Solar & battery
Solar Array Calculator
Configures an array, where string design is constrained at both voltage limits.
Result
The cold open-circuit voltage is the number that damages inverters, and it is checked against the record low for the site rather than a typical winter morning. Exceeding the inverter maximum can destroy it and is not covered by warranty. Panel datasheets give an exact temperature coefficient; the 0.3% figure here is a common approximation for crystalline silicon.
Get a Free Business AuditHow the calculation works
Modules arranged into strings, with the string voltage checked against the inverter's maximum input at the coldest expected temperature and against its minimum operating voltage at the hottest. Both limits bind: too long a string exceeds the maximum in cold weather, too short a string drops below the minimum in heat.
What moves the number
- Module open circuit and maximum power voltages.
- Coldest and hottest expected cell temperatures at the site.
- Inverter maximum input and operating voltage window.
- Number of strings the inverter accepts.
Where this one goes wrong
Designing to only one of the two limits is the usual error, and the consequences differ: exceeding the maximum input voltage can damage the inverter, while dropping below the operating minimum simply stops production on the hottest afternoons when it was most wanted. Check both using the site's temperature extremes. The code and the adopted edition also govern the calculation method, so the design should be done to the text the inspector will apply.
Before you quote
- Check string voltage at both temperature extremes.
- Use site-specific record temperatures, not regional averages.
- Design to the code edition the local authority has adopted.
Where the rules come from
Questions
About this calculator
What limits how many modules go in a string?
Two limits. The inverter's maximum input voltage, checked at the coldest expected temperature, and its minimum operating voltage, checked at the hottest. Designing to only one is the usual error and both have real consequences.
What happens if a string is too short?
It can drop below the inverter's operating voltage on the hottest afternoons, stopping production exactly when it was most valuable. That failure is less dramatic than exceeding the maximum and it quietly costs energy every summer.
Which temperatures should be used?
Site-specific records rather than regional averages, because the extremes are what bind. Published temperature data for the location is available and using an average instead is how strings end up outside the window on the few days it matters.
The formula
string voltage = panels in series x Vmp; cold Voc adds roughly 0.3% per degree C below 25
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How to use this calculator
- Panel rating (watts). Enter your own figure; the page starts at 400.
- Panel voltage at max power (Vmp). Enter your own figure; the page starts at 34.5.
- Open circuit voltage (Voc). Enter your own figure; the page starts at 41.2.
- Panel current at max power (Imp). Enter your own figure; the page starts at 11.6.
- Panels in series per string. Enter your own figure; the page starts at 12.
- Parallel strings. Enter your own figure; the page starts at 2.
The result updates as you type, so you can try a figure, see what it does to array power, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Panel rating (watts) | 400 |
| Panel voltage at max power (Vmp) | 34.5 |
| Open circuit voltage (Voc) | 41.2 |
| Panel current at max power (Imp) | 11.6 |
| Panels in series per string | 12 |
| Parallel strings | 2 |
| Coldest expected temperature (F) | 10 F |
What comes out
| Result | Value |
|---|---|
| Array power main result | 9,600 |
| String voltage at max power | 414.0 |
| Open circuit voltage when cold | 549.6 |
| Array current | 23.2 |
| Total panels | 24 |
9,600 watts from 24 panels. Strings run at 414.0 V in operation, but reach 549.6 V open circuit at 10 F. That cold figure is the one that must stay under the inverter's maximum.
How sensitive the answer is
Holding everything else steady and changing only panel rating (watts), this is what happens to array power. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Panel rating (watts) | Array power |
|---|---|
| 200 | 4,800 |
| 300 | 7,200 |
| 400 example above | 9,600 |
| 600 | 14,400 |
| 800 | 19,200 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Panel rating (watts)
- A number, between 0 and 1000, starting at 400.
- Panel voltage at max power (Vmp)
- A number, between 0 and 200, starting at 34.5.
- Open circuit voltage (Voc)
- A number, between 0 and 250, starting at 41.2.
- Panel current at max power (Imp)
- A number, between 0 and 40, starting at 11.6.
- Panels in series per string
- A number, between 0 and 40, starting at 12.
- Parallel strings
- A number, between 0 and 60, starting at 2.
- Coldest expected temperature (F) (F)
- Measured in F, between -60 and 90, starting at 10 F.
What the results tell you
| Result | What it is |
|---|---|
| Array power | The headline result. A number. |
| String voltage at max power | A decimal figure. |
| Open circuit voltage when cold | A decimal figure. |
| Array current | A decimal figure. |
| Total panels | A number. |
A design still needs a site visit
Output and sizing figures use the assumptions you enter, not measured irradiance, shading or roof orientation at your address. Real production depends on all three, plus inverter behaviour and local interconnection rules.
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