Efficiency of a solar system is generally defined as the ratio of electrical energy out of the system to incoming solar energy.
We can then consider four major factors:
However when we are trying to work out how well a solar system is performing not all of these elements are useful as we cannot control or change them. So, we often define a separate metric, performance ratio (also sometimes called system efficiency), as the ratio of total energy produced by the system to the total possible energy produced by the system.
Panel efficiency is how well the panel converts the solar energy falling on it to electrical energy and is typically around 17-22%. If you have a 20% efficient 1*1.65m^2 panel at so called "1-sun" standard conditions of 1000W/m^2 solar irradiance, 25C, sea level etc, you get 330W DC, which is its nameplate power. Most of the remaining energy is converted to heat. As there are many different solar panel manufacturers, and you cannot change the panels once they are in the system, the efficiency at standard conditions is usually excluded from the performance ratio calculation.
Incoming Solar Energy
The incoming solar energy is impacted by time of day, time of year, location and weather which all affect the sun angle, intensity and spectrum. It's also impacted by the orientation and pitch of the solar system. Ideal orientation and pitch can increase it (e.g. if you have average 5 PSH/day in Perth on the flat, you will have average 5.5 at 30deg pitch north facing) but poor orientation and pitch can significantly decrease it as well. As this is so dependent on the system's location and set up, this generally isn’t considered as part of the performance ratio (so values can be compared for different locations). However, other metrics like energy yield (the amount of energy expected in a year per kW capacity) include this so they can compare the overall solar output.
The sub-system efficiency includes the efficiencies of everything between the solar panels and the load ( e.g. wiring losses, inverter losses etc) and is typically around 95%. If you add a battery, you might be looking at anywhere between 60-90% efficiency depending on charge regimes and battery efficiency.
The solar energy * panel efficiency * sub-system efficiency is what you get out of the inverter in AC at standard conditions.
Now for deviations from standard conditions. These can all be attributed to one of the above groups, but for simplicity, we can break them into a separate group as they all are incorporated into the performance ratio
Temperature, panel mismatch, degradation all impact panel efficiency. A 5yo system with typical mismatch in typical Perth temperatures, will produce about 85% of what an ideal system at standard conditions would. Shading and soiling reduce the incoming solar energy that reaches the cells so further reduces the efficiency. These effects can have a large impact on the output of the solar system but can all be managed (e.g. good ventilation, panel cleaning, removing shady trees, etc.) so hence are incorporated into the performance ratio.
Of course another key aspect for actual yield is "availability" - defined as how often is the system actually working? For decent systems with Solar Analytics to help you resolve the issues, this is probably around 97-99%. But of course without SolA you're flying blind!
All of that is a long way of saying, you might get between around 70% and 85% as a performance ratio depending on a lot of these factors, most notably cell temperature and shading. Or 50%-80% if you have a battery.