Series adds voltage. Parallel adds current. The watts come out the same either way, and every string decision in solar is choosing where to sit on that tradeoff, with the inverter's input limits as the fence around the choices. Get this one idea and you can look at any string layout on any solar drawing and say why it was drawn that way.
The two wiring rules
Series means daisy-chained, plus to minus, one path through everything. Voltages add. Current stays the same everywhere in the chain, because there is only one path for it.
Parallel means side by side, all the pluses tied together and all the minuses tied together. Currents add. Voltage stays the same, because every branch sees the same two points.
That is the whole physics. Batteries in a flashlight are series: two 1.5V cells make 3V. Jumper cables are parallel: two batteries, same 12V, more cranking amps. Solar just runs the same two rules at bigger numbers.
A real string, panel by panel
Take the Canadian Solar 395W panel, the one on a real customer job we designed. One panel: Voc 36.6V, Imp 12.91A, Isc 13.77A, straight off the datasheet.
Wire 11 of them in series and the voltages add: 11 times 36.6 is 402.6V open-circuit at STC. The current does nothing. The string still moves 12.91A at max power, the same as one panel alone, because series is one path. That is the high-voltage, low-current shape long wire runs want, and it is why ordinary 10 gauge PV wire can carry a whole rooftop array.
Now parallel. That job lands two of those 11-panel strings on one MPPT input of an EG4 FlexBOSS21, which parallels them internally. Voltage stays 402.6V. Currents add: two times 12.91 is 25.82A. And that number matters, because the input is rated 26A. The design clears it by 0.18A, and the job notes say so out loud: substitute a higher-current panel on that design and the math breaks. The EG4 manual also caps any single string at 13A on a shared input, and this panel's 12.91A slides under by 0.09A. Margins this thin are why strings get designed on paper, never guessed in the field.
The fence: what limits string length
Series voltage cannot grow forever. The FlexBOSS21 tops out at 600V absolute maximum, stops operating at a 550V protection point, and delivers full power in a 250V to 440V window.
Here is the trap: Voc rises when panels get cold. This panel's Voc climbs about 0.26 percent for every degree the cells drop below 25°C, so a string reading 402.6V at STC corrects to about 439.2V on a morning ten below zero Celsius. Still under the fence. The same math kills longer strings. A 14-panel string corrects to about 551V at just four below zero, tripping the protection point on a near-freezing Gulf Coast morning, and 15 panels sit past the full-power window before the cold even gets a vote. On that job the roof wanted rows of 11, the inverter allowed 13 at most, the design used 11, and everyone stayed friends. String length is set by the inverter and the coldest morning of the year, never by the roof.
Batteries do it too
Parallel batteries follow the same current rule. That same job banks two EG4 16kWh indoor wallmounts. Each is 51.2V and 314Ah. In parallel the voltage stays 51.2V and the amp-hours add: 628Ah total, 32.16kWh. EG4's guidance for that inverter wants at least 600Ah of battery behind it, and 628 clears it. One battery would not. Parallel capacity is how a bank meets the inverter's appetite without changing the DC voltage the whole family of gear is built around.
The weakest-link rule
One more consequence of series being one path: the whole chain moves at the current of its weakest member. Shade one panel in an 11-panel string and you have not lost one panel's worth. You have throttled all 11, because every electron has to get through the shaded one. Bypass diodes inside the panel soften this, and how they do it is a datasheet topic for another guide. This is why string layout cares about shade lines and roof planes. The job above runs one string per roof row on purpose, so no string crosses a boundary where conditions change.
Parallel has its own failure shape. Branches are independent, so a lost branch costs only its own current. What parallel demands instead is matching. Tie mismatched voltages together and current circulates between branches trying to even them out, which is why EG4 parallels identical batteries with matched cabling, and why nobody should mix old and new strings casually.
Try this at home
If you have a solar proposal or drawing in hand, reverse-engineer its string math. Multiply the per-panel Voc by the string length and check it against the inverter's max input voltage. Then multiply the Imp by the number of paralleled strings and check it against the MPPT's current rating. Both numbers come off the panel datasheet and the inverter manual. If either margin surprises you, that is a conversation to have with the designer now, on paper, instead of a fault code in January. Or send us the layout and we will run it with you.
Sources
- Canadian Solar HiKu6 All-Black datasheet, models CS6R-380 to 405MS-HL, v1.1C25, Aug 2022: Voc 36.6V, Imp 12.91A, Isc 13.77A, and the Voc temperature coefficient of 0.26 percent per degree C. The string figures (402.6V at STC, about 439.2V at ten below zero C, about 551V for 14 panels at four below zero) are arithmetic from those values.
- EG4 FlexBOSS21 manual: MPPT input ratings including the 26A input, the 13A per-string cap on shared inputs, 600V absolute maximum, 550V protection point, and the 250V to 440V full-power window. Check the current manual before designing.
- EG4 WallMount Indoor 16kWh battery datasheet: 51.2V, 314Ah per unit, and EG4's minimum battery bank guidance for the paired inverter.
- The series and parallel rules, circulating current between mismatched parallel branches, and the weakest-link behavior under shade are circuit physics, no document needed.