Photovoltaic system sizing

Calculating a photovoltaic array

It is absolutely essential to take into account the location (geographic location, latitude, altitude, shade, etc.) and installation factors (direction faced, angle, etc.).

Firstly, the approximate power output may be calculated based on the available surface area:

10 m² = 1 kWp

7140 m² (= football ground) = 700 kWp

The PV array should always be arranged around the inverter. The calculations involved should compare the characteristics of the modules and those of the inverter with a view to identifying the optimal configuration.

• String composition:

NB: Number of modules x Voc (at t° min) < inverter Vmax

The no load voltage of the string (Voc x number of modules in series) at the minimum temperature of the installation location must be lower than the inverter’s maximum input voltage.

This must be strictly observed. Otherwise the inverter may be destroyed. Apart from the aforementioned rule for preventing destruction of the inverter

Number of modules x Voc (at t° min) < inverter Vmax – two other limits must be observed:

• Number of modules x Vmpp (at t° max) > inverter Vmin

The operating voltage (Vm x number of modules in series at all temperatures at the installation location) should fall within the inverter’s MPPT voltage range. Otherwise, the inverter will stall and energy supply will cease.

• Isc strings < inverter I max

The total Isc current for strings in parallel must be lower than the maximum input current for the inverter. Otherwise, the inverter limits the supply of energy delivered to the network.

Inverter specifications

• In Europe, the power level of the inverter must be between 0.8 and 1 times the power of the array:

0.8 < Pinverter / Parray < 1

• Below this (under 0.8 Parray), the inverter limits power significantly. The energy sold to the network will thus be inferior to that which the panels are capable of supplying and therefore it will take longer to secure a return on investment.
• Above this (over Parray), the inverter is too large for the power level of the array. Again, it will take longer to secure a return on investment.

• Single-phase or three-phase

A decision should be made over these two options in consultation with the local energy distributor based on the devices available in manufacturers’ product ranges, often within the following limits:

• Inverter Pn < 10 kW ≥ single phase inverter
• 10 kW < Pn < 100 kW ≥ either three-phase inverter(s) or single-phase inverters split between the three phases and neutral. The management of unbalances between phases needs to be checked in this instance.
• Pn > 100 kW => three-phase inverter(s)

• Configuration software

Manufacturers of inverters help design offices and installers to size strings for residential and service sector installations based on the equipment available by supplying sizing software.