instead, use consistent weather and loss assumptions where appropriate, then identify what the design differences explain.
Keep three evidence categories distinct:
- Vendor projection: a forecast based on disclosed inputs.
- Independent review: a check of inputs, design and calculations.
- Measured performance: operating data from an installed system.
For the initial bid review, compare production separately from savings. Require a distinct financial model documenting tariff, self-consumption, export and financing assumptions.
5. Make roof work and interconnection exclusions explicit
Ask for a scope matrix using four labels: included, excluded, allowance, or unresolved.
For rooftop projects, request explicit treatment of structural assessment, roof repairs, reroofing, attachments or ballast, waterproofing, drainage, access and coordination with the roof-warranty provider.
For electrical and utility work, request treatment of:
- Application and study fees.
- Metering, protection and export controls.
- Switchgear or transformer work.
- Site electrical upgrades.
- Utility-required upgrades.
- Shutdown coordination and temporary arrangements.
DOE’s procurement template explicitly addresses site electrical modifications, utility interconnection studies, approvals, fees and upgrades. Use it as a scope prompt, not as evidence that those responsibilities automatically belong to your contractor. Your signed agreement must allocate them. (energy.gov)
For every allowance, ask: what does it cover, what evidence supports the amount, and who bears an overrun?
For every unresolved item, request a decision date and change-control process. “Interconnection included” is insufficient if the contract excludes studies or upgrades.
Do not insert an invented upgrade cost to make a spreadsheet look complete. Mark the exposure as unresolved and obtain a priced clarification before award.
6. Separate warranties, monitoring and maintenance obligations
Build a warranty schedule rather than comparing the longest advertised term.
Separate module product coverage, module power-output coverage, inverter coverage, installation workmanship and any contractual system-performance guarantee.
A manufacturer document illustrates why this matters: Canadian Solar’s September 2019 double-glass-module warranty separates product and performance coverage, verifies module output under standard testing conditions, and generally leaves removal and reinstallation costs with the buyer unless otherwise agreed in writing. This is a dated example—not the warranty to assume for a current proposal. Require the document applicable to the offered model and sale. (canadiansolar.com)
Ask each bidder:
- When does each warranty start?
- Who submits and manages claims?
- Who pays for testing, access, freight, labor and reinstallation?
- What exclusions and transfer conditions apply?
- What remedy is available, and who provides it?
Then specify the operating service separately. Request monitoring-access rights, data-export capability, subscription charges, alarm responsibility, response commitments and maintenance exclusions.
DOE recommends preparing the operations-and-maintenance plan during design and considering access, connected-inverter capabilities, staff training and spare parts. A dashboard alone should not be treated as your maintenance plan. (energy.gov)
7. Use a bid worksheet and test apparent bargains
Copy this worksheet into your procurement spreadsheet. Add a source-document reference beside every entry.
| Comparison item | Bid A | Bid B | Clarification needed |
|---|---|---|---|
| DC module capacity and AC inverter capacity | — | — | Reconcile ratings and quantities |
| Export limit | — | — | Confirm operating assumption |
| Exact equipment and substitution terms | — | — | Obtain model-specific documents |
| First-year energy and modeling inputs | — | — | Explain forecast differences |
| Gross installed price | — | — | Match scope and tax treatment |
| Roof and structural work | — | — | Identify owner-funded work |
| Interconnection studies and upgrades | — | — | Identify allowances and exclusions |
| Monitoring and recurring fees | — | — | Define access and responsibilities |
| Warranty coverage and claim costs | — | — | Identify coverage gaps |
| Commissioning and handover | — | — | Define completion evidence |
| Schedule and change-order rules | — | — | Allocate dependencies and risks |
Worked example—illustrative arithmetic only
Assume two hypothetical U.S. rooftop purchase bids, each offering 200 kW DC and 160 kW AC. All amounts below are fictional USD figures, before incentives, on the same tax basis. This is a scope comparison, not a market-price or payback estimate.
| Item | Bid A | Bid B |
|---|---|---|
| Headline price | $360,000 | $390,000 |
| Headline price per watt DC | $1.80 | $1.95 |
| Required roof work | Excluded | Included |
| Added roof-work quotation | $40,000 | No addition |
| Clarified comparable cost | $400,000 | $390,000 |
| Clarified price per watt DC | $2.00 | $1.95 |
Bid A initially looks cheaper. Once the required roof work is added, its comparable cost is $10,000 higher.
That calculation does not establish a winner. Equipment, production, service and acceptance obligations still require review.
Other discrepancies to flag include a generation model using a different inverter size from the equipment schedule, monitoring described as included but carrying an undisclosed subscription, or a price dependent on unapproved equipment substitution.
8. Define acceptance before signing
Agree on the evidence required for payment and handover before construction begins.
DOE recommends establishing a commissioning plan and budget ahead of construction. Its commissioning guidance covers system documentation, array testing and whole-system performance testing, and recommends third-party verification. (energy.gov)
Ask your engineer and contract adviser to define:
- Required inspections and utility authorization.
- Commissioning tests, pass criteria and reports.
- Equipment verification against approved submittals.
- Monitoring setup and owner access.
- As-built drawings, manuals, serial-number records and warranties.
- Operator training and emergency information.
- Defect-list closure and responsibility for retesting.
If the contract includes a performance test or energy guarantee, specify the measurement boundary, test conditions, weather adjustments, exclusions and remedy. Do not leave “system operating satisfactorily” as the only acceptance language.
Before award, confirm that every material clarification is incorporated into the contract and its schedules.
The decision rule: choose the proposal with the best justified combination of comparable cost, credible design, disclosed uncertainty and enforceable obligations—not simply the lowest headline price per watt.