Hello A,
When PV panels are new, and conditions are “just right,” each of your panels will indeed put out 500 (or very close to 500) Watts. As they age, their output dops off steadily till it reaches approximately 80% of the rating on the label. Degradation rates vary, but typically fall in the range of 0.3 to 0.5% per year.
So what are those “just right” conditions? Here’s a brief explanation:
PV panels produce their rated output at Standard Test Conditions which is:
- Solar irradiance of 1,000 W/m2
- Cell temperature is held constant at 25°C (77°F).
- Air mass coefficient is 1.5.
However, STC are ideal conditions that don’t really occur in the real-world.
(e.g. when you put solar panels on a roof). To more precisely account for real-world conditions, the NOCT and later NMOT test conditions were introduced.
Temperature affects PV panels as well. As they heat up, their output voltage drops.
More info here:
So it’s quite likely your array isn’t putting out 3.5kW.
On another note…
You’d be much better served if you configured your system for 24 Volt operation
vice 12 Volt. At 24 Volts, the current will be half that of a 12 Volt system.
Mr Ohm tells us that losses in a conductor increase with the square of the current.
e.g. if the current is doubled, the losses increase by a factor of four.
That means larger conductors for a 12 Volt system vice what’s needed for a 24 Volt setup. Translation: That cabling will cost more. Possibly much more.
Food for thought.