A Performance Ratio (PR) is a relative indicator of total system losses which is based on the sunlight (insolation) that reaches your system over a period of time. The higher the PR, the more energy efficient your system is, but it can never be 100% due to things like thermal losses, inverter efficiency and resistive cable losses.

Autarco’s calculations of PR take in to account various factors so that we expect your system to have a fairly constant PR over it’s entire lifetime.

  • The measured insolation and temperature on your roof. Typically based on satellite data for small systems.
  • The direction and tilt of your solar modules to convert the horizontal satellite measurements to a “plane-of-array” measurement which is the sunlight directly hitting the face of your solar panels.
  • The shade of your system at every month/hour of the day.
  • The age of your system (module degradation)
  • The exact components used in your system and how they were installed (system stringing).

If your PR drops it indicates module soiling, new shade (tree growth) or perhaps some product defect.

The major differences in expected PR’s for different systems comes down to the system size and temperature losses.

Based on everything we know about your system we know what is should produce every 15 minutes. The Energy Performance Index (EPI) is the actual output divided by our expected output. If the EPI > 100% then happy days. If the EPI is less than 100% for a long enough period for us to be certain that something isn’t right, then alerts will automatically be raised.

Power Performance Index (PPI) considers how much energy is produced for every kWp of solar modules installed. It does not require an advanced yield model to calculate, but is simply output (kWh) / Rated Power (kWp).

For example, a system with lots of shade or installed in the north of Scotland will have a much lower PPI (~800kWh/kWp) than a completely shade free system or a system installed in sunny Australia (~1500kWh/kWp)

This is different to PR and EPI that look at how your system is performing relative to what is expected from it.