Strings and MPPT (PV field design) explained simply

Strings and MPPT (solar array design) explained simply

We are going to design the solar array in a simple way so that you do not exceed the voltage, you do not lack amperes, and your inverter/controller can operate comfortably in its “optimal zone” (MPPT window = range where it delivers maximum output).

Quick translation of acronyms (in 1 sentence):

• Voc: panel “open-circuit” voltage (no load). Increases with cold.
• Vmp: voltage when the panel delivers its power. Decreases with heat.
• Isc: “peak” current in short circuit. Used to check limits.
• NOCT: typical panel temperature in sunlight. Helps estimate actual heat.
• Temperature coefficient: how much Voc/Vmp changes per degree (approx. 0.3%/°C).

What it’s for

• Avoid surprises on cold mornings (the Voc increases and you may exceed the limit).
• Do not “overload” the MPPT with excessive Isc when connecting in parallel.
• Ensure there is enough Vmp in summer for the equipment to start.
• The same principles apply to off-grid (controller) and self-consumption/hybrid (inverter).
• Leave realistic margins: it’s better to have a safety margin than to be on the edge.

What you need to check in the datasheets

From the panel: Voc, Vmp, Isc, temp. coefficient of Voc and Vmp, and NOCT.
From the equipment: max Vdc, MPPT window (min–max), and maximum currents per input/MPPT.

How many panels in series (without complicated formulas)

Idea: when it’s cold, the panel’s Voc increases. You must check that No. of panels × cold Voc does not exceed the equipment’s max Vdc.

1. Note the max Vdc of the inverter/controller (e.g.: 600 V, 1000 V).
2. Take the panel’s Voc and your typical minimum temperature.
3. Quick rule: for every 10 °C below 25 °C, the Voc increases by ≈3%.
4. Divide max Vdc by that “cold” Voc and round down. That is your maximum in series.

Quick multipliers (cold Voc ≈ Voc × …):
−10 °C: ×1.03 · −20 °C: ×1.06 · −30 °C: ×1.09 · −40 °C: ×1.12

Example: Max Vdc 600 V, Voc 49.5 V, Tmin = −5 °C (30 °C colder than 25).
Voc “cold” ≈ 49.5 × 1.09 = 53.9 V → 600 / 53.9 = 11.1 → 11 in series (if it's tight, remove 1 for margin).

Does it fall within the MPPT window in summer?

Idea: with heat the panel Vmp drops. You must check that number of panels × Vmp in heat remains within the MPPT range.

1. Check the MPPT window (e.g.: 120–580 V).
2. If you want to avoid complications: T_panel ≈ T_ambient + 25 °C in full sun.
3. Quick rule: for every 10 °C above 25 °C, Vmp drops by ≈3%.
4. Multiply the “hot” Vmp × number in series and check that it falls between MPPT min and max.

Short example: Vmp 41.5 V; hot day 40 °C → panel ≈ 65 °C (40 °C more than 25) → drops ≈12% → Vmp “hot” ≈ 36.5 V.
With 11 in series → ≈402 V → within 120–580 V ✔.

Parallels: do not exceed the current

In parallel voltage does not increase, amperage increases. Check with Isc (not with Imp).

1. Note the panel Isc (e.g.: 13.2 A).
2. Count how many strings in parallel go to the same input/MPPT.
3. Sum Isc: Isc_total = Isc × number of strings.
4. Check that Isc_total does not exceed the limit per input and/or per MPPT as specified in the datasheet.

Mini example: Panel Isc 13.2 A; MPPT limit 30 A and 2 inputs of 18 A each.
2 strings: per input 13.2 ≤ 18 ✔; total 26.4 ≤ 30 ✔ → OK.
3 strings: total 39.6 > 30 ✘ → distribute to another MPPT or change equipment.

Temperature: two easy reminders

• Cold → Voc rises → watch out for the max Vdc of the equipment.
• Heat → Vmp drops → watch the MPPT min so it “catches”.

Quick estimation of panel temperature:
Without complications: T_panel ≈ T_amb + 25 °C.
More precise with NOCT: T_panel ≈ T_amb + (NOCT − 20) (at 800 W/m²).

Oversizing (DC/AC) without fear

Having panel power a bit higher than the inverter (1.1–1.3) is normal. Just respect Vdc and currents. Assume overall system losses of 15–20% for quick estimates.

Short practical examples

Off-grid (regulator 250 V / MPPT 60–230 V / 60 A)

With the example panel: in cold 4 in series max; in heat Vmp_string ≈ 146 V (within range). With 60 A MPPT and Isc 13.2 A → up to 4 parallels (≈52.8 A) ✔.

Self-consumption (inverter 600 V / MPPT 120–580 V / input Isc 30 A)

Max 11 in series. With 2 strings in parallel → Total Isc ≈ 26.4 A (≤30 A) ✔.

Solar pumping (dedicated controller)

The manual requires power and voltage. Adjust the number in series to not exceed max Vdc and to ensure the Vmp in heat exceeds its minimum start value. In many cases, parallels are not used.

Quick checklist (mark ✔/✘)

Typical mistakes (and how to avoid them)

• Calculating only with catalog data (25 °C) → Include cold/hot conditions.
• Going to the Vdc limit → Leave a margin (remove 1 panel if you are close).
• Adding Imp instead of Isc for limits → Use Isc.
• Mixing different modules in the same MPPT → Avoid it.
• Forgetting the minimum MPPT in summer → the inverter may not start properly.

FAQs

Can I connect several strings to one MPPT? Yes, if the sum of Isc stays within the limits (per input and per MPPT if applicable).

What minimum temperature should I use? A reasonable minimum for your area (better to be cautious).

Does this apply to off-grid and self-consumption? Yes: same checks for Vdc, MPPT window, and Isc.

Does oversizing damage the inverter? No, as long as you respect voltage and current limits; the inverter will clip peaks.

What exactly is NOCT? A reference to estimate the panel's actual temperature under sunlight.

Related products

• Collection: Solar Panels
• Collection: MPPT Regulators
• Collection: Single-phase Inverters
• Collection: Three-phase Inverters

Need help? Tell us your location (cold/hot), panel model and inverter/regulator, and whether it is off-grid, self-consumption, or pumping. We will send you the calculation and the list of materials.