Solar Inverter Sizing Guide for B2B Projects

This guide walks through the six steps of sizing a commercial solar inverter so that the unit, its DC strings, its AC breaker, and its cabling all stay within their datasheet limits at the site’s worst-case temperature extremes. Whether you are quoting a 50 kW rooftop or a 1 MW ground-mount farm, the same six steps apply — only the numbers change.

Step 1: Set the DC/AC overload ratio

Modern panels are oversized vs the inverter on purpose. Because the array rarely produces nameplate STC power (only on cool, perfectly clear days at solar noon), the inverter can be smaller than the array DC nameplate and still capture most of the energy.

• Cool / temperate sites: 1.20 – 1.30
• MENA, Sub-Saharan Africa, Southeast Asia: 1.35 – 1.50
• Bifacial panels on white-roof reflective surface: drop ratio by 0.10 to leave room for rear-side gain

Above 1.55 the inverter starts clipping more than 2% of annual production, which usually exceeds the capex saved on the smaller unit.

Step 2: Pick the panel and read the four key numbers

From the panel datasheet at STC (1,000 W/m², 25 °C cell):

• Voc (open-circuit voltage)
• Vmpp (max power point voltage)
• Isc (short-circuit current)
• Impp (current at Vmpp)

Then read the temperature coefficient of Voc (typically −0.27% to −0.30% per °C for monocrystalline modules).

Step 3: Calculate worst-case string voltage

The inverter MPPT upper limit must not be exceeded at the site’s coldest expected morning (when irradiance is already present but cells are still cold).

Voc_adj = Voc * (1 + (T_min - 25) * tcVoc)
N_max  = floor(Vmpp_inv_max / Voc_adj)
N_min  = ceil(Vmpp_inv_min / Vmpp_adj_hot)

Worked example (10 kW inverter, MPPT 200–800 V, panel Voc 41.8 V, tcVoc −0.28%/°C, site T_min = −5 °C in winter):

• Voc_adj = 41.8 × (1 + (−5 − 25) × −0.0028) = 41.8 × 1.084 = 45.3 V
• N_max = floor(800 / 45.3) = 17 panels per string

At hot extreme (cell 65 °C):

• Vmpp_adj = 34.5 × (1 + (65 − 25) × −0.0042) = 34.5 × 0.832 = 28.7 V
• N_min = ceil(200 / 28.7) = 7 panels per string

Operating range: 7–17 panels per string. Use 12–15 for headroom.

Step 4: Allocate strings to MPPTs

Group strings by orientation, tilt, and shading profile. One MPPT per group. If your inverter has 4 MPPTs and you have 4 orientations, one string per MPPT is correct. If you have a single orientation with 6 strings, you can parallel 2 strings per MPPT (verify each MPPT’s per-channel current limit — typically 13–15 A per MPPT for commercial inverters).

Step 5: Size AC cable and breaker

AC current at full output: I_ac = P_inv / (V_ac × √3 × cosφ) for three-phase, or I_ac = P_inv / V_ac for single-phase. Add 25% headroom per IEC 60364-7-712.

For a 50 kW three-phase inverter at 400 V:

• I_ac = 50,000 / (400 × 1.732 × 0.95) = 76 A
• MCCB / fuse: 76 × 1.25 = 95 A → use 100 A
• Cable: 35 mm² Cu (4-core) for ≤ 30 m run, voltage drop < 2%

Step 6: Pick the inverter SKU

Cross-check the inverter datasheet for:

• Rated AC output (P_inv) matches the array’s DC kWp ÷ overload ratio
• Number of MPPTs ≥ number of orientation/shading groups
• MPPT range covers the calculated worst-case string voltage at both extremes
• DC overload ratio supported ≥ your chosen ratio (1.35 needs an inverter that allows 1.35× DC input)
• IP65 minimum + operating ambient ≥ site design T_max
• Grid-code certification matches destination country (SASO, SONCAP, SNI, etc.)

For more on hybrid vs on-grid topology selection and string vs micro trade-offs, see our companion articles.

Four worked examples

Ready to spec?

Browse the Solar Inverter catalog 3 kW – 100 kW, or skip the math and send us your panel datasheet, string layout, and site coordinates via Request a Quote — our application engineer returns a sized BOM, recommended cable, and breaker selection within 24 hours.