From Panels to Strings: Sizing the Solar Array for Large Pumps (1.5 kW to 18.5 kW)

The short answer: the 1.3x rule sizes a solar pump array in watts, but above roughly 2 kW the real work stops being arithmetic and becomes architecture — how many modules per string, against which voltage window, and whether an AC input should be allowed to shrink the array at all. A 200 W garden pump and one panel are a purchase. An 11,000 W irrigation machine and its twenty-six modules are a small power plant, with string lengths, cold-morning voltage spikes and land area to plan. This guide walks the array side of the 2DPC/3DPC, 3DSC/4DSC and 6DSC/8DSC ladder that our model-number guide decoded — every model name below verified against the live catalogue of 1,045 real pumps.

The 1.3x rule tells you watts. It does not tell you wires.

Our spec sheets state the array rule directly: at least 1.3 times the pump power. Applied across the ladder, with modules of the common 550 W class as the counting unit:

Two honest observations fall straight out of this table. At the bottom, the rule and the module market disagree politely: one modern 550 W module already doubles the target of a 200 W pump, and the real constraint becomes the controller input, not the wattage — see the FAQ below. At the top, the rule quietly becomes a construction project: forty-four modules do not hang on a roof rack, they occupy land, face a calculated tilt, and feed a three-phase machine.

What the table deliberately does not tell you is how those modules connect to each other. Four modules can be four in series, or two parallel pairs — and which one is right depends on the voltage window of the controller, the coldest temperature your site sees, and the cable run ahead. That is the rest of this article.

After setting the string voltage, account for PV shading and bypass diodes before treating the array wattage as available pump power.

Same watts, different voltage: reading the fourth field as an architecture choice

Compare two real 1,500 W pumps from the catalogue:

• 4DSC9.5-90-110-1500 — 110 V system, 9.5 m3/h, 90 m head
• 4DSC4.8-203-200-1500-A/D — 200 V system, 4.8 m3/h, 203 m head

Same power, same 1,950 W array target, different machines. The voltage field is not a trivia answer — it halves the current for the same watts. A 1,500 W draw at a 200 V bus moves roughly half the current of the same draw at 110 V, and half the current over the same cable run means less voltage loss in the copper — or the same loss through thinner, cheaper cable. Over the 100 m and 150 m drops of a deep borehole, that difference is real money; our cable sizing guide walks the drop formula.

The voltage field also shapes the array. Higher system voltage wants more modules in series per string, which is exactly where the next two sections live: the HV window that caps string length, and the 520 V bus that the largest pumps are built around.

String length against the 80 V to 430 V window: a worked example

The catalogue marks wide-voltage models with an HV suffix — for example 3DSC5-135-150-1500-HV, a 1,500 W stainless-steel pump whose controller accepts an input from 80 V to 430 V. That window is not a spec to admire; it is two equations that size your string.

Take a module of the common 550 W class, whose open-circuit voltage VOC sits near 50 V in the datasheets of that class. Crystalline silicon VOC rises by roughly 0.3 percent per degree below the 25 degree test condition, so a cold morning at about minus 13 degrees pushes the string about 15 percent above the datasheet number. Now count:

The window also has a floor, and the floor bites at noon, not at dawn. The operating voltage of a hot module on a clear midday sags well below its rated maximum-power voltage; for this module class, call it near 37 V each. Two modules in series sag to roughly 74 V — under the 80 V floor, where the controller stops tracking. Three modules hold about 111 V and stay comfortably in range.

For a 550 W-class module, this window lands at 3 to 7 modules per string. Redo both sums with your own module datasheet before ordering: a different VOC or a colder site moves the answer, and the cold-morning spike is the single most common way an array that looked fine on paper trips its controller in winter. (The same physics, and the Andean data behind it, appear in our high-altitude guide.)

The 520 V bus: where 26 modules refuse to fit

Now step up to the farm tier. The 6DSC and 8DSC machines carry AC380/DC520 in the voltage field: a three-phase 380 V AC output, built around a 520 V DC bus on the panel side. Apply the 1.3x rule to the 11,000 W 6DSC36-210-380/520-11000-A/D and you need 14,300 W — twenty-six 550 W-class modules.

Here the arithmetic starts fighting itself. Twenty-six modules do not divide into tidy strings under the ceiling: nine in series reach 450 V at test conditions and about 517 V on that cold morning — touching the 520 V bus with no margin at all. Eight in series keep the cold voltage near 460 V, but three strings of eight are twenty-four modules, 13,200 W — only 1.2 times the pump power, under the 1.3x bar the catalogue itself sets.

This is not a mistake in the table; it is a real design tension that nobody notices until the modules are on the rack. The honest way out runs through one question: is 520 V the controller true maximum input, or just the nominal bus? The model name does not say. A datasheet — or a message to Trista — does. If the true ceiling has headroom, nine-module strings fit. If it does not, the practical answer is an eight-module string layout plus the A/D input to bridge the shortfall on clear days when the array runs a little under. Either way, you want that answer before the panels are ordered, not after.

What A/D really changes: the array becomes optional at the margin

The A/D suffix appeared 537 times in the catalogue — and on every model from 1,500 W upward it changes what the array is for. A DC-only pump must survive its worst month on panels alone, so the array is sized for the worst month and idles through the good ones. An A/D pump can be sized for the clear-season average and let the AC input carry the cloudy weeks:

The 20 to 40 percent range is climatology, not marketing: in strongly seasonal monsoon climates the worst month and the annual average diverge the most, and in equable climates barely at all. The trade is real — bridged days burn fuel or tariff money — so the honest comparison is array cost against fuel cost, which our solar-pump-versus-generator guide prices out. If the sun alone can carry your worst month, stay DC-only; running without batteries keeps the system simplest.

The 6DSC/8DSC tier: your roof is now a power plant

From 7,500 W upward the array stops being an accessory and becomes site infrastructure. Eighteen modules for the 7,500 W tier, twenty-six for 11,000 W, forty-four for the 8DSC150-37-380/520-11000-A/D — whose spec sheet lists 18,500 W (the power field in the model name predates a catalogue revision; the spec table governs). Plan on roughly 70 square metres of glass per 14 kW of array, and a fenced footprint of 100 to 150 square metres once row spacing, tilt and access paths are counted — about 5 square metres of land per kilowatt is the planning figure that has held across our deployments.

Three things change character at this tier:

• Ground mount, not roof. Forty-four modules need structural wind loading calculations no roof rack was designed for, and rows on open land are cheaper to build, clean and replace.
• Three-phase 380 V AC output. The machine feeds irrigation and farm equipment directly — no inverter stage between the sun and the load, which is why these pumps are the workhorses of the large-farm ladder.
• The array is the main asset. Panels, not the pump, are now most of the project cost — which is exactly why the string-length arithmetic above and the curve-verification habit matter most at the top of the ladder.

If your daily energy math is still at the litres-per-day stage, start instead with what a watt actually buys and how many panels a small pump needs; this page assumes those numbers are already settled.

The pre-order checklist

Five lines to confirm before any array above 1.5 kW is ordered:

1. Array watts — at least 1.3 times the pump power; A/D models may intentionally sit lower, by decision rather than accident.
2. String voltage, cold — series count times module VOC, times 1.15 if your site sees frost, inside the controller ceiling with margin.
3. String voltage, hot — series count times hot-midday operating voltage, above the window floor (80 V on HV models).
4. Cable — sized for the string current over the real run length; voltage drop inside 5 percent, per the cable guide.
5. The 520 V question — for 6DSC/8DSC machines, confirm the controller true maximum DC input from the datasheet before fixing the string length.

How many modules will your pump really need? Send Trista the pump model, your water level and the module you plan to buy — the string arithmetic takes minutes, and it is cheaper on WhatsApp than after delivery. Fill your numbers into the sizing tool, or WhatsApp Trista directly.