| Step
|
What to check
|
Formula / Rule
|
What it determines
|
Notes
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| 1
|
Gather panel data
|
Need: Voc, Vmp, Isc, Imp, panel wattage, Voc temperature coefficient
|
Inputs for all later calculations
|
Use the actual module datasheet, not only the sales listing.
|
| 2
|
Gather inverter PV input data
|
Need: max DC voltage, MPPT voltage range, max input current per MPPT/input, max short-circuit current if listed, max PV wattage per MPPT, total max PV wattage
|
Inputs for inverter-side limits
|
Treat each MPPT/input separately unless the manual clearly says inputs are internally paralleled.
|
| 3
|
Choose design minimum temperature
|
Use lowest expected cell or ambient temperature for conservative design
|
Needed for maximum string voltage
|
Cold increases panel voltage, so this is a safety-critical step.
|
| 4
|
Correct panel Voc for cold weather
|
temp coefficient of Voc| × (25°C - Tmin)]
|
Real worst-case open-circuit voltage per panel
|
If coefficient is given in %/°C, convert to decimal first. Example: 0.28%/°C = 0.0028/°C.
|
| 5
|
Determine maximum panels in series
|
Max series panels = floor(inverter max DC voltage / Voc,cold)
|
Maximum safe string length
|
This is based on Voc, not Vmp.
|
| 6
|
Check MPPT operating window
|
String Vmp = number of series panels × panel Vmp
|
Confirms whether inverter can track the string
|
String Vmp should fall within the inverter MPPT voltage range under normal operating conditions.
|
| 7
|
Check low-end MPPT startup
|
String Vmp should be comfortably above the inverter MPPT minimum
|
Confirms inverter will wake up and track well
|
Designing too close to the MPPT minimum can cause poor performance in hot weather.
|
| 8
|
Check high-end MPPT limit
|
String Vmp should stay below the inverter MPPT maximum
|
Confirms normal operating voltage is usable
|
This is different from absolute max DC voltage, which is checked with Voc.
|
| 9
|
Determine current of one string
|
For one series string: string current is approximately panel Imp for operating current, and panel Isc for fault/current-limit checks
|
Current contribution of one string
|
Series adds voltage, not current.
|
| 10
|
Determine maximum parallel strings by operating current
|
Max parallel strings by Imp = floor(inverter max input current per MPPT / panel Imp)
|
Maximum number of strings per input from current limit
|
This usually controls how many strings can be placed on one MPPT.
|
| 11
|
Determine maximum parallel strings by short-circuit current
|
Max parallel strings by Isc = floor(inverter max short-circuit input current per MPPT / panel Isc)
|
Safety check on parallel string count
|
Use this if the inverter datasheet lists max PV short-circuit current. If not listed, at minimum respect the operating current limit.
|
| 12
|
Determine maximum parallel strings by wattage per MPPT
|
Max strings by MPPT power = floor(max PV watts per MPPT / string wattage)
|
Maximum strings per input from power rating
|
String wattage = series panel count × panel wattage.
|
| 13
|
Determine maximum parallel strings by total inverter PV wattage
|
Total allowed strings = floor(total max PV watts / string wattage)
|
Whole-inverter cap on string count
|
After finding total allowed strings, distribute them across MPPTs without exceeding per-input limits.
|
| 14
|
Final allowable strings per input
|
Allowable strings per input = smallest result from Step 10, Step 11, and Step 12
|
Final answer for each MPPT/input
|
Then confirm the total across all inputs also satisfies Step 13.
|
| 15
|
Check symmetry and MPPT grouping
|
Keep strings on the same MPPT electrically similar
|
Proper inverter tracking
|
Same orientation, tilt, shading profile, and module type should generally be grouped on the same MPPT.
|
| 16
|
Add design margin
|
Stay somewhat below absolute limits where practical
|
Better reliability and fewer edge-case failures
|
Do not design right at the 500 V maximum or current maximum unless the manufacturer explicitly supports it and conditions are well characterized.
|
| 17
|
Final design output
|
Report: panels per string, strings per MPPT, total strings, total DC watts
|
Complete PV stringing layout
|
Also report cold-corrected string Voc and nominal string Vmp.
|
| 18
|
Final rule summary
|
Strings per input = minimum of current limit, short-circuit current limit, per-MPPT wattage limit, and any manufacturer string-count limit
|
One-line decision rule
|
Series count is set by voltage; parallel count is set mainly by current and power.
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