Matching the array to the controller
A pump selection is only half the job. The array that feeds it has to stay inside the controller input window at both ends of the year: high enough on the hottest afternoon to keep running, low enough on the coldest morning not to trip the limit.
This page works through the arithmetic with numbers you can check, and lists the five ways it usually goes wrong.
Cold sets the ceiling
Open-circuit voltage rises as the module cools. The coldest morning is what decides the longest string you can build.
Heat sets the floor
Maximum power voltage falls about 15% at 70 °C cell. Too short a string drops under the start voltage in the afternoon.
Both, at your site
Neither limit is a datasheet number. They come from the record temperatures where the array will actually stand.
Why temperature, and not just power
A module datasheet quotes its voltages at standard test conditions: 25 °C cell temperature, 1000 W/m². Nothing outdoors is at standard test conditions. Two coefficients on the same datasheet tell you what happens when it is not.
| Voc temperature coefficient | Typically around −0.25 to −0.29 %/°C. Negative, so voltage goes up as temperature comes down. This is the one that can break the controller. |
| Vmp temperature coefficient | Typically around −0.30 to −0.40 %/°C. Same direction, larger. This is the one that decides whether the array can still drive the pump when it is hot. |
| Cell temperature, cold case | Close to air temperature on a clear morning before the sun is on the array. Use the record low for the site. |
| Cell temperature, hot case | Well above air temperature, commonly 25 to 35 °C above it in still air. 70 °C cell is a normal working assumption in a hot climate. |
The ranges above are the usual order of magnitude for current crystalline modules. Use the coefficients printed on the datasheet for the module you are actually buying.
A worked example
A drive with a 350 – 780 V input window, the higher of the two ranges we offer. Modules rated Voc 49.5 V and Vmp 42.0 V at standard test conditions, with a Voc coefficient of −0.27 %/°C and a Vmp coefficient of −0.35 %/°C. Cold case taken at −5 °C cell, hot case at 70 °C cell.
Vcold = Voc × ( 1 + β ÷ 100 × ( Tcold − 25 ) ) × N
Vhot = Vmp × ( 1 + γ ÷ 100 × ( Thot − 25 ) ) × N
β = Voc coefficient · γ = Vmp coefficient · N = modules in series
| String length | Voc at −5 °C cell | Vmp at 70 °C cell | Verdict |
|---|---|---|---|
| 13 modules | 696 V | 460 V | Works, but leaves array capacity on the table |
| 14 modules | 749 V | 495 V | The most you can fit inside the window |
| 15 modules | 803 V | 531 V | Over the 780 V limit on a cold morning |
So 14 modules per string is the answer for that module on that drive. Change the module and it changes. Move the site somewhere colder and it changes again. This is why we ask for the location before quoting anything, and why the string arrangement goes on the quotation with the two voltages written out.
DC pumps: the same check, smaller numbers
Small DC brushless pumps require a compatible controller. Its input limits cannot be inferred from the motor voltage alone. Keep the equipment part numbers and applicable manual revisions with the design calculation.
A motor voltage is not a PV input specification. DC and AC/DC hybrid controllers can have different limits even with the same motor rating. Use the exact controller part number and manual revision.
| Required controller evidence | Check separately |
|---|---|
| Maximum input Voc | Cold-corrected array open-circuit voltage must remain within the specified limit, including required design margins. |
| MPPT operating range | Check array operating voltage over the design temperature range; do not confuse Vmp with Voc. |
| Minimum input and start conditions | Confirm the documented start and operating requirements, including available power. |
| Current and power limits | Check parallel strings, module electrical data and the controller input ratings. |
| Motor and supply compatibility | Confirm approved motor type, rated voltage, controller configuration and any AC backup arrangement. |
Array-to-pump power ratios and panel counts are not universal installation rules. A qualified designer must check the selected equipment and site conditions; a catalogue wiring illustration is not a project wiring plan.
Five ways this goes wrong
Sizing on the STC number
Voc on the datasheet is measured at 25 °C cell temperature. Your coldest morning is nowhere near that, and the array voltage rises as it gets colder. The limit has to hold at the cold extreme, not at STC.
Using air temperature instead of cell temperature
On a clear cold morning with the array still dark, cell temperature is close to ambient. That is the worst case for Voc, and it is why the record low matters more than the average low.
Forgetting the hot end
At 70 °C cell the maximum power voltage falls by around 15%. If the string is too short, the array drops below the controller start voltage in the middle of a hot afternoon, which is exactly when you want the water.
Treating an array-to-pump ratio as a complete design
A larger array may extend useful operation, but a fixed ratio cannot prove daily delivery. Check seasonal solar conditions, duty point, losses and controller input limits for the selected system.
Ignoring cable voltage drop on the DC side
A long run from array to cabinet costs voltage exactly when the array is already weak. Size the DC cable for the run, not for the current alone.
What a tender usually asks for here
Buyers running a formal procurement often ask for proof that the array stays inside the controller limits at the minimum and maximum design temperatures, one calculation per pump and controller combination. It is the same arithmetic as above, written out and signed. Worth preparing once rather than per bid.
Send the module datasheet and the site
Voc, Vmp, both temperature coefficients and the city. We come back with the string arrangement and the array voltage at your record low and high, written on the quotation.
Array and controller matching FAQ
Why does the PV array voltage have to be checked at the coldest temperature?
Because open-circuit voltage rises as the module gets colder. A module rated 49.5 V Voc at 25 °C reaches about 53.5 V at −5 °C cell temperature with a typical −0.27 %/°C coefficient. Multiply that by the number of modules in series and a string that looked safe on the datasheet can exceed the controller input limit on the first cold morning.
How many modules can I put in one string on a solar pump controller?
As many as keep the cold-morning open-circuit voltage under the controller limit. On a 780 V drive with 49.5 V Voc modules, 14 in series gives about 749 V at −5 °C cell temperature and 15 gives about 803 V, which is over. So 14 is the answer for that combination, and it changes with a different module or a different site.
What cell temperature should I use for the cold case?
The record low air temperature at the site is the usual basis, because a module that has been dark all night sits close to ambient. Using the average winter low instead is the most common way a string ends up over the limit two or three mornings a year.
How much bigger than the pump should the array be?
There is no universal ratio that guarantees output. Size the array against the required daily water delivery, design-month solar conditions, pump duty point, losses and exact controller limits. A catalogue ratio is a starting assumption to check, not a performance guarantee.
Does module temperature matter on the hot end as well?
Yes, in the opposite direction. At 70 °C cell temperature the maximum power voltage falls roughly 15%. If the string is too short, the array drops under the controller start voltage on hot afternoons. The string has to fit between both limits, not just under the top one.
What do you need from me to check this?
The module datasheet with Voc, Vmp and the temperature coefficients, the controller or drive input window, and the site city or coordinates so we can take the temperature extremes. We send the string arrangement with the array voltage at your record low and high temperature stated on the quotation.
Technical Guides
Products & Systems
Markets & Services
Need project-specific engineering support?
Free CAD layout, wind/snow load calculations, and BOM review included with project orders.