Strings & voltage (series)
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Now the first real sizing decision: how many modules go in a string? Wire too few and the inverter won’t run; wire too many and you can exceed its limits and trip (or damage) it. The answer comes down to keeping the string’s voltage inside the inverter’s MPPT window — in every temperature your roof sees.
Series adds voltage
From Module 1: modules in series add their voltages. Five panels at ~40 V each make a ~200 V string. The inverter’s datasheet (Module 2) gives the window that voltage must land in: a minimum to operate well and a maximum you must never exceed.
The twist: voltage moves with temperature
Panel voltage isn’t fixed — it rises as panels get cold and falls as they get hot (that’s the temperature coefficient on the datasheet). So:
- Coldest morning → highest voltage. This sets the maximum modules per string: the string’s Voc on the coldest expected day must stay under the inverter’s max input voltage.
- Hottest afternoon → lowest voltage. This sets the minimum: the string’s Vmp when blazing hot must stay above the MPPT’s minimum, or the inverter drops off its best operating point.
The classic, expensive mistake is sizing for a nice 25 °C day and forgetting the cold-morning Voc spike — which can push voltage past the inverter’s limit.
A worked example (Texas-ish)
Say a panel has Voc = 41 V at 25 °C, an inverter max input of 600 V, and a record-cold morning of about −5 °C. Cold pushes Voc up a few percent — call it ~45 V per panel. 600 ÷ 45 ≈ 13.3, so 13 modules is the safe maximum for that string. On the hot end, you check that 13 panels’ Vmp at, say, 65 °C cell temperature still sits above the MPPT minimum. Land between those two bounds and you’re good. (Exact numbers come from the datasheet; this is the method.)
Try it in the editor
In the system editor, set modules in series for a sub-array and watch it stay within the chosen inverter’s window — the tool does this temperature-bounded check for you, so you can feel how the min/max move as you change panel or inverter.
Takeaway
- String length is bounded by voltage, not by “how many fit.”
- Cold sets the max (Voc), heat sets the min (Vmp).
- Always size for your record cold and hot, not an average day.
Next: MPPTs — what they do, and how paralleling strings adds current you must watch.
This course teaches you to design and model a system. Physical wiring, MPPT/battery connection, and grid interconnection are licensed-electrician work under the NEC — design here, build with a pro.