Sizing the inverter is only half of a solar pumping design. The other half is the array: how many watts of panels, wired in what configuration, so the pump actually receives the power the inverter is rated for. An undersized array leaves a good pump crawling at midday. An oversized or badly wired one can push string voltage outside the inverter window and shut the system down. This guide covers array sizing only: panel wattage, series and parallel wiring, and the voltage window. If you still need the inverter side, start with our guide on what size solar pump inverter you need, then come back here for the panels.
Step 1: Start from the motor nameplate, not the panel catalogue
Every array calculation begins with the pump motor: rated power in kW, rated voltage and phase, and full-load current. Those three values decide the inverter class, and the inverter class decides the DC window the array has to fit. On the QB100-PV solar pump inverter range, for example, the 220V classes (S2 three-phase and SS2 single-phase, 0.4-7.5kW and 0.4-5.5kW) accept 180-450V DC, while the 400V three-phase class (0.75-37kW) accepts 250-900V DC. Write the motor data and the window down first; every later step refers back to them.
Step 2: Size array watts at 1.3 to 1.6 times motor power
Panels almost never deliver nameplate watts in the field. Output falls with cell temperature, dust, cable losses and any hour when the sun is not square overhead, which is most of the day. To keep the pump near its rated point through the useful part of the day, size the array at roughly 1.3 to 1.6 times the motor rated power. Use the lower end for cool, high-irradiance sites with short cable runs, and the upper end for hot climates, deep wells with long cables, or sites where morning and evening water matters.
| Motor rated power | Recommended array (1.3x) | Recommended array (1.6x) |
|---|---|---|
| 0.75kW | 1.0kWp | 1.2kWp |
| 1.5kW | 2.0kWp | 2.4kWp |
| 2.2kW | 2.9kWp | 3.5kWp |
| 4kW | 5.2kWp | 6.4kWp |
| 7.5kW | 9.8kWp | 12kWp |
| 15kW | 19.5kWp | 24kWp |
| 37kW | 48kWp | 59kWp |
Oversizing within this band is safe because the inverter only draws what the motor needs; the extra watts buy longer pumping hours, not extra stress. Going far beyond 1.6x adds cost without useful gain, and going below 1.3x is the most common reason a correctly chosen pump underperforms.
Step 3: Wire in series until the string voltage sits inside the window
Watts set the array size; wiring sets the voltage, and voltage is the hard constraint. Modules in series add voltage, modules in parallel add current. Two module values matter: Vmp (voltage at maximum power, the working voltage) and Voc (open-circuit voltage, the highest voltage the string can present). The design targets are simple to state: string Vmp should sit comfortably inside the inverter DC window across the working temperature range, and string Voc must stay below the inverter maximum DC input even on the coldest morning, because Voc rises as temperature falls. Check the module datasheet temperature coefficient and do the cold-morning sum explicitly; this is the calculation most often skipped and most often regretted.
A worked example with typical datasheet values: a 2.2kW motor on a 220V-class drive (window 180-450V DC) with an array target of about 3.3kWp. Using common 550W modules with Vmp around 42V and Voc around 50V, six modules in series give 3.3kWp, a working voltage near 252V, well inside the window, and a cold Voc near 300V before temperature correction, safely below the 450V ceiling. The same six modules in parallel would give only about 42V, far below the window, and the pump would never start. Series first, then parallel strings if more current is needed.
Step 4: Add parallel strings for current on larger motors
Once one string reaches the right voltage, larger arrays are built by paralleling identical strings. Each additional string adds current at the same voltage. Before paralleling, check the rated input current of the exact inverter model in its model table and keep total array current within it, use identical modules in every string, and keep string cable lengths similar so the strings share load evenly. Never mix module types or orientations within one string.
Step 5: Derate honestly for the site
Finally, walk the site factors that eat into the budget: high ambient temperature (the LX200 platform, for instance, is specified for -10 to +50 C with derating above 40 C, and arrays behave the same way), dust and soiling between cleanings, long DC cable runs, tilt and orientation away from ideal, and partial shading from trees or structures. If several apply, stay at the 1.6x end of the band from Step 2. If the site is cool, clean and unshaded, 1.3x is enough and the saving is better spent on mounting and cable quality.
Common array sizing mistakes
- Matching array watts 1:1 to motor watts. The pump then reaches rated flow only near solar noon on the best days.
- Counting watts but not volts. A string outside the DC window will not run, however many watts it has.
- Forgetting cold-morning Voc. A string that is legal in summer can exceed the inverter maximum on a frosty dawn.
- Mixing old and new modules in one string. The weakest module drags the whole string down.
- Ignoring cable losses. Long, thin DC runs waste the margin the oversizing was meant to buy.
How to Size the Array: Quick Checklist
Run through this checklist in order before ordering panels. For the full series-count calculation behind step 3, see How Many Solar Panels Do I Need for a Solar Water Pump?
- Record the motor data. Note rated power in kW, rated voltage and phase, and full-load current from the pump nameplate.
- Fix the DC window. Confirm the inverter DC input window for that voltage class — for QB100-PV, 180–450 V DC on 220 V classes and 250–900 V DC on the 400 V class.
- Size array watts at 1.3–1.6 times motor power. Use the lower end for cool, high-irradiance sites and the upper end for hot climates, deep wells or long cable runs.
- Count the series string. Choose a series count whose string Vmp sits inside the window and whose cold-morning Voc stays below the inverter maximum.
- Add parallel strings for current. Parallel identical strings for larger motors, keeping every string the same module type and count.
- Derate for the site. Allow for temperature, dust, tilt, shading and DC cable losses before finalising the count.
Frequently asked questions
Can I add more panels later?
Yes, if the string voltage stays inside the inverter window and total input current stays within the model rating. Adding a parallel string of identical modules is usually the cleanest expansion path, so leave combiner and cable capacity for it at design time.
What happens if the array is too big?
Within reason, nothing bad: the MPPT controller takes only the power the motor needs, and extra watts extend pumping hours. The real limits are voltage and current, not watts, so an oversized array must still respect the DC window and the input current rating.
Do I size panels differently for a deep well?
The method is the same, but deep wells punish weak hours harder because the pump must overcome the full head before any water arrives. Favour the 1.6x end of the band and a string voltage in the upper half of the window so the pump reaches working speed earlier in the day.
Does more panels always mean more water?
No — this is the trap. Extra watts only help while the string voltage stays inside the inverter DC window and the input current stays within rating. A string wired outside the window will not run at all, however many watts it has, and going far beyond 1.6 times motor power adds cost without useful gain, as this guide explains.
How do I work out how many panels to put in series?
Divide the target string voltage by the module Vmp for a minimum count, and divide the inverter maximum by the cold-morning Voc for a maximum count. The full method, with minimum and maximum series counts, is in How Many Solar Panels Do I Need for a Solar Water Pump?
Send us your pump nameplate and the module datasheet you plan to use, and we will check the string configuration against the inverter window before you buy anything: contact the IDEEI team.