Easy Solar Calc

What happens with two 100W solar panels in series?

Short answer

Two 100W panels in series give 36 V × 5.56 A = 200 W at maximum power. Their open-circuit voltage is 43.2 V at 25 °C and about 48.5 V on a 14 °F (−10 °C) morning — well inside a 100 V MPPT controller.

How to work it out

  1. Series adds volts: 18 V × 2 = 36 V; parallel adds amps: 5.56 A × 1 = 5.56 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)

These numbers use Victron’s example 100 W, 36-cell panel: Voc 21.6 V, Vmp 18 V, Imp 5.56 A, and a Voc temperature coefficient of −0.35 %/°C. Put in your own panel’s data-sheet figures — they vary by model.

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 × 5.56 A = 200 W
Array Vmp
36 V
Array Imp
5.56 A
Array power (STC)
200 W
Array Voc at 25 °C (STC)
43.2 V
Array Voc at the coldest temperature
48.5 V
Array Isc
6.12 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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MPPT solar charge controllerA series string at 36 V needs an MPPT controller whose PV voltage limit clears the cold Voc of 49 V.
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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

What to know about two panels in series

  • It needs an MPPT controller on 12 V. A PWM controller would pull the 36 V string down to battery voltage and waste most of the second panel. An MPPT controller converts it: about 200 W ÷ 13 V ≈ 15 A into the battery in full sun.
  • Half the current of parallel. 5.56 A instead of 11.1 A means less voltage drop in the cable from the roof.
  • Shade hits both. If one panel is shaded, the string’s current drops toward the shaded panel’s. On a van roof with a vent shadow, parallel can be the better choice.
  • Plenty of voltage headroom. Even at −22 °F (−30 °C) the string is about 51.6 V, so a 75 V or 100 V controller is fine; three in series would be about 73 V at −10 °C — too close for a 75 V controller.

Four panels, compared both ways, are on four solar panels in series vs parallel. The controller current for 200 W is on what size charge controller for a 200 watt solar panel, and the daily output on how much power a 200W 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 about two 100W solar panels in parallel?

18 V × 11.12 A = 200 W — the same 200 W at 18 V, with a cold Voc of only 24.3 V.

What is the voltage of three 100W panels in series on a cold morning?

About 72.8 V at 14 °F (−10 °C) — OK — 27.2 V below the 100 V limit.

Other common sizes

Sources