The MPPT window is the DC voltage range inside which a solar pump inverter can track the maximum power point. Keep the array voltage inside it and the pump gets full power; let voltage drift outside it and output falls even on a sunny day.
What the MPPT Window Is
MPPT tracking and the voltage window, in plain terms: MPPT tracking follows the point where the array gives its maximum power, and the MPPT window is the DC voltage range inside which the inverter can do that tracking. For the difference between MPPT control and a standard VFD, see the MPPT solar pump inverter vs VFD article.
Why Voltage Moves
Voltage moves with temperature and light: cold mornings push string voltage up, hot afternoons pull it down, so a string sized only for one condition fails in the other.
When to Check the Window
Check the window at design time, when strings are sized; re-check it when panels are added or a pump is moved to a hotter or colder site. For array sizing, see how to size solar panels for a solar water pump.
Where the Window Applies
The window applies to every solar pump array, and the site reference is the QB100-PV 400V three-phase models’ DC 250-900V MPPT window used in the 380V article.
How to Keep the Array Inside the MPPT Window
- Read the inverter’s MPPT window from its data: for the QB100-PV 400V three-phase models it is DC 250-900V, as the 380V article sets out.
- Work out the string voltage at the coldest expected morning: cold-corrected open-circuit voltage must stay below the window’s upper limit.
- Work out the string voltage on the hottest afternoon: working voltage must stay above the window’s lower limit.
- Choose the series count so the string sits inside the window in both extremes, using the panel-series article’s method.
- After installation, watch the pump speed track the light through a full solar day, starting early and slowing gracefully in the evening, as the 380V commissioning advice sets out.
- If the pump underperforms at midday despite sun, re-check the window before blaming the pump.
What Moves Voltage
| Condition | Voltage effect | Risk |
|---|---|---|
| Cold morning | String voltage rises | Exceeds the upper limit, inverter cannot track |
| Hot afternoon | String voltage falls | Drops below the lower limit, output falls |
| Wrong series count | Voltage outside the window in one extreme | Poor output in that season |
Window Check
| Check | Pass sign |
|---|---|
| Cold-corrected open-circuit voltage | Below the upper limit |
| Hot-afternoon working voltage | Above the lower limit |
| Full-day speed tracking | Speed follows the light, no midday flat spot |
Frequently Asked Questions
What is the MPPT window in one sentence?
The DC voltage range inside which the inverter can track maximum power.
Why does cold weather raise voltage?
Panel voltage rises as temperature falls, so the coldest morning gives the highest string voltage.
What happens outside the window?
The inverter cannot track maximum power, so output falls even in sun.
If I add more panels in series, will output always rise?
No: too many in series can push cold-morning voltage above the window, so series count must be checked, not just added.
How do I know my string is inside the window?
Check cold-corrected open-circuit voltage against the upper limit and hot-afternoon working voltage against the lower limit, then watch full-day speed tracking.
To check your array, send IDEEI your panel model and series count for a window check.