Easy Solar Calc

Four solar panels: series or parallel?

Short answer

Wired as two strings of two (series-parallel), four 100 W panels give 36 V × 11.12 A = 400 W with a cold open-circuit voltage of about 48.5 V — comfortable for a 100 V MPPT controller. All four in series would reach about 97 V on a 14 °F morning.

How to work it out

  1. Series adds volts: 18 V × 2 = 36 V; parallel adds amps: 5.56 A × 2 = 11.12 A
  2. Cold Voc per panel = 21.6 V × (1 + −0.35% × (−10 − 25) °C) = 24.25 V × 2 in series = 48.5 V (rounded up)

Four panels give you three ways to wire them: all in parallel (18 V, 22 A), all in series (72 V, 5.6 A) or two strings of two (36 V, 11 A). The best one depends on your controller’s voltage limit, your cable run and your shade.

Adjust for your project

From the panel’s data sheet or label (STC). Default: Victron’s example 100 W panel.
°F
Record low where the panels are. Open-circuit voltage rises as panels get colder.
On the data sheet, e.g. −0.35 %/°C. Enter it as a negative number.
The first number in names like “100/30” on many MPPT controllers. Blank to skip the check.
More options: Controller max Isc
Many MPPT data sheets list a maximum PV short-circuit current separately from the charge current. Blank to skip the check.
Array at maximum power36 V × 11.12 A = 400 W
Array Vmp
36 V
Array Imp
11.12 A
Array power (STC)
400 W
Array Voc at 25 °C (STC)
43.2 V
Array Voc at the coldest temperature
48.5 V
Array Isc
12.24 A
Controller voltage check
OK — 51.5 V below the 100 V limit
Check before you wireFor permanent installations, NEC 690.7 governs how maximum PV system voltage is determined, and wire and fuse sizes come from NEC/ABYC ampacity tables and the equipment manual — have a qualified installer check them.

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MC4 branch connectors (Y splitters)Branch connectors join parallel strings without a combiner box on small arrays — check their current rating against the total.
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Cold open-circuit voltage at other temperatures

Cold open-circuit voltage at other temperatures
Coldest expected temperatureArray Voc at the coldest temperatureController voltage check
50 °F45.5 VOK — 54.5 V below the 100 V limit
32 °F47 VOK — 53 V below the 100 V limit
14 °F48.5 VOK — 51.5 V below the 100 V limit
-4 °F50.1 VOK — 49.9 V below the 100 V limit
-22 °F51.6 VOK — 48.4 V below the 100 V limit

The three options compared

  • All parallel (4P): 18 V and about 22 A. Works with a PWM controller on 12 V, but the current needs heavy cable on a long run, and with three or more strings in parallel an installer should check whether each string needs a fuse.
  • All series (4S): 72 V and 5.56 A at maximum power, 86.4 V open-circuit — and about 97 V on a cold morning, which leaves almost no margin on a 100 V controller. A 150 V controller is the safer match.
  • Two strings of two (2S2P): 36 V and 11.12 A, cold Voc about 48.5 V. A common middle ground for 12 V systems with a 100 V MPPT controller.
  • Shade decides ties. Put panels that get shaded at the same time of day in the same string, so one shadow doesn’t pull down a string of sunny panels.

The two-panel version is on two 100W solar panels in series; the controller for 400 W is on what size charge controller for a 400 watt solar panel. Daily output of 400 W is on how much power a 400W solar panel produces per day.

Different size? Use the full Solar Panel Series & Parallel Calculator.

What you’ll need

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Frequently asked questions

What voltage are four 100W panels all in series on a cold morning?

About 97 V at 14 °F — OK — 3 V below the 100 V limit.

What current do four 100W panels all in parallel make?

18 V × 22.24 A = 400 W, with a short-circuit current of 24.48 A.

Other common sizes

Sources