This is the math that protects YOUR investment. String design means wiring your panels so the voltage and current they produce fits inside the MPPT window of your inverter. Get it right and your system runs for 25 years. Get it wrong and you’re buying a new inverter — out of pocket, no warranty, because it’s installer error.
FIVE MINUTES OF MATH. That’s all it takes. Do it.
Series vs. Parallel
Series wiring — positive of one panel to negative of the next. Voltages ADD. Current stays the same. Four panels at 37V wired in series = 148V. Same current as one panel.
Parallel wiring — all positives together, all negatives together. Current ADDS. Voltage stays the same. Four panels at 11A wired in parallel = 44A. Same voltage as one panel.
Most patriots building their own systems use series strings — or series strings wired in parallel. Series is simpler, uses thinner wire (lower current), and the higher voltage travels farther with less loss. YOUR roof. YOUR inverter. YOUR money on the line if you get the wire sizing wrong.
Reading a Panel Datasheet
Every panel ships with a datasheet. Here are the four numbers that MATTER:
Voc (Open Circuit Voltage) — Maximum voltage with nothing connected. Highest when cold and in full sun. Your combined string Voc at the coldest day must stay UNDER your inverter’s max input voltage. No exceptions.
Vmp (Voltage at Maximum Power) — Operating voltage at rated output. Your combined string Vmp at the hottest day must stay ABOVE your inverter’s minimum MPPT voltage. If it drops below, the inverter stops — right when the sun is strongest.
Isc (Short Circuit Current) — Maximum current output. Determines your fuse and wire ratings.
Imp (Current at Maximum Power) — Current at peak output. Used for parallel string calculations.
Temperature coefficients — How much voltage shifts per degree Celsius. COLD panels = MORE voltage. HOT panels = LESS voltage. The datasheet gives the exact number — usually something like -0.27%/°C. This is the key input for BOTH temperature checks.
The Cold Weather Rule: Don’t Fry Your Inverter
Temperature correction:
Voc_cold = Voc_STC × n_panels × (1 + |αVoc| × ΔT)ΔT = how far your coldest local temperature drops below 25°C STC. Use your actual record low. Leave MARGIN. Do not design to the exact limit.
The Hot Weather Rule: Don’t Sleep Through Peak Sun
Temperature correction:
Vmp_hot = Vmp_STC × n_panels × (1 - |αVmp| × ΔT)Use your record high air temperature, add 20°C for roof panel heating, and that’s your ΔT above STC. Run this check. EVERY DESIGN. No exceptions.
Never Mix Different Panels in a Series String
American patriots build things RIGHT the first time. One order. One model. No mixing.
Matching Strings to MPPT Inputs
For every MPPT input on your inverter, check ALL FOUR limits:
- Combined Voc at your record low temperature stays BELOW MPPT max input voltage
- Combined Vmp at your record high temperature (plus 20°C roof heating) stays ABOVE MPPT minimum voltage
- Total current stays BELOW MPPT max input current
- Total wattage stays WITHIN MPPT max PV input power
Two MPPT inputs? Run two independent strings — different roof orientations, different shade situations, different panel counts. Each optimized separately. That’s how you squeeze maximum production out of YOUR roof.
Worked Example: The Full Calculation
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):
Voc_cold = 8 × 37.5V × (1 + 0.0027 × 32) = 300V × 1.086 ≈ 326V326V — well under the 500V max! ✓
HOT WEATHER CHECK (panel temp 45°C = 20°C above STC):
Vmp_hot = 8 × 31.2V × (1 - 0.0027 × 20) = 249.6V × 0.946 ≈ 236V236V — well above the 120V minimum! ✓
CURRENT CHECK: 13.9A — under 15A max! ✓
What’s Next
String design done. Now: where does the power go when the sun goes down?
Next: Batteries →
See also: Panels | Inverters | Batteries
DATA SOURCED FROM: We’re using THEIR data against them. 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. Every number sourced. Every claim verifiable. 2026.