String design is the math that makes your investment work. You’re wiring panels together to produce voltage and current that fits inside your inverter’s MPPT window — the operating range where your inverter can actually convert power. Get this right and your system runs for 25 years. Get it wrong and you’re buying a new inverter before you’ve paid off the first one.
This takes five minutes. Do the math.
Series wiring (positive of one panel to negative of the next): voltages add, current stays the same. Wire four panels with 37V open-circuit voltage in series and you get 148V — at the same current as a single panel.
Parallel wiring (all positives together, all negatives together): current adds, voltage stays the same. Wire four panels with 11A short-circuit current in parallel and you get 44A at the same voltage as one panel.
Most DIY builds use series strings — or series strings wired in parallel. Series wiring is simpler, uses thinner wire, and the higher voltage travels farther with less loss. That matters when your panels are on the roof and your inverter is in the garage.
Every panel ships with a datasheet. These are the four numbers that matter for string design:
Voc (Open Circuit Voltage) — Maximum voltage the panel produces, with nothing connected. Peaks when the panel is cold and in full sun. Your combined string Voc at coldest temperature must stay below your inverter’s max input voltage — or you damage it.
Vmp (Voltage at Maximum Power) — The operating voltage at rated wattage. Your combined string Vmp at hottest temperature must stay above your inverter’s minimum MPPT voltage — or your inverter stops working right when the sun is at its strongest.
Isc (Short Circuit Current) — Maximum current output. Drives fuse and wire sizing.
Imp (Current at Maximum Power) — Current at peak output. Used for parallel string calculations.
Temperature coefficients — How much Voc and Vmp shift per degree Celsius. Cold panels produce more voltage. Hot panels produce less. The datasheet gives the exact coefficient — usually expressed as a percentage per degree, like -0.27%/°C.
Temperature correction formula:
V_oc,cold = V_oc,STC × n_panels × (1 + |α_Voc| × ΔT)Where ΔT is how many degrees your coldest local temperature drops below 25°C STC. Plug in your actual numbers. Keep it under the inverter’s limit with margin.
Temperature correction formula:
V_mp,hot = V_mp,STC × n_panels × (1 - |α_Vmp| × ΔT)Use your record high air temperature, add 20°C for roof heating, and subtract from 25°C STC to get ΔT. Run this check before you finalize your string length.
One order, one pallet, one model. That’s the right way to do it.
For each MPPT input on your inverter, check all four limits:
If your inverter has two MPPT inputs, you can run two independent strings — different roof orientations, different shade profiles, different panel counts — each hitting its own optimal window.
Array: 8 x 400W panels
| Spec | Value |
|---|---|
| Voc | 37.5V |
| Vmp | 31.2V |
| Isc | 13.9A |
| Imp | 12.8A |
| Temperature coefficient (Voc) | -0.27%/°C |
Inverter MPPT window: 120—500VDC, 15A max.
Cold weather check (record low -7°C = 32°C below 25°C STC):
V_oc,cold = 8 × 37.5V × (1 + 0.0027 × 32) = 300V × 1.086 ≈ 326V326V — under the 500V max with room to spare. ✓
Hot weather check (45°C panel temperature = 20°C above STC):
V_mp,hot = 8 × 31.2V × (1 - 0.0027 × 20) = 249.6V × 0.946 ≈ 236V236V — above the 120V minimum. ✓
Current check: 13.9A — under the 15A max. ✓
String design locked in. Next up: where to store the energy your system produces.
Next: Batteries →
See also: Panels | Inverters | Batteries
DATA SOURCED FROM: Panel datasheet specifications and temperature coefficient standards per IEC 61215; inverter MPPT voltage window specifications from manufacturer datasheets; temperature correction methodology per NEC 690.7 and NREL PV string design guidance; individual system results vary based on local climate and equipment specifications, 2026.