The Installer's Checklist: How to Evaluate a Solar Proposal for Your Next B2B Project

Who This Checklist is For

If you're a project developer or an EPC contractor reviewing a proposal for a mid-size commercial solar installation—say 200 kW to 2 MW—you've probably got a stack of quotes that look similar on paper, but you suspect the differences are in the details.

This checklist isn't for the theoretical side of solar. It's for what you do after the site survey, when you're sitting down to compare a SunPower proposal against two or three others. I've reviewed roughly 40+ large-scale solar procurement packages in the last year as a quality manager, and I've seen the same three or four mistakes sink projects. Here's what I check on every single proposal before it goes to the procurement committee.

Step 1: Demand a 'What's NOT Included' Statement

The first thing I look for isn't the price. It's the list of what the price doesn't cover. Every standard quote will include modules, inverters, and racking. The trap is in the balance of system (BoS) and soft costs.

I've learned to ask 'what's NOT included' before 'what's the price.' Seriously—if the vendor doesn't offer a line-item list of exclusions before you sign, that's a red flag. In my Q1 2024 quality audit, we saw a proposal that looked 18% cheaper until the addenda came in for trenching, conduit, and a specific type of combiner box requested by the utility. The real delta was under 3%.

Action item: After you get the quote, ask for every 'standard assumption' they made. If they say 'standard electrical work,' ask for a list of what constitutes 'standard.' Trust me on this one—it's the number one source of post-award change orders.

Step 2: Verify the Degradation Curve, Not Just the Efficiency Number

Everyone loves to point at the headline efficiency number. For a module like the SunPower Performance or Maxeon series, that's a strong talking point. But for a 25-year project model, the rate of degradation is what actually determines the payback.

Like most beginners, I used to just compare the '25-year performance guarantee' and call it done. Learned that lesson the hard way when a vendor claimed 'low degradation' but the fine print used a non-linear model that nosedived after year 10. Now I run a simple comparison: I take the 0.25% and 0.50% degradation rate scenarios and run a 25-year output model. The difference on a 500 kW system can be over $150,000 in lifetime energy value.

I went back and forth between two module quotes for weeks for a large community project. One had a linear 0.25% degradation guarantee with a 92% retention at year 25. The other was 0.5% but 4% cheaper. On paper, the cheaper one looked fine. But my gut said the long-term reliability of the premium module was worth it for the client's financing terms. Ultimately chose the premium module because the bank required a 24-year NPV positive model, and the cheaper one didn't quite hit the hurdle rate. Looking back, I should have run that model earlier—would have saved two weeks of back-and-forth.

Action item: Ask for the module's warranty document, not just the spec sheet. Look for the specific Pmax (maximum power) degradation limit each year. If it's not a linear graph, flag it.

Step 3: Check the 'System Balance' vs. 'Panel Only' Performance Guarantee

This is the one step most people overlook. A module might be great, but if the inverter clips during the middle of the day or the DC/AC ratio is set too aggressively, you're losing energy.

I reviewed a proposal last year that offered top-tier SunPower modules but paired them with an inverter that had a string-level limitation in a partial-shading scenario. The overall system efficiency was theoretically there, but the real-world output modeling showed a 4.2% energy loss due to clipping that the installer 'assumed' was acceptable. That's the kind of detail that kills a project's IRR (Internal Rate of Return).

I ran a blind test with our engineering team: same 1.3 DC/AC ratio, same module, but one proposal had a multi-MPPT (maximum power point tracking) inverter and the other had a cheaper single-input model. 80% of our engineers identified the multi-MPPT system as 'more reliable' without knowing the cost difference. The cost increase was about $0.02 per watt. On a 1 MW run, that's $20,000 for measurably better performance and fewer curtailment issues.

Action item: Don't just compare module specs. Ask for a full system simulation (PVsyst is standard) showing annual kWh output, not just kW rating. Look at the clipping ratio specifically.

Step 4: Scrutinize the 'Soft Cost' and Permitting Fee Schedule

Here's where the transparency stick hits. The vendor who lists all fees upfront—even if the total looks higher—usually costs less in the end. An 18-month timeline with a 'standard' permitting fee might sound okay, but if the local AHJ (Authority Having Jurisdiction) requires specific structural calculations or fire access pathways that aren't in the base quote, you get a change order.

I've seen this pattern many times. But when I say 'many,' I do not mean just a few—I mean consistently across 20+ utility-scale inspections. Saved $15,000 by accepting a 'permit-ready' proposal. Ended up spending $28,000 on a re-design and three months of schedule delay because the plan didn't meet the local seismic code. The 'standard permit' choice looked smart until our structural engineer flagged it. Net loss: $13,000 plus opportunity cost.

Action item: Ask for a list of all assumed permits and fees by name. Ask them to confirm if the local building department requires a stamped PE (Professional Engineer) review. If they can't commit, build a 10% contingency into your budget and ask for a 'do not exceed' clause on soft costs.

Step 5: Demand the Thermal and Hail Testing Data (Yes, for Commercial)

Most commercial proposals focus on performance, not durability. But if you're specifying for a 25-year system, you need to know how it handles 140°F roof temps and that rogue hail storm. SunPower modules, for example, have a specific composite frame and tempered glass construction. I ask to see the test results for Hail Test (IEC 61215) and High Temperature (to 85°C).

I rejected a batch of modules from a budget vendor last year because their 'industrial grade' label wasn't backed by IEC 61730 certificate I requested. Normal tolerance is to see a valid cert. The vendor claimed it was 'within industry standard.' We rejected the batch, and they swapped the supply chain at their cost. Now every contract includes 'IEC 61215/61730 cert must be provided with batch shipment.'

Action item: In your proposal specification, write: 'All modules must be certified to IEC 61215 and IEC 61730. Provide certificate with proposal.' If they can't produce it, cross them off the list.

Step 6: Get the Battery Safety and Warranty Details in Writing

If the proposal includes storage—like the SunPower SunVault or a dual-voltage hybrid system—you need to know the fire suppression protocol and the lithium battery chemistry. I won't go into full safety training here, but you need to know the state of charge limits for transport and the proper method for extinguishing a battery fire. Per basic safety standards, a lithium-ion fire requires a Class D or specialized dry-powder extinguisher, or a high-volume water spray for specific thermal runaway events. Ask what the warranty covers for capacity fade. A standard U.S. residential storage warranty is 70% capacity retention at year 10.

The SunVault cost number I see quoted often is around $12,000 to $18,000 installed for a small system, but that's before site-specific electrical work. Make sure the quote separates the battery cost from the balance of system. I read a review on CNET that mentioned unexpected costs for the SunVault installation upgrade—that's the kind of thing that comes from not having this checklist.

Action item: Ask for the battery chemistry type and the 'state of charge' limit at delivery. Also, ask for their fire safety plan for the installation site. If they look confused, that's a red flag.

Final Thoughts: Why This Checklist Matters for Your Project

I've seen too many projects go over budget because an 18-month facility plan didn't account for a $22,000 redo on a mis-specified inverter or a $15,000 change order on a main panel upgrade. This isn't about being paranoid. It's about being prepared. From a quality perspective, a spec written with full transparency at the start will always beat a spec written with hidden assumptions. The vendor who answers 'what's NOT included' clearly is the one you want in the partnership.

Take this checklist. Run it on every proposal. It'll save you more than the time it takes to read it.