Losses in solar electric system?
The rated power of the solar panel is under standard test conditions (STC) that don't usually exist in the real world. The panel is kept at 25C, and the incident light is 1000W per square meter. As you can imagine, the panel does not want to stay at 25C when it is getting hit with 1000W per square meter, so it has to be cooled during the test, or, the test must be very brief so that it doesn't have time to heat up.
Perhaps in some cool but sunny place this can be achieved in the real world. But in many places it is impossible in the field.
Many panels also come with a more realistic rating called NOCT  (normal operating cell temperature) rating. This is the output when cell temperature is 47.5C and incident light power is 800W per square meter. In reality the panel temperature will vary based on location and installation details (such as how exposed the back side of the panel is to circulating air).
To sum up, assuming constant light conditions, temperature is by far the biggest factor in power generation. In general, when cell temperature goes up, the Vmpp and output power go down.
The inverter or charge controller also has losses. Most grid tie inverters seem to be 90 or 95 percent efficient (roughly). Charge controllers vary widely. I would say that 80 to 95 percent may be the range.
If you direct connect the panel to the battery with no charge controller, then you also have the problem of the panel not being at its MPP (maximum power point). This may be OK for panels with 36 cells and a 12V lead acid battery (as long as you avoid over-charge). But in some cases the solar panel output will be greatly reduced if the battery voltage is not a good match for the number of cells in the panel.
There are losses in the wiring, but these can be kept under control by using larger diameter wires.