Capacity
When using the World API PV Power endpoint, you should provide your AC capacity in Watts. This is most commonly the size of your inverter.
As The World API uses a Simple PV Power model, there are some cases where using the DC capacity may give you more accurate results during peak sunlight hours. If your DC capacity is greater than the AC capacity by 10%, you will likely see better alignment with observed data if you use the DC capacity in Watts (the sum of the rated capacity of the PV modules).
Azimuth
The azimuth of your solar PV system is the direction it is pointed with respect to the directions of a compass. This is important for determining what time of the day the power output from your solar PV system will peak. The numerical values for each direction are listed in the image below (red text).
Read more about azimuth in this support article.
Tilt
The tilt of your solar PV system is a measurement of how far your PV panels are from horizontal. If you have a horizontal (lying flat) tilt, you have a 0 degree tilt. Conversely, a 90 degree tilt is where the panel would be completely upright, pointing towards the horizon.
Estimating the tilt of your PV system doesn't not have to be an exact science. An estimate to within 5 degrees should be sufficient for any use case.
You can dive deeper on estimating PV system tilt in this article
Efficiency
Efficiency may also be called the 'Loss Factor' in various pages + other articles and our API documentation
This is the factor by which to reduce your output forecast from the full capacity based on characteristics of the PV array or inverter. This is effectively the non-temperature loss effects on the nameplate rating of the PV system, including inefficiency and soiling.
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.
It is challenging to estimate the efficiency/loss factor without comparing to measurements, but a general rule is that PV modules lose 0.5-2% efficiency per year. Therefore the efficiency of 10 year old panels may be as low as 80% (.8).