How to use this calculator
- Enter module datasheet values. Use module STC power, Voc, Vmp, Isc, Imp and signed voltage temperature coefficients.
- Enter design temperatures. Use the project cold design temperature for Voc and a realistic hot cell temperature for Vmp.
- Enter inverter limits. Use max DC voltage, MPPT min/max voltage, startup voltage and max input current from the checked MPPT channel.
- Set planned string layout. Enter modules in series and parallel strings on that MPPT.
- Review margins. Check the min/max series window, cold Voc, hot Vmp, input current utilization and DC:AC ratio.
How it works
A solar panel string sizing calculator starts with the module datasheet
voltage at STC, then applies the signed temperature coefficient:
V(T) = V_stc x (1 + beta x (T - 25) / 100)
where beta is percent per degree C. Negative crystalline-module
coefficients make voltage rise in cold weather and fall when the cell is hot.
Cold string Voc is N_s times cold module
Voc, and it must stay below the inverter or controller absolute
max DC voltage. Hot string Vmp is checked against MPPT minimum
and startup voltage. Cold string Vmp is also checked against the
MPPT high operating limit. For energy, battery and inverter capacity, use the
off-grid solar calculator. For
conductors and raceways, use
voltage drop,
conduit fill and
grounding conductor sizing.
Worked example
Verified against the live calculator
With a 400 W module at Voc = 41 V,
Vmp = 34 V, betaVoc = -0.29%/C,
betaVmp = -0.35%/C, -10 C cold design temperature
and 70 C hot cell temperature, cold module Voc is
about 45.16 V and hot module Vmp is about
28.65 V. A 10-module string has cold Voc near
451.6 V and hot Vmp near 286.5 V.
For a 600 V inverter with a 120-550 V MPPT window
and 150 V startup, the series window screens as
6 to 13 modules. Two parallel strings at
Isc = 10.5 A use 80.8% of a
26 A MPPT current limit.
Reference data
Use this as a PV string voltage worksheet before choosing final wiring, protection and product-specific settings.
| Worksheet step | Formula | Use the result for |
|---|---|---|
| Module datasheet | Pmod, Voc, Vmp, Isc, Imp, betaVoc, betaVmp | Inputs for voltage/current correction. |
| Temperature correction | V(T) = Vstc x (1 + beta x (T - 25) / 100) | Cold Voc, hot Vmp and cold Vmp. |
| Series window | Nmin = ceil(max(Vmppt_min, Vstart) / Vmp_hot) | Minimum modules in series. |
| Upper voltage limit | Nmax = min(floor(Vdc_max / Voc_cold), floor(Vmppt_max / Vmp_cold)) | Maximum modules in series. |
| Parallel current | Isc_array = Np x Isc_module | MPPT input current screen. |
| DC:AC ratio | Rdcac = Pdc_array / Pac_inverter | Nameplate loading check. |
Source: Formula-only worksheet. Module and inverter values must come from the actual datasheets for the project.
Frequently asked questions
How many solar panels can I put in series?
Use the lower of two maximums: inverter max DC voltage divided by cold-corrected module Voc, and MPPT max voltage divided by cold-corrected module Vmp. Then check that the chosen string also clears the MPPT minimum and startup voltage at hot cell temperature.
Why does cold Voc matter for PV string sizing?
Module open-circuit voltage rises when temperature falls. A string that is safe at STC can exceed the inverter or charge-controller absolute max DC voltage on a cold morning, so cold Voc sets the upper module count.
Why does hot Vmp matter?
Module maximum-power voltage falls as cell temperature rises. Hot Vmp is used to confirm the string can still reach the MPPT operating and startup voltage when the array is hot.
What is MPPT start voltage?
Startup voltage is the minimum input voltage the inverter or controller needs to wake up or begin tracking. It may be higher than the normal MPPT minimum, so this calculator checks both.
Does this size PV fuses, wire or rapid shutdown?
No. It is a string voltage and current worksheet only. Fuses, conductors, disconnects, rapid shutdown, grounding, product listings, adopted code and permits need separate design checks.
How is DC:AC ratio used?
DC:AC ratio is array DC nameplate watts divided by inverter AC watts. It is a planning check for inverter loading and clipping, not a substitute for the inverter manufacturer sizing tool.
Method & assumptions
- Uses user-entered module and inverter datasheet values; it does not embed module or inverter catalogs.
- Temperature coefficients are signed percent per degree C. Verify the datasheet sign and whether the coefficient is for Voc, Vmp or Pmax.
- Cold Voc uses the entered cold design temperature. Hot Vmp uses the entered hot cell temperature, not ambient air temperature.
- Parallel-string current uses module Isc times the entered number of parallel strings on the checked MPPT channel.
- Does not size fuses, OCPD, conductors, combiner boxes, disconnects, rapid shutdown, grounding, batteries, structural mounting, utility interconnection or permits.