How to use the solar string sizing calculator
- Enter the module numbers from its data sheet: Voc, Vmp, Isc and the temperature coefficients of Voc and Vmp as percent per degree.
- Enter the site's lowest temperature and the hottest module temperature, which is above the air temperature in sun.
- Enter the inverter's maximum DC voltage and MPPT minimum voltage. In the advanced options, add the MPPT maximum voltage, the input current limit and a current margin factor.
- Read the fewest and most modules per string, the temperature-adjusted voltages and the table of strings near the limits.
Formula
Voc cold = Voc x (1 + Voc coefficient x (lowest temperature - 25))
Vmp hot = Vmp x (1 + Vmp coefficient x (hottest temperature - 25))
most modules = round down(maximum DC voltage / Voc cold), also limited by MPPT maximum / Vmp cold
fewest modules = round up(MPPT minimum / Vmp hot)
strings in parallel = round down(current limit / (Isc x margin factor))
The coefficients are in percent per degree Celsius and are normally negative. The voltage limits come from the inverter data sheet.
A 40 V module and a 600 V inverter
A module with Voc 40 V, Vmp 33 V, Isc 10 A and coefficients of -0.30 and -0.38 percent per degree, at a site that falls to -10 C and where cells reach 65 C. Voc at -10 C is 40 x (1 + 0.003 x 35) = 44.20 V. A 600 V inverter allows round down(600 / 44.2) = 13 modules.
Vmp at 65 C is 33 x (1 - 0.0038 x 40) = 27.98 V, so an MPPT minimum of 200 V needs round up(200 / 27.98) = 8 modules. The strings can have 8 to 13 modules. A 13-module string measures 574.6 V when coldest and 363.8 V when hottest. With an input limit of 25 A, two strings can run in parallel.
Why temperature drives the limits
A solar module's voltage rises when it is cold and falls when it is hot. The highest voltage of a string occurs on the coldest sunny morning and must stay under the inverter's maximum, with a safety margin that your rules set. The lowest voltage occurs on a hot afternoon and must stay above the level at which the inverter tracks the maximum power point. The two limits set the range of string lengths that work.
To judge the benefit and cost of a system, see the solar payback calculator. For batteries and cable runs, the battery bank calculator and the wire voltage drop calculator help.
Planning aid only
Always check the inverter manual, the module data sheet and your local electrical code before choosing a string length. Safety factors and limits come from those documents, not from this tool. The factors are yours to enter, and nothing from a code is built in. Work on solar systems carries a risk of shock and fire, so have a qualified person design and install it.
Assumptions and limits
- All modules in a string are identical, and the temperature coefficients are linear over the range.
- The hottest module temperature is entered by you; the page does not add a rise above the air temperature.
- The current margin factor is an example value of 1, which means none. Use the figure your rules require.
- Shading, mismatch, wiring loss and inverter clipping are not modeled.
Frequently asked questions
Which temperature do I use for the cold limit?
Use the lowest temperature you plan for at the site. Some designers use a record low and others a low that occurs a certain share of years. Your rules or the inverter maker may name the figure.
What is the MPPT window?
It is the voltage range in which the inverter can track the maximum power point. A string below the minimum may not start, and above the maximum the inverter cannot track, so it must also stay within the absolute maximum voltage.
What if no string fits?
The page says "No string fits" when the fewest modules needed is above the most allowed. Try a different inverter with a wider window, or a module with a different voltage.
Does this check the current?
Only through the optional input current limit, which sets how many strings can share a tracker. Cable sizes and fuses are not covered.