Starting from the existing PV array: translating your setup into inverter sizing data
Record the module values: Pmpp, Voc, Vmpp, Isc and temperature coefficients (%/°C)
For clean inverter sizing, start from the module datasheet, not from meter readings. Note the STC values for Pmpp, Voc, Vmpp, and Isc. Add the temperature coefficients, in %/°C, at least for Voc and Pmpp. Voc is the value that rises in cold conditions. It is the one that drives the risk of exceeding DC voltage.
Describe the strings: number of modules in series, number of strings in parallel, wiring and cable sizes
Document Ns (modules in series) and Np (strings in parallel). Record the round-trip lengths, the PV1-F cable type or equivalent, the cross-sections (mm²), and any string protection devices if present. This survey sets the DC current to be carried, and therefore the voltage drop and the acceptability of the inverter inputs.
Identify orientations and shading: why one MPPT input per orientation changes inverter sizing
Map the orientations (azimuth, tilt) and any masking. A different orientation or structural shading in practice requires a dedicated MPPT. Otherwise, the most penalized string limits the whole array, and you either oversize the inverter unnecessarily or degrade its MPPT range. To quantify the impact, rely on suitable orientation coefficients.
Adjusting array power vs inverter power: targeting the right ratio without overselling
Understanding the difference between Ppeak (array) / Prated (inverter): peak is rarely reached
The array’s Ppeak is given under STC conditions. On site, irradiance, incidence angle, DC losses and especially cell temperature all pull instantaneous power down. Inverter Prated, by contrast, is a continuous AC power rating. In inverter sizing, you are therefore comparing a DC “catalogue” value with a real AC capacity.
Choosing a slight array oversizing: effects on production and clipping
A field that is a little more powerful than the inverter raises production over most hours, without changing the AC ceiling. The downside is called clipping. When the array exceeds the AC limit, the inverter clips the output. This is acceptable only if clipping remains marginal over the year and is documented in the study. To make this concrete, rely on a production estimate that quantifies annual losses and clipping.
Validating AC/DC matching: practical limits according to site, tilt, temperature and customer objectives
Set a DC/AC ratio that matches the resource, tilt, rear ventilation and strategy. Self-consumption, export, grid limitation, or yield priorities. Keep a margin on AC power in summer, and also check DC input capability to avoid surprises in operation.
Securing voltage: the cold-weather calculation that prevents inverter damage
Calculating string open-circuit voltage in deep cold: Voc corrected by the temperature coefficient
For proper inverter sizing, start from the module Voc at STC and its βVoc coefficient (%/°C) on the datasheet. Voc increases as the cell cools. Calculate the cold Voc using the linear correction. Use a realistic minimum cell temperature for the site, or otherwise the minimum outdoor temperature increased with a safety margin.
Determining the maximum number of modules in series: stay below the maximum admissible DC voltage
The criterion is simple. Nmax = integer part of inverter VDC max divided by module Voc_cold. Also compare this with the admissible voltage of DC disconnects, surge protection devices and connectors. The goal is to stay below the limit in the worst case.
Identifying the governing case: the classic mistake “I size on power” and forget cold Voc
In the field, failures happen when the inverter is chosen by kW and the strings are validated by feel. On a frosty morning, string Voc exceeds VDC max, and the electronics take the overvoltage hit. Final point: different orientations must go to separate MPPTs, otherwise you lose production and make checks more complex.
Ensuring summer operation: staying within the MPPT range despite heat
Calculating string Vmpp at the highest cell temperature: apply the coefficient (%/°C)
For the summer operating point, start from the module datasheet at STC. Record Vmpp_STC and the temperature coefficient of Vmpp, in %/°C (negative). Then estimate the site’s maximum cell temperature and calculate string Vmpp: Vmpp_mod(T) = Vmpp_STC × (1 + coef/100 × (Tcell - 25)). Multiply by Ns.
Setting the minimum number of modules in series: Vmpp must stay within the inverter’s MPPT window
Derive Ns_min = ceiling(VMPPT_min inverter / Vmpp_mod(Tcell_max)). In inverter sizing, keep a margin for DC voltage drops and ageing. If Ns is too low, the inverter leaves MPP tracking and clips, or even drops out.
Managing multi-string configurations: lengths, voltage drops and input balancing
In multi-string setups, align Ns on the same MPPT input when orientations are identical. For different roof planes, separate the strings onto separate inputs or distinct MPPTs. Check cable voltage drop at Impp and current sharing in parallel. Method references: NF C 15-712-1 and IEC 62548.
Checking currents and inputs: avoiding saturation, especially in multi-string systems
Comparing Isc increased by the normative safety coefficient with the admissible input current
In inverter sizing, do not stop at power. Take the module short-circuit current (Isc), apply the normative safety coefficient, then compare it with the maximum input current listed on the inverter datasheet (per MPPT or per input).
Checking current per input and total current: impacts of parallel strings
As soon as you put strings in parallel, currents add up. Check the limit per input and the device’s total DC limit. A classic mistake is to be compliant on each string, but over the limit once they are combined.
Correctly assigning strings: one orientation per MPPT and the pitfalls of mixing on the same input
Assign one coherent orientation and layout per MPPT. Mixing east/west or different lengths on the same input creates diverging maximum power points, and you lose production while making troubleshooting harder. To go further on the impact of orientation, see PV panel orientation and tilt.
