The word 'factor' here can be confusing. Let's get back to the basics of what the efficiency factor does.
First a little review on percentages. A percentage is represented by 0 - 100%, which is well known. However what is less commonly understood is that mathematics uses decimal places to represent percentages in math formulae. So, when you use the efficiency factor in any API requests, you'll need to use a value from 0.01 (1%) to 1.0 (100%). For example, 85% efficient would be a value of 0.85. 70% efficient would be a value of 0.70.
Now, onto what it means and how to estimate your efficiency factor.
The Solcast API uses the efficiency factor to account for all non-temperature driven losses in the PV system. This includes wiring losses, degradation and soiling (dust/dirt on panels).
It is generally difficult to estimate an efficiency factor without comparing your observed data versus that produced by our API. We suggest you use the efficiency factor to adjust the API data until it closely matches the energy generation or power output from your own solar PV system (paying attention to the magnitude of power output estimates across the day).
However, if you need a place to start, we recommend you apply a 0.5% to 2.0% reduction in the efficiency factor for each year that has passed since your solar system was installed. If you have a 4 year old PV system, and want to assume a 2.0% loss per year, you would use an efficiency factor of 0.92. How did I get this value? You start with 100% (1.0), and subtract 8% (4 years x 2% per year) to get 92%, which is an efficiency factor of 0.92.
Lastly, note that the default efficiency factor is 0.9 in the "Simple PV Power" (legacy) API requests.