Yes and no...
Yes, your math and basic assumptions are correct.
No, in the sense that many devices do not use their name plate ratings...
For example, many ratings have maximum current and maximum voltage listed... However, for many devices, they are "constant power" devices (many electronics and motors)--So, at minimum voltage (of say 10.5 volts) it will use maximum current (example: 10.5v*4amps=42 watts). At the other end, of the limits, high voltage--the current may be less (example: 42w/15v=2.8amps, but nameplate may look like; 4a*15v=60w vs 42 watts actual).
Other examples are devices that use power in cycles (like a refrigerator), or use at different levels (fridge in hot weather vs fridge in cold weather, monitor back-light on high vs low, washer mostly empty vs mostly full).
In the end, you usually will have to measure the actual current/voltage/time profile for your needs and plug those into the equations.
And, the amount of sun you gather can easily vary by 10%-20% over time (weather patterns, pollution, etc.). So, getting "too accurate" does not help much either.