Count solar panels for a solar water pump by voltage window first, then by watts. The string voltage must stay inside the inverter PV input class in weak morning light and on a cold morning, while the total array watts must be enough to run the motor at useful speed. A count based on watts alone can leave a pump that starts late, stops on every cloud, or never starts at all, even though the array looks large on paper.
Vmp, Voc and the MPPT Window, in Plain Terms
Two voltages on the module datasheet decide the string count. Vmp is the voltage at maximum power under test conditions — the voltage the string aims to work at while pumping. Voc is the open-circuit voltage, the highest voltage the string reaches when it is connected but not yet loaded. The inverter can only use power while the working string voltage sits inside its PV input class, and the string Voc must never exceed the inverter maximum.
For the verified QB100-PV range, the product page lists DC 180–450V for 220V models and DC 250–900V for 400V models. Treat the lower figure as the floor the working voltage must clear and the upper figure as the ceiling the cold-morning Voc must not cross. For the wider watts method, see How to Size Solar Panels for a Solar Water Pump, and for the motor kW behind the array size, see How Do I Convert HP to kW for a Solar Pump Inverter?.
How to Calculate the Series Count
- Read the module datasheet. Note Vmp, Voc and watts-peak for the exact module you will buy. Do not borrow another module voltage, because one volt per module multiplied by a long string becomes a large error.
- Find the minimum series count. Divide the inverter PV input floor by Vmp and round up: Nmin = ceiling(floor ÷ Vmp). The string must clear this floor even in hot, weak light, when Vmp sags.
- Find the maximum series count. Divide the inverter PV input ceiling by Voc and round down: Nmax = floor(ceiling ÷ Voc). Use Voc corrected for the coldest morning if the datasheet gives a temperature coefficient; if it does not, leave headroom below the arithmetic maximum instead of running at the limit.
- Choose a count inside the band. Pick a whole number between Nmin and Nmax. A count near the middle of the band is more forgiving than a count pressed against either edge.
- Size total watts, then split into strings. Plan the array at roughly 1.3 to 1.6 times motor power, divide that target by the module watts-peak, and round to whole strings of the series count you chose. Add parallel strings, never mixed modules, for larger motors.
- Re-check both edges. Confirm the chosen string Voc stays below the ceiling on a cold morning and the working voltage stays above the floor in hot, weak light. If either check fails, change the series count or the inverter class before ordering.
Worked Example with the QB100-PV Windows
The table below uses a worked example only — a typical module datasheet with Vmp 41V and Voc 49V — applied to the verified QB100-PV input classes. Your own module voltages replace these figures; the method does not change.
| QB100-PV class | PV input class | Minimum series (floor ÷ 41V) | Maximum series (ceiling ÷ 49V) | Practical string |
|---|---|---|---|---|
| 220V models (S2 / SS2) | DC 180–450V | 5 panels (205V at Vmp) | 9 panels (441V at Voc) | 5–8 panels, leaving cold-morning headroom below 450V |
| 400V models | DC 250–900V | 7 panels (287V at Vmp) | 18 panels (882V at Voc) | 7–17 panels, leaving cold-morning headroom below 900V |
Notice what the example proves. A 220V string of only four such modules gives about 164V at Vmp, below the 180V floor, so the inverter may not start or may hunt in weak light. A 220V string of nine modules is arithmetically legal at 441V Voc, yet a cold morning can push Voc above 450V, so eight is the safer practical choice. The same logic makes a 400V string of six modules too low at about 246V at Vmp, while seventeen leaves headroom under the 900V ceiling.
Why a Wrong Voltage Window Means a Late Start and Cloudy Stops
MPPT tracking needs a working voltage to track. In the morning, irradiance is low, module current is low, and the string voltage the inverter sees can sit below its floor. The drive then waits, or starts and sleeps again, until the sun is strong enough to lift the string into its window. On a cloudy day the same mechanism repeats: the voltage sags, the pump slows below the speed a deep well needs to lift its water column, and the system stops until light recovers.
A string that is too long fails in the opposite direction. Its Voc can exceed the inverter ceiling on a cold, bright morning, when the array is cold and the drive has not yet loaded the string. That is a hard limit, not a performance loss: the inverter must protect itself, so the correct response is a shorter string or a different inverter class, not more panels in the same string.
Oversizing by 10–20%: What It Buys and Where It Stops
Once the string voltage is correct, adding roughly 10–20% more array watts — for example, one extra parallel string of the same modules — usually buys earlier starts, later stops and steadier flow on partly cloudy days, because the motor reaches its needed power earlier and holds it longer. The inverter only draws what the motor needs, so moderate oversizing does not force extra power into the pump.
Beyond that, extra watts stop buying water. The inverter input current limit, the PV voltage ceiling and the motor rating still cap the system, and extra panels add cost, roof or ground area, cabling and fault current without extending pumping hours. Oversizing is therefore a watts decision made inside a fixed voltage window, not a substitute for a correct series count.
| Motor rated power | Recommended array (1.3×) | Recommended array (1.6×) | What to check next |
|---|---|---|---|
| 0.75 kW | 1.0 kWp | 1.2 kWp | String count inside the QB100-PV class |
| 1.5 kW | 2.0 kWp | 2.4 kWp | String count inside the QB100-PV class |
| 2.2 kW | 2.9 kWp | 3.5 kWp | String count inside the QB100-PV class |
| 4 kW | 5.2 kWp | 6.4 kWp | Parallel strings of identical modules |
| 7.5 kW | 9.8 kWp | 12 kWp | Parallel strings of identical modules |
For the hardware behind these windows, see the QB100-PV solar pump inverter (0.4–37kW), with DC 180–450V on 220V models and DC 250–900V on 400V models and MPPT tracking rated up to 99%. Send your module Vmp, Voc and watts-peak, motor kW and planned string layout for a string-count check.
Frequently Asked Questions
Are more solar panels always better for a solar water pump?
No. This is the trap in panel counting. More panels in one string can push Voc over the inverter ceiling, and more watts beyond the motor and inverter current limits add no extra water. More panels help only when the series count stays inside the voltage window and the extra watts are added as correct parallel strings.
Do I count panels by Voc or by Vmp?
By both, for different edges. Use Vmp for the minimum count, because the string must work above the floor while loaded, and Voc for the maximum count, because the string must never exceed the ceiling when open-circuited on a cold morning.
Why does my pump start late in the morning even with a large array?
Because the string voltage, not the total watts, is the morning gate. If the series count is too low, the working voltage stays below the inverter floor until irradiance rises. Adding watts in parallel will not fix a series count that is too short; adding a module in series, within the ceiling, will.
Can I mix old and new modules in one string?
No. The weakest module limits the whole string, and mixed Vmp and Voc values make both edge checks unreliable. Keep each string to identical modules, and parallel only identical strings.
How many strings do I need for a larger motor?
First fix one correct series count for your inverter class and module, then add identical parallel strings until the total watts reach roughly 1.3 to 1.6 times motor power, subject to the inverter input current limit. Check the current limit on the inverter datasheet before adding a third or fourth string.
Not sure your string count clears both edges? Send IDEEI your module datasheet, motor kW and inverter class. Contact IDEEI for a string-count check.