Solar Inverter Rating vs Module Rating

Solar inverters have a nominal or rated AC power as specified by the manufacturer. Solar Modules have a rated DC power at standard test conditions (STC) which is 25 degrees centigrade.

Factors Solar Inverter Rating vs Module Rating

There are a number of factors suggesting that to optimise a PV system, the inverter rating should be significantly less than the module rating. Much research indicates that in the UK an inverter’s rated AC power should be between 80% and 90% of the module rating at STC. Larger inverters may function perfectly well but may operate most of the time at lower efficiency and will cost more for little or no benefit.

Factors To Take Into Account

• At normal operating cell temperature, typically 50 degrees centigrade, the module rating will be reduced by about 10% due to the rise in temperature and this is the prime reason for designing with an apparently lower rated inverter than the modules.
• There will also be a reduction in power between the DC input and the AC output depending on the inverter’s efficiency. 
  
• The inverter manufacturer specifies limits for minimum and maximum DC voltage, current and power at the extremes of low and high temperature and these must always be adhered to.
• An sub-optimal array orientation, e.g. facing South West or located in Scotland, may indicate that a lower rated inverter would be more appropriate as the system will operate for longer periods at less than maximum power.
• Compliance with the grid connection standards, e.g. limiting output to 16A per phase, can significantly reduce the risk and cost of a project.

Example Small System

A system with 6 X 250W Poly modules with a total rated power of 1.5kWp could use inverters with an AC rated power between 1.2kW and 1.8kW. Typically the 1.2kW sized inverters will perform best and be the lowest cost, but where the only inverter from your chosen manufacturer is larger, e.g. 1.8kW, this can be used and will work but may not be the most cost effective option.

Example Larger Systems

With larger residential systems, e.g. 16 X 250Wp modules the suitable range would be between 3.6kW and 4.0kW as there are many options from all the manufacturers suitable for a 4kWp system with to need to choose an over rated inverter. A 3kWp PV array is ideally suited to a 2.5kW AC rated inverter.

Sometimes customers will expect to get a 4kW inverter with a 4kWp solar PV system and it is tempting for the installer to give the customer what they want, but rarely will this be the best solution in terms of cost or yield, not to mention grid connection compliance.i

It is more common to use a higher proportion of inverter rated power to DC module power with larger systems, e.g. 50kW, but in fact the same guidelines apply and typically 40kW of AC rated power is ideal for a 50kWp system. Often though the cost differential between say 40kW and 50kW of inverters is so small people often choose to more closely match the inverter rating to the module rating.