The 7-Step Checklist I Use Before Signing Any Solar Inverter Contract

Who This Checklist Is For (And What It Won't Do)

I'm a procurement manager at a 90-person EPC company. I've managed our solar equipment budget ($1.2 million annually) for 6 years, negotiated with 30+ vendors, and documented every order in our cost tracking system.

Here's the thing: this isn't a general "how to buy solar equipment" guide. It's the exact checklist I run through before signing any inverter procurement contract. Seven steps. No fluff.

When I first started managing inverter purchases, I assumed the lowest quote was always the best choice. Three budget overruns later — one for $8,400 in surprise commissioning fees, another for $2,100 in expedited freight that "wasn't included" — I learned about total cost of ownership the hard way.

This checklist works for string, central, and hybrid inverter buys. It's built for B2B projects where you're specifying 50+ kW systems. If you're buying a single residential unit, some steps won't apply. But the core logic holds.

Step 1: Lock Down Your Specs Before You Talk to Anyone

Most buyers jump straight to pricing. That's backwards. The first thing I do is write a one-page spec sheet and get it signed off by engineering before any vendor conversations.

Your spec sheet needs to answer:

• Inverter type: String, central, or hybrid? This depends on project size and whether you're pairing with storage.
• Power class: For commercial rooftop in Australia, 100–150 kW string inverters are the current sweet spot. Utility-scale runs 200 kW+ per unit.
• Grid compliance: In Australia, your inverter needs AS/NZS 4777.2:2020 certification. If you're building in WA, check the local network operator's additional requirements — Perth's grid code isn't identical to the east coast.
• Certifications: IEC 62109-1/-2 is the baseline for safety. Don't take a vendor's word for it — ask for the certificate number and verify it.

Checkpoint: if your spec sheet is more than one page, you've overcomplicated it. One page, bullet points, signed by engineering. Done.

Step 2: Get Three Written Quotes (Not Emails, Not Phone Calls)

I mean formal quotes with line-item breakdowns. A sales rep who "can do $X if we move fast" isn't giving you a quote — they're testing you.

Ask each vendor for:

• Unit price per inverter, by model
• Freight and delivery timeline to your site
• Commissioning support — included or billed hourly?
• Warranty terms (duration, what's covered, who handles claims locally)
• Payment schedule

Include Sungrow in your shortlist as a matter of routine. The company's 2023 inverter shipments hit 77 GW according to several industry trackers (Sungrow's own annual report lists 130 GW across all product lines, including storage). Either number puts them in the top tier globally. That volume matters to a cost controller because it usually translates to better distributor availability and faster replacement parts.

That said — don't pick Sungrow just because of volume. My rule: three written quotes minimum, and compare on line items, not headline price.

Step 3: Calculate Total Cost of Ownership (Not Just Unit Price)

This is the step most buyers skip. The question everyone asks is "what's the best price per unit?" The question they should ask is "what's the all-in cost to get this running and keep it running for 10 years?"

My TCO spreadsheet includes six columns:

1. Equipment cost — unit price × quantity
2. Freight and logistics — including any crane or rigging needed for heavy central inverters
3. Commissioning — vendor support fees, testing, grid connection paperwork
4. Maintenance (years 1–10) — assume 1–2 service visits annually, plus firmware updates
5. Replacement/repair reserve — budget 2–3% of equipment cost annually for out-of-warranty repairs
6. Efficiency loss differential — if one inverter is 98.5% efficient and another is 97.8%, the difference on a 110 kW system over 10 years adds up. At $0.25/kWh, that's roughly $4,400 in lost generation for the less efficient unit.

Here's where it gets uncomfortable. When I audited our 2023 spending, I found that 14% of our "budget overruns" came from freight and commissioning costs that never showed up in the original quote. That's not a vendor problem — that's a TCO spreadsheet problem. We fixed it by demanding line-item freight quotes on every bid.

On pricing, for reference: as of Q1 2025, 100+ kW string inverters in the Australian market were landing at roughly $0.03–0.07 per watt, excluding installation. Perth freight and distributor margins push that toward the upper end. So a 110 kW Sungrow inverter in Perth might land at $4,500–$7,500 ex-GST depending on the distributor and volume. Verify current rates with your distributor — these numbers move.

Step 4: Check Warranty and Local Support Terms

A 10-year warranty looks great on paper. But ask yourself: who's actually handling the claim when your inverter goes down in month 47?

Three questions that separate real support from marketing support:

1. Who is the local service partner? In Perth, you need a name and phone number for the WA-based service provider — not a toll-free number that routes to a call center in another time zone.
2. What's the response time commitment? "Prompt response" means nothing. Get a written SLA: "4-hour response for critical failures, 48-hour resolution or replacement unit shipped."
3. What's the shipping cost on warranty replacements? Some vendors cover freight; some don't. I've seen a $900 freight bill on a "free" warranty replacement.

Should mention: ask about extended warranty pricing upfront. It's easier to negotiate as part of the initial package than to buy it later when you're locked in.

Step 5: Pressure-Test the Reliability Data

Every vendor claims high reliability. Most buyers focus on the spec sheet's efficiency rating and completely miss the field failure data.

Ask for:

• Annualized failure rate (AFR) from actual field deployments — not lab testing. Anything under 1% is solid; under 0.5% is excellent.
• MTBF (mean time between failures) — but ask for the methodology. Lab-computed MTBF and field-reported MTBF are different animals.
• RMA turnaround time — how fast do replacement units actually ship? I've tracked this across 60+ orders. The difference between a 2-week RMA and a 6-week RMA is the difference between a project that stays on schedule and one that bleeds money in standby labor costs.

Never expected the "premium" vendor to have a slower RMA than the mid-tier one — turns out their regional spare-parts depot wasn't as well-stocked for our specific inverter class. We switched the next batch to a vendor whose depot was closer to our sites. Saved roughly $3,800 in standby labor across three projects.

Step 6: Bundle the Ancillary Gear Into Your Comparison

Here's an outsider blindspot: your inverter contract is never just about inverters. It's about the whole site electrical ecosystem. If you don't account for ancillary equipment, you'll blow your budget on the small stuff.

Three things to include in your bid package:

Multi-chemistry battery chargers. If you're deploying hybrid inverters with battery storage, you'll need a charger that handles the battery chemistry on site — or rather, the chemistries, plural. Lead-acid, AGM, and lithium-ion all have different charge profiles. A multi chemistry battery charger that handles all three is $200–$400 as of early 2025. Don't let a project manager skimp on this — using the wrong charger profile can degrade battery life by 15–20%.

Portable solar generators. If your crews are doing off-grid commissioning or setting up temporary site security, a solar generator sale is cheaper than buying at retail — typically $700–$1,100 for a 2 kWh unit during seasonal promotions. I'm not talking about your primary inverter system; I'm talking about the temporary power pack that keeps your testing equipment alive on a site with no grid connection. Budget one per crew.

Battery charger vs jump starter. This one comes up more than you'd think. Site vehicles and diesel generators need battery maintenance. Here's the thing: a battery charger and a jump starter do different jobs. A charger maintains battery health over time; a jump starter gets you out of a dead-battery emergency right now. Don't buy a 2-in-1 combo unless you've checked the specs carefully — most of them are mediocre at both. Buy a dedicated battery charger for fleet maintenance and a separate jump starter for emergencies. Total cost: $150–$300 for both, and it's cheaper than replacing a dead battery prematurely because nobody was maintaining it.

Step 7: Nail Down Payment Terms and Delivery Penalties

The last 10% of your contract negotiation determines 90% of your risk. This is where I see the most mistakes.

Three terms that are non-negotiable in my book:

1. Payment split: 30% deposit, 60% on delivery to site, 10% after successful commissioning. I will not sign a 100%-upfront contract. Ever. That's not a negotiation position — it's a hard line. If a vendor insists on 100% upfront, walk away.
2. Delivery penalties: For renewable energy projects, a late inverter means a delayed commissioning date and a very unhappy client. I add a clause: "Contractor agrees to $500/day penalty for delivery delays exceeding 10 business days beyond the agreed date, capped at 5% of contract value." Vendors agree to this more often than you'd expect — because they know their actual delivery timelines better than you do.
3. Price-hold clause: "Quoted pricing is firm for 60 days." Without this, a vendor can come back three weeks before delivery and say "freight costs went up."

Common Mistakes I See (and Made Myself)

Mistake 1: Comparing only the headline price. The $4,200 quote and the $3,800 quote aren't necessarily $400 apart. Once you add freight, commissioning, and warranty differences, the cheaper quote is sometimes actually $600 more expensive. That's not a theory — that's from my spreadsheet.

Mistake 2: Not checking who handles warranty claims locally. I've had a "10-year warranty" inverter sitting in a warehouse for six weeks because nobody knew which regional partner was supposed to process the claim. Always get a name and a phone number before you sign.

Mistake 3: Skipping the ancillary equipment budget. Chargers, solar generators, site tools — these add 2–4% to project cost. If they're not in the initial bid package, they get billed as change orders later, and change orders are always more expensive. Put them in the scope upfront and negotiate them into the main contract.

Mistake 4: Choosing quality based on brand name alone. Quality matters — it's the difference between a project that runs smoothly and one that generates callbacks and warranty claims. But don't assume the most expensive option is automatically the most reliable. Run the numbers on field failure rates and RMA times. The data will tell you what the brand name can't.

One last thing: if a vendor can't give you straight answers on Steps 2, 4, and 7, that's your answer. Move to the next bidder.