C&I vs Utility-Scale Module Specs: What Changes in the RFQ
Key Takeaway
A commercial rooftop RFQ and a utility ground-mount RFQ can name the same module and still need different columns. Mechanical load, operating current, packaging, and the bifacial energy assumption move with the site. Open-circuit voltage, short-circuit current, outline, and mass stay on the datasheet. This note uses three Weltrus TOPCon bands as the module-side inputs. It does not assign a project size to either segment.
Table of Contents
When One Spec Sheet Is Pasted into Two RFQs
Procurement teams often keep a single module tab and duplicate it. One copy goes to a rooftop commercial and industrial bid. The other goes to a ground-mount bid. The price per watt is compared. The columns that actually change the design are left on last year’s template: design load, fuse current, pallet handling, and whatever bifacial percent happened to be in the cell.
Those columns fail in different places. On a roof, the failure is a clamp zone the installation manual does not allow, or a pallet the hoist cannot lift, or a rear-gain percent copied from an open field onto a dark membrane. On a ground-mount, the failure is a series count set at standard test conditions and then installed in a cold climate, or a string fuse sized to front-side short-circuit current while the rear side adds current for much of the year. Neither failure is fixed by a higher nameplate wattage.
The module datasheet and the site are different authors. The datasheet can state voltage, current, outline, mass, and the load the module was tested at for a stated support method. The site states wind, snow, roof structure or pile design, albedo, minimum cell temperature, and how the modules will be delivered and staged. An RFQ that asks the factory to “confirm utility spec” without those site fields is asking for a label, not a design.
Split the Columns by Who Owns the Number
Write the RFQ in two blocks. Block one is the module identity: model code, power bin, and the datasheet revision. Block two is the installation identity: rooftop or ground-mount, the support method, the minimum site temperature you will use for voltage, the current the fuse must cover, the bifacial assumption the yield model uses, and the delivery constraint (hoist, stair, container, or laydown yard). If block two is blank, the quotation cannot be compared with another quotation that filled it in.
Mechanical load is a pair of documents, not a single pascal number copied between projects. The module side is the tested load for a stated clamp position or bolt pattern, printed on that model’s datasheet and limited by the installation manual. The site side is the structural calculation: roof member capacity, ballast, penetrations, or the tracker and pile design. This note does not print a pascal rating for the Weltrus bands below. If the page you hold does not print one, the datasheet for that model controls. Do not borrow a ground-mount test setup and call it valid for a rooftop clamp zone, or the reverse.
Current is where bifacial assumptions enter the electrical RFQ. Front-side short-circuit current on the three tables below sits in a narrow band, about 16 A. A ground-mount with a bright rear side can operate at a higher current than a module sitting close to a dark roof. The fuse, the connector, and the string conductor have to cover the current the datasheet requires once the rear contribution the manufacturer specifies is included. Do not scale that current by the ratio of nameplate watts. On these outlines, watts change more than short-circuit current does.
Packaging changes even when the model code does not, because the path from the container door to the roof deck is not the path from the container door to a field laydown. Module length and mass decide pallet length and pallet gross weight. A roof job may be limited by a hoist, a parapet, and the number of modules staged on the deck. A ground-mount job may be limited by yard space, forklift rating, and how long pallets sit before installation. Piece count per container is a result of the pallet plan for that shipment. It is not a Weltrus constant, and it is not different merely because one RFQ says “utility” and the other says “C&I.”
Bifacial energy is a yield-model input. The module tables print a bifaciality factor, not a site gain. A close-mounted commercial roof usually has a low rear irradiance. An elevated ground-mount usually has more, and a high-albedo surface has more still. Put the assumption you used in the RFQ so two bidders are priced against the same kilowatt-hours. Leave it out, and the lower price may be the bid that assumed no rear gain, or the bid that assumed a field gain on a roof.
What Changes, Band by Band
The module-side numbers below come from the internal parameter tables for three TOPCon bands. They are inputs to the RFQ. They are not a rooftop design and not a ground-mount design. Maximum system voltage on each table is 1500 V DC. The temperature coefficient of maximum power on each is -0.26% per degree C.
| RFQ field | 465-490 W | 540-560 W | 645-670 W | What the site still adds |
|---|---|---|---|---|
| Outline and mass | 1762 x 1134 x 30 mm, 28.0 kg | 1990 x 1134 x 30 mm, 25.0 kg | 2382 x 1134 x 30 mm, 34.0 kg | Hoist limit, roof staging, tracker table length, forklift rating |
| Open-circuit voltage | 38.20 V | 43.19 V | 51.40 V | Cold-temperature rise. Use the Voc coefficient on the datasheet, not the power coefficient |
| Short-circuit current | 15.92 A | 16.09 A | 16.16 A | Rear-side contribution the datasheet specifies, then the fuse and connector |
| Bifaciality | 80% plus or minus 10 percentage points | Same factor | Same factor | Albedo, height, and row shade. Roof gain and field gain are different yield inputs |
| Mechanical load | Not printed in this note | Not printed in this note | Not printed in this note | Tested load for the support method, plus the site structural case |
Series count at standard test conditions is only the first cut
A 1500 V maximum system voltage and the printed open-circuit voltage set an upper count at 25 degrees C cell temperature, before tolerance and before cold weather. For the 645-670 W table, 29 times 51.40 V is 1490.6 V, and 30 times 51.40 V is 1542 V. Thirty modules in series already exceed 1500 V at standard test conditions. For the 540-560 W table, 34 times 43.19 V is 1468.46 V, and 35 times 43.19 V is 1511.65 V. For the 465-490 W table, 39 times 38.20 V is 1489.8 V, and 40 times 38.20 V is 1528 V. The allowed count on a real site is lower once open-circuit voltage rises in the cold. This arithmetic is not a Weltrus string-design approval. It shows why the voltage column cannot be shared across the three bands, and why a rooftop in a mild climate and a ground-mount in a cold climate do not keep the same series count even on the same band.
The -0.26% per degree C coefficient does not close that gap. It applies to maximum power, not to open-circuit voltage. Using it as a stand-in will point the series count in the wrong direction: power falls as the cell heats up, while open-circuit voltage rises as the cell gets colder. Ask for the open-circuit-voltage temperature coefficient on the datasheet of the model in the RFQ. Until that number is in the file, do not publish a cold-weather module count as if it came from these tables.
Mass does not follow the segment label
The 540-560 W band is the lightest of the three at 25.0 kg, even though its nameplate is higher than the 465-490 W band at 28.0 kg. The 645-670 W band is both the longest and the heaviest, at 2382 mm and 34.0 kg. A rooftop RFQ that says “use the lighter commercial module” needs a mass, not a segment adjective. A ground-mount RFQ that says “use the utility module” needs a length the tracker table can accept. Width is 1134 mm and thickness is 30 mm on all three, so those two dimensions do not tell the bands apart. Length, mass, and open-circuit voltage do.
Efficiency on the 465-490 W table is 23.3-24.5%. On the other two tables it is 23.9-24.8%. That difference matters when the roof area is fixed and the energy model is sensitive to watts per square meter of outline. It does not, by itself, decide rooftop versus ground-mount. Degradation on all three tables is first year 1.0% or less, then 0.35% per year. Applied as 29 steps after a first-year floor of 99.0%, the end of year 30 is 88.85% of nameplate, if the limited warranty for that model prints the same rates. The warranty schedule is not one of the columns that flips between a roof RFQ and a field RFQ. The energy model still changes, because temperature, soiling, and rear irradiance change.
Selection Boundaries
- No project size is stated here. A small ground-mount and a large roof can use the same model. Do not invent a megawatt cutoff that these tables do not contain.
- Higher wattage is not a utility classification. The 34.0 kg outline may be a poor fit for a roof with a low hoist rating. It may also be a poor fit for a tracker that was drawn around a shorter module. Check the drawing.
- Do not copy one bifacial gain into both RFQs. The shared bifaciality factor is a module test result. The gain percent is not.
- Do not invent a mechanical load. Pascal ratings, clamp zones, and tracker torque limits come from the manual and the structural calculation for the support you will build.
- Do not extend these rows to every wattage from 100 W to 700 W. A model outside 465-490 W, 540-560 W, and 645-670 W needs its own voltage, current, outline, and mass. The datasheet for that model controls.
- IEC 61215 and IEC 61730 remain type tests. They support qualification of a design. They do not replace the site load case, and they are not a 30-year outdoor dataset.
A usable comparison sheet has one row per bidder and separate cells for model code, open-circuit voltage, the cold-weather series count, the current used for the fuse, the tested mechanical load and its support method, pallet mass, and the bifacial gain percent with the installation type written next to it. Price per watt is the last cell. If the installation type cell is empty, the prices are not yet comparable.
Send the RFQ Columns With the Model Code
Tell Weltrus whether the array is a rooftop or a ground-mount, and send the model code. The reply can match the datasheet row and list which load, current, and packaging fields still have to come from the site.
Frequently Asked Questions
Is a higher-wattage module automatically a utility module?
No. The 645-670 W outline is longer and heavier than the 465-490 W and 540-560 W outlines, at 2382 mm and 34.0 kg. Whether it belongs on a roof is a structural and handling question. A ground-mount project can also use a shorter outline. Segment follows the site, not the nameplate alone.
Can the same model code be quoted for a rooftop and a ground-mount?
Yes, if the mechanical load method, the current the fuse must cover, the packaging plan, and the warranty territory all match that site. The datasheet row can stay the same while the RFQ columns for load, bifacial energy, and delivery change. Copying one energy-gain percent into both RFQs is the step to avoid.
Why does open-circuit voltage change the series count between these bands?
At standard test conditions, 30 modules at 51.40 V is 1542 V, which is already over a 1500 V system, while 29 modules is 1490.6 V. The 43.19 V band reaches 1511.65 V at 35 modules and 1468.46 V at 34. The 38.20 V band reaches 1528 V at 40 modules and 1489.8 V at 39. Cold weather raises open-circuit voltage further. The -0.26% per degree C figure is the maximum-power coefficient, not the open-circuit-voltage coefficient. The datasheet supplies that second coefficient.
Should a rooftop RFQ and a ground-mount RFQ use the same bifacial gain?
No. Bifaciality on these three tables is a module factor, 80% plus or minus 10 percentage points. Energy gain is a site result. A module close to a dark roof membrane usually sees less rear light than an elevated ground-mount. Put the gain in the yield model for that installation. Do not treat one percent as a Weltrus promise for both.
Where does the mechanical load number come from?
From the datasheet and the installation manual for that model, for a stated support method, plus the structural calculation for the site. This note does not print a pascal rating. Do not borrow a load figure from a different wattage or from a ground-mount drawing and apply it to a roof clamp zone.
