String Design
String design is where your panel specs and inverter specs meet. You’re figuring out how to wire the panels together so the voltage and current they produce fits inside the window your inverter’s MPPT can accept.
Get this wrong in one direction and your inverter can’t track power from your array. Get it wrong in the other direction and you can damage it permanently. Five minutes of math prevents both outcomes.
Series vs. Parallel
When you wire panels in series (positive of one panel to negative of the next), voltages add and current stays constant. Wire four panels with 37V open-circuit voltage in series and you get 148V at the same current as a single panel.
When you wire panels in parallel (all positives together, all negatives together), current adds and voltage stays constant. Wire four panels with 11A short-circuit current in parallel and you get 44A at the same voltage as a single panel.
Most DIY systems use series strings — or a combination of series strings wired in parallel. Series wiring is simpler, uses thinner wire (lower current), and produces higher voltage that travels longer distances with less loss.
Reading a Panel Datasheet
Every panel has a datasheet. Here are the four numbers you need for string design:
Voc (Open Circuit Voltage) — The panel’s maximum voltage, produced when nothing is connected. Peaks when the panel is cold and in full sun. This number — multiplied by your string length — must stay below your inverter’s maximum DC input voltage.
Vmp (Voltage at Maximum Power) — The operating voltage at peak wattage. This number — multiplied by your string length — must stay above your inverter’s minimum MPPT voltage, even on the hottest day.
Isc (Short Circuit Current) — Maximum current the panel can produce. Used for fuse and wire sizing.
Imp (Current at Maximum Power) — Current at peak output. Use this for parallel string current calculations.
Temperature coefficients — How much Voc and Vmp shift per degree of temperature change. Panels produce higher voltage when cold, lower voltage when hot. The datasheet gives the exact coefficient — usually a percentage per degree Celsius, like -0.27%/°C for Voc.
The Cold Weather Rule
Temperature correction for cold weather:
V_oc,cold = V_oc,STC × n_panels × (1 + |α_Voc| × ΔT)
Where ΔT is the temperature drop below 25°C STC (the standard test condition temperature). If your record low is -7°C, that’s 32 degrees below STC.
Check this against your inverter’s max DC input spec with some margin. This takes five minutes and protects a very expensive piece of equipment.
The Hot Weather Rule
Temperature correction for hot weather:
V_mp,hot = V_mp,STC × n_panels × (1 - |α_Vmp| × ΔT)
Where ΔT is the temperature rise above 25°C STC. Panels on a dark roof in summer run 20-25°C above air temperature. Use your local record high air temp, add 20°C for roof heating, and correct from there.
Never Mix Different Panels in a Series String
Buy all your panels at once from one order if you can. Same model number, same batch, same string.
Matching Strings to MPPT Inputs
If your inverter has one MPPT input, all your panels go on one string (or one set of parallel strings). If it has two MPPTs, you can run independent strings — different orientations, different shade profiles, different panel counts — each optimized separately.
For each string, verify:
- Combined Voc at coldest expected temperature stays below MPPT max input voltage
- Combined Vmp at hottest expected temperature stays above MPPT minimum voltage
- Total current stays below MPPT maximum input current
- Total watts are within MPPT maximum PV input power
Worked Example
8 x 400W panels. Datasheet specs:
| 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 (Portland record low: ~-7°C = 32°C below STC):
V_oc,cold = 8 × 37.5V × (1 + 0.0027 × 32) = 300V × 1.086 ≈ 326V
326V is well under the 500V max. ✓
Hot weather check (hot roof day: ~45°C = 20°C above STC):
V_mp,hot = 8 × 31.2V × (1 - 0.0027 × 20) = 249.6V × 0.946 ≈ 236V
236V is well above the 120V minimum. ✓
Current check: 13.9A Isc is under the 15A max. ✓
What’s Next
String design done. Now you need somewhere to store what you produce.
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, 2026.