Three suppliers quote the same pump motor. Each proposal contains roughly the same total PV wattage, yet one array struggles to start in hot weather, one risks exceeding the controller’s DC limit on a cool morning, and only one keeps the operating voltage inside a useful range.
The difference is string design. Solar pump inverter DC voltage cannot be checked by multiplying panel quantity by rated watts. A pump controller responds to voltage and current at changing irradiance and temperature, while the motor and hydraulic system determine how much power is actually needed to move water.
This design note works through three illustrative PV arrangements for a 380 V-class WLD280. The module values are examples, not a quotation. The method is reusable; the numbers must be replaced with the chosen module datasheet, site temperature and exact WLD280 model.
Before the Panel Count: Fix the Water Duty
A distributor should resist the temptation to begin with the number of panels visible in a customer’s photo. The electrical design has to serve a hydraulic requirement.
Record these inputs first:
- required water volume per day in cubic metres;
- static water level and expected seasonal drawdown;
- discharge elevation;
- pipe length, diameter and fittings;
- required pressure at the outlet;
- selected pump curve and motor nameplate;
- available solar window by season;
- whether water can be stored for use outside pumping hours.
The total dynamic head is the sum of static lift, delivery elevation, required outlet pressure expressed as head, and friction losses at the design flow. If the head is understated, adding panels later cannot make an incorrectly selected pump efficient.
The NREL Renewable Energy Water Pumping Systems Handbook treats water source, water demand, power source, system sizing and storage as connected design questions. Its most useful principle here is simple: in many solar-pumping applications, energy can be stored as pumped water rather than requiring the pump to match consumption at every moment.
Read Four Module Values, Not One Wattage Number
The PV module datasheet provides four electrical values at standard test conditions:
| Symbol | Meaning | Why it matters |
|---|---|---|
Voc |
Open-circuit voltage | Used to check the maximum cold-condition array voltage |
Vmp |
Voltage at maximum power | Used to estimate whether the inverter has enough operating voltage |
Isc |
Short-circuit current | Used in protection and conductor review |
Imp |
Current at maximum power | Used to estimate operating current and parallel-string contribution |
It should also provide temperature coefficients, normally expressed as a percentage change per degree Celsius. Module voltage rises as cells become colder and falls as cells become hotter. That is why the same series string can approach the controller’s maximum in cool conditions and fall below a useful operating range in hot conditions.
Solar Pump Inverter DC Voltage: The WLD280 Window
According to the WLD280 manual used for this article:
| WLD280 voltage class | Maximum DC input | Recommended DC range | Recommended working voltage |
|---|---|---|---|
| 220 V class | 400 VDC | 220-370 VDC | 305 VDC |
| 380 V class | 800 VDC | 420-720 VDC | 540 VDC |
The maximum value is a hard design boundary, not a target. The recommended range is where the array should remain useful under expected operating conditions. The final controller model must also be checked for motor power, rated current and any model-specific PV input requirements.
Design Notebook: One Module, Three String Arrangements
Assume the selected module datasheet states:
Voc = 49.5 VVmp = 41.5 VIsc = 13.8 AImp = 13.0 AVoctemperature coefficient-0.28%/°CVmptemperature coefficient-0.35%/°C
For this illustration, the lowest expected cell temperature is 5°C and a hot operating cell temperature of 70°C is used as a screening condition. Standard test condition is 25°C.
Step 1: Correct the Cold Open-Circuit Voltage
The temperature difference is 25 - 5 = 20°C. Because Voc rises in colder conditions:
Cold Voc per module = 49.5 x [1 + (0.0028 x 20)] = 52.27 V
This value is used to check that the series string cannot exceed the WLD280’s 800 V maximum under the assumed cold condition. The actual site design may need a lower recorded temperature, an additional design margin or a standard-specific calculation.
Step 2: Screen the Hot Operating Voltage
The hot-cell difference is 70 - 25 = 45°C:
Hot Vmp per module = 41.5 x [1 - (0.0035 x 45)] = 34.96 V
This is an estimate, not a full PV model. It shows why using STC voltage alone can hide weak hot-weather operation.
Step 3: Compare the Three Quotations
| Series arrangement | Cold string Voc |
Hot string Vmp |
380 V WLD280 screen | Finding |
|---|---|---|---|---|
| 10 modules in series | 523 V | 350 V | Hot voltage below 420 V recommended range | Insufficient operating-voltage margin |
| 14 modules in series | 732 V | 489 V | Inside 420-720 V operating range and below 800 V maximum | Workable voltage candidate |
| 16 modules in series | 836 V | 559 V | Cold Voc exceeds 800 V maximum |
Reject this series count |
The middle arrangement passes this simplified voltage screen. It has not yet passed the full design. Current, array power, motor duty, site solar resource, protection, conductor sizing and the exact WLD280 rating still need review.
Series Changes Voltage; Parallel Changes Available Current
Modules connected in series add voltage while string current remains approximately the current of one module. Identical strings connected in parallel add current while voltage remains approximately the string voltage.
For the illustrative 14-module string:
- one string has about
13.0 Aat maximum power; - two identical parallel strings have about
26.0 Aat maximum power; - three identical parallel strings have about
39.0 Aat maximum power.
Parallel strings should use the same module type, orientation and series count. The combined current must remain within the selected WLD280 input specification and the ratings of the DC isolator, protection, connectors and conductors. Array short-circuit current, not only Imp, is relevant to protection design.
Adding parallel strings can provide more power in adequate sunlight. It cannot rescue a series voltage that remains below the controller’s useful range. Conversely, adding more modules in series to fix low voltage can breach the maximum cold Voc. This is the central reason panel wattage alone is an unreliable comparison.
Nine Checks Before a String Is Approved
- Confirm the pump duty. Use the pump curve at the actual total dynamic head.
- Confirm the motor. Record rated voltage, current, frequency and power.
- Choose the WLD280 voltage class and model. Match motor current and system input limits.
- Obtain the exact module datasheet. Similar wattage does not guarantee similar voltage.
- Calculate maximum cold
Voc. Include the site’s design minimum temperature and required margin. - Estimate hot operating voltage. Use a defensible cell-temperature assumption and module coefficient.
- Check parallel current and array power. Include protection and conductor ratings.
- Review the cable route and surge environment. The WLD280 manual requires a DC circuit breaker and calls for a Type II lightning protector at the DC input when PV modules are more than 10 m from the controller.
- Commission against water output. Record irradiance condition, DC voltage, output frequency, motor current, flow and head instead of checking only whether the pump rotates.
The manual also recommends an output reactor when the pump is more than 50 m from the inverter. That is a separate motor-cable issue from PV string length and should appear as a distinct item in the design drawing.
What WLD280 Contributes After the Array Is Correct
The BEDFORD WLD280 solar pump inverter accepts solar DC input and can also support AC input arrangements appropriate to the project. Its solar-pumping functions include automatic operation and water-level-related control options. These functions help the system use changing solar availability, but they do not authorise an array outside the published voltage range.
For distributor quotations, the most valuable service is often not selecting a larger inverter. It is catching a wrong series count before panels are delivered to a remote site.
The Quotation Request That Prevents Guesswork
Ask the customer for the following before pricing a complete solar pump system:
- site location and minimum/maximum ambient conditions;
- daily and seasonal water requirement;
- static and pumping water levels;
- delivery elevation, pipe details and required outlet pressure;
- pump and motor nameplate photographs;
- pump curve or proposed pump model;
- PV module datasheet, not only wattage;
- available installation area and shading information;
- distance from array to controller and controller to motor;
- storage tank volume and operating strategy;
- backup AC source, if expected;
- local electrical protection and earthing requirements.
With those inputs, solar pump inverter DC voltage becomes a transparent design decision. Without them, two quotations with the same number of kilowatts may describe systems with completely different operating margins.