What is the source of the 15.8VA and the 15W spec?
Is it from the documentation of the camera or the power supply, or from a measurement made?
If the number is from the camera documentation, then you must account for
- Power Factor (As Brian mentioned)
- Efficiency of the power supply
- Power loss due to cable
also, the number is likely the peak power the camera might draw, which could be substantially more than actual.
If the number is from the power supply documentation, its likely overstated, since it will only list the max draw of the power supply.
Best bet is direct measurement, if you really need to be accurate is to get a Kill-o-Watt or something like it. Let it run for a day and it does all the math for you in a real use case and accounting for all variables.