If you buy PV inverters in bulk—or if you're adding battery chargers to a product line—this checklist is for you. I've been a procurement manager at a mid-size solar equipment distributor for six years. We spend roughly $180,000 a year on inverters, chargers, and related components. I've compared more vendors than I can count, and I've built my whole process around one principle: the quote you see should be the price you pay.
This is the checklist I actually use. It's not perfect, and it's not universal. I can only speak to our model: stable, recurring orders with a primary plus backup supplier. If you're a one-off project buyer, the calculus might be different.
Before you start: what to have ready
Don't ask for prices first. Ask for specs first. Have your technical baseline written down before you contact any supplier. For a string inverter order, that includes input voltage range, MPPT operating window, max DC/AC ratio, efficiency curve, protection class, communication interface, and warranty terms.
If you're looking at a Growatt inverter 5kW unit, the model family covers a range of configurations—5kW, 6kW, 10kW, and 12kW—so the catalog matters more than a single datasheet. I make it a habit to ask for the full PV inverter catalog, not just one model page. A supplier that publishes detailed specs across the range is usually more transparent about what's included.
What I mean by "full catalog" is simple: specifications, dimensions, protection ratings, certifications, and installation notes for every model in the family. If a supplier hesitates on that, it's an early red flag.
Step 1: Get a price breakdown, not a price
I've learned to ask "what's NOT included" before "what's the price." The first quote you receive will almost never be the final cost. Freight, palletizing, import documentation, payment terms, minimum order quantity, and warranty claim logistics all have a price.
Three years ago, I compared eight vendors for a bulk string inverter order. One vendor had the lowest per-unit price by 12%. Another was higher but listed freight, insurance, and a fixed lead-time buffer on the quote. I almost placed the order with the first vendor until I calculated the total: with palletizing fees, container drayage, and a 'rush production' charge that appeared only after the PO, the 'cheap' option was actually 9% more expensive. That's not an unusual story.
From the outside, a lower quote looks like a lower cost. The reality is the cheapest quote often moves costs to another line. That's why my first checklist item is: require a line-item breakdown. I want to see product cost, freight, insurance, payment processing, and any optional service fees separately. If a vendor says "we'll work that out later," I move them to the bottom of the list.
Step 2: Check the certifications yourself
Every supplier will tell you their inverters are certified. Ask for the test report number and confirm it against the issuing body's database. For grid-connected string inverters, the standards I check first are IEC 62109-1 and IEC 62109-2 for safety, and the relevant grid interconnection standard for the target market—EN 50549 in Europe, UL 1741 in North America, IEEE 1547 for DER interconnection. According to IEC 62109-1, the standard covers safety requirements for power converters in PV systems. A certificate that's still current and issued by a recognized test lab is worth more than a PDF of a claimed compliance statement.
Honestly, I'm not sure why some suppliers don't include certificate numbers in their datasheets. My best guess is that they'd rather you didn't verify. That's a guess, but it's been consistent over the years.
Step 3: Calculate total cost over the warranty period
Per-unit price tells you almost nothing about per-watt cost over the system's life. Warranty is where costs hide.
For a 100kW bulk string inverter order, if 2% of units fail in year three, who pays for return shipping? Who pays the labor to replace the unit? Who pays for the downtime while the installer waits for an RMA? I ask every supplier to explain their warranty claim process in writing. I want the contact person, the response-time commitment, and the replacement policy for failed units.
I once had a supplier that offered a 10-year warranty but required failed units to be returned to the factory in another country before any replacement was shipped. The shipping cost for one 12kW inverter was more than the repair. Not ideal. Worse than expected. We changed suppliers.
What I mean is that the warranty length doesn't matter if the claim process costs more than the product. We now require that replacement parts are shipped before the failed unit is returned, or at least within the same week. That's a negotiable point, and it matters more than the warranty length.
Step 4: Test support before you sign
Before I place any bulk order, I test the supplier's support process. I send a technical question about MPPT sizing. I ask for a dimension drawing. I ask about spare parts availability. If the response takes five days, that tells me what support will look like after the PO.
I have mixed feelings about support response times. On one hand, a 24-hour response is not always realistic for a midsize factory. On the other, I've seen the operational chaos that comes from unanswered questions during an installation. I compromise: I expect a same-day acknowledgement, and a detailed answer within two business days.
One more thing: I ask about minimum order quantities for spares. In Q4 2024, we ordered spare PCBs and communication modules for a 5kW hybrid inverter line. The supplier's minimum spares order was actually higher than the number of units we planned to sell in the first year. That changed our decision.
Step 5: How to evaluate charger manufacturers
If you're evaluating battery charger manufacturers, don't let the simpler product fool you. The same process applies. A buyer can choose a charger solely on price, only to discover that the manufacturer's charging curve didn't match the battery chemistry they were supporting. That's exactly why the checklist includes: confirm the charging profile matches your target batteries, not just "compatible with all."
With chargers, I add a few specific checks:
- Charging voltage and current range, and whether it's adjustable in the field.
- Temperature compensation behavior.
- Cooling design and derating at high ambient temperatures.
- Certifications for the target markets—CE, UL, or other relevant marks.
- Spare parts availability and repair turnaround time.
That last one is the one most people ignore. A charger that costs $40 less but has a six-week repair cycle is not a bargain if your customer's system is down in the middle of summer.
Common mistakes I still see
A few final notes, in no particular order:
Mistake 1: Quoting a spec sheet instead of a datasheet. A spec sheet gives you the headline numbers. A real datasheet gives you the details: efficiency curves, ambient temperature limits, derating behavior, and terminal sizes. If a supplier isn't willing to share the full datasheet before you order, that's a red flag.
Mistake 2: Ignoring the cost of inventory risk. When you buy bulk string inverters, you are holding inventory. If the model is discontinued or the connector type changes, you're stuck. I ask about product lifecycle and connector compatibility before I commit. (Should mention: we got burned on this in 2021 when a manufacturer changed connector types without notice. Our installer had to buy adapter cables.)
Mistake 3: Assuming higher price equals better quality. It's not that simple. I've seen budget brands outperform premium brands on real-world derating tests. Price is not quality. Tested performance is quality.
Mistake 4: Forgetting to check the supplier's financial health. It's uncomfortable to ask, but if you sign a three-year supply agreement, you need the supplier to exist in three years. We now ask for a basic company registration number and, for major orders, a reference from another distributor. We learned that one by paying for it.
Three things keep a bulk order from going sideways: specs confirmed, timeline agreed, payment terms clear. In that order.
