It is similar a mile-per-hour rating.
When a system is described as a 5 kilowatt system, that means that it is estimated that it operates at 5 kilowatts per hour when it is at full power. That's not an indication of how much power it will generate every hour, but what it's rated at its very best performance.
Determining the kilowatt rating of a system usually involves multiplying the STC wattage of the panels by the number of panels included in the system.
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