Ten panels? Twelve? Fourteen? A string is right when four checks pass. A string that passes three of them is still wrong.
Part of the series The electrical checks on every solar job.
Panels per string is not one number. It is a window with a top and a bottom.
Ask an electrician how many panels go on a string and the honest answer is "between this and that". The top of the window is set by voltage on a cold morning, the bottom by voltage on a hot afternoon. Inside it, current decides how many strings one tracker will take, and the size of the whole array decides whether the inverter is a good match and which route the network operator wants. Those are the four checks. A calculator runs them in a second. It pays to know what it is doing, because the day it matters is the day you are typing figures from a datasheet you have not seen before.
The checks below are the ones the Installa string sizing calculator runs, and the codes are the ones Installa reports on a design.
Check one: on the coldest morning, open-circuit voltage stays under the inverter's DC limit
Open-circuit voltage rises as cells get colder. The datasheet's coefficient for Voc is a negative percentage per degree, and it works both ways: below 25 °C the voltage goes up. The string's Voc on a clear frosty morning, with the sun on the panels and the inverter not yet loading them, is the highest voltage the inverter will ever see.
For one panel the arithmetic is Voc × (1 + βVoc × (T − 25)), with βVoc as a fraction. Multiply by the panels in series and the result must stay under the inverter's maximum DC input voltage. Divide the DC limit by the cold Voc of one panel and round down, and you have the most panels the string can take. Installa reports a breach as PV003, an error, because this is the one that damages inverters.
Check two: on the hottest afternoon, operating voltage stays inside the MPPT window
Operating voltage, Vmp, falls as cells get hotter. A dark panel in still summer air runs well above the air temperature, on the afternoon the array produces the most, so it is the wrong moment for the inverter to lose the maximum power point. The string's Vmp at the hottest cell temperature must sit above the tracker's MPPT minimum. Divide the MPPT minimum by one panel's hot Vmp and round up, and you have the fewest panels the string can take. Below that is PV001, an error.
The window has a top too. At standard test conditions the string's Vmp must be under the MPPT maximum. That is PV002, a second cap on string length alongside check one, and the lower cap wins. A string under the MPPT minimum at STC but above it once hot gets a warning, PV004: it runs, but not at its best in ordinary conditions.
Check three: the strings on one tracker must not add up to more current than it takes
Strings in parallel on one MPPT add their current. The tool compares the panel's Imp, multiplied by the number of strings, with the tracker's maximum input current. Over that and the inverter limits the array, which is PV005, an error. Divide the tracker's input current by one panel's Imp and round down for the most strings a tracker will take. With a 13 A panel on a 15 A tracker, the answer is one.
The tool also reports the DC design current, Isc × 1.25 per string. That figure is not compared with the inverter. It is the current the DC cables and string fuses are chosen for under BS 7671 section 712, and it belongs on the design sheet next to the string voltages. If the panel's Isc alone is above the tracker's input current, the tool warns you to check the datasheet's maximum short-circuit current per MPPT.
Check four: the array against the inverter, and which route the network operator wants
Once the strings themselves work, two things are judged for the system as a whole.
The first is the ratio of array DC watts to inverter AC watts. Above 1.5 the tool warns of heavy clipping, PV008. Above 1.3, some clipping on bright days, PV009. Below 0.8, an inverter oversized for the array, PV010. All three are warnings, not errors: a high ratio on an east-west roof is a design choice, and the tool says so rather than stopping you. Among layouts that work, it prefers a ratio closest to 1.1.
The second is the grid connection route, which follows from the inverter's AC rating and the supply phases. Under Engineering Recommendation G98, an inverter up to 3.68 kW on single phase, or 11.04 kW on three phase, is fitted and then notified to the DNO within 28 days of commissioning. Anything larger needs a G99 application before the install, and above 50 kW a full application with a network study. It is not a string check, but it is the one that changes the install date, so the tool shows it beside the string window.
Worked example: twelve 440 W panels on a 5 kW single-phase hybrid
The panel is the Aiko Neostar 2 AIKO-A440-MAH54Mb from the tool's library: 440 W, Voc 40.82 V, Vmp 34.38 V, Isc 13.92 A, Imp 12.80 A, βVoc −0.22 %/°C, γPmax −0.26 %/°C. The inverter is the GivEnergy Hybrid Inverter 5.0 Gen 3: 5,000 W AC, single phase, maximum DC voltage 580 V, MPPT window 120 V to 550 V, 15 A per tracker, two trackers. The example assumes a coldest cell temperature of −10 °C and a hottest of 70 °C. Those are the tool's UK defaults and this example's own assumptions. Change them for the site and the window moves.
- Cold Voc. −10 is 35 degrees below 25. 40.82 × (1 + (−0.0022 × −35)) = 40.82 × 1.077 = 43.96 V per panel. 580 ÷ 43.96 = 13.19, so 13 panels at most.
- Hot Vmp. 70 is 45 degrees above 25. 34.38 × (1 + (−0.0026 × 45)) = 34.38 × 0.883 = 30.36 V per panel. 120 ÷ 30.36 = 3.95, so 4 panels at least. At STC, 550 ÷ 34.38 = 15.99, so the top of the MPPT window allows 15 and check one's cap of 13 binds. The window is 4 to 13 panels per string.
- Current. 15 ÷ 12.80 = 1.17, so one string per tracker. Isc of 13.92 A is under 15 A, so no warning. DC design current: 13.92 × 1.25 = 17.4 A per string.
- Twelve panels. Six per string, one string on each tracker. Cold Voc 6 × 43.96 = 263.8 V, under 580. Vmp at STC 6 × 34.38 = 206.3 V, inside 120 to 550. Hot Vmp 6 × 30.36 = 182.1 V, above 120. Twelve on one tracker also passes: 527.6 V cold, 412.6 V at STC, 364.3 V hot. The array is 12 × 440 = 5,280 W against 5,000 W AC, a ratio of 1.06. No warnings.
- Grid route. 5 kW on single phase is over the 3.68 kW G98 limit, so this job is a G99 application before the install, not a notification afterwards.
Push the same kit to fourteen panels and the arithmetic changes shape. Fourteen in one string gives 615 V on the cold morning, over the 580 V limit, PV003. Seven and seven passes every check with a ratio of 1.23. The window did not move. The layout did.
The tool gives you a window. The roof gives you the number.
Write three things on the design sheet: the window (4 to 13 in the example), the most strings per tracker (one), and the cold Voc, hot Vmp and design current for the string you chose. Then let the roof decide the count inside the window. Two strings of six on separate trackers means a shaded array does not drag the other down. Twelve on one tracker leaves the second free for an extension later. Both pass. The tool ranks the layouts that work for your count, and the choice stays yours.
Three habits keep it honest. Type the figures for the exact variant on the pallet, because one model name covers several wattages with different Voc. Leave a little room above the MPPT minimum, because the hot-side sum is slightly optimistic. And run the manufacturer's configurator before the kit is ordered, because it knows things about the inverter that the datasheet does not say.
Installa does the four checks for you
- Stop working the window out by hand. Pick the panel and inverter on the electrical design screen and Installa shows the window and the strings each tracker will take, and fills each tracker with a string of the suggested length in one click.
- Stop counting panels off the drawing. The arrays laid out in the 3D designer come through as strings, one per array, spread across the trackers.
- Stop checking one temperature. Every string is checked at −10 °C against the DC limit and at 70 °C against the MPPT window, with the same PV codes as the tool. On current the design screen is stricter: it compares Isc × 1.25 with the tracker's input current.
- Stop wondering whether a fault matters. An error you must fix is listed apart from a warning you can live with, and the last validation is saved with the design.
- Stop guessing the route. The system summary says whether the design is a G98 notification or a G99 application, against thresholds held in a rule book rather than buried in the code. Once the route is set on the job, the job file lists the DNO form, the single line diagram, the submission and the approval before install, each with a due date.
Cold Voc under the limit, hot Vmp inside the window, current the tracker can take, and an array that matches the inverter. Four checks on every string, on every design. See how electrical validation in Installa works, or try the string sizing calculator with your own kit.