What is the sign convention for generated power?

Generally speaking, generated power is denoted as negative, correct? If I understand correctly, this is just the convention.

Conceptually, I think of it kind of like a force, it has a magnitude and direction. In this case, the direction is either positive or negative, effectively.

If that is correct, is it necessary to write that a source generates -15 W (for example)? It seems redundant. I wouldn't write that a force is -10 N up, that just doesn't make sense. I would write that it is 10 N down. So similarly, isn't it sufficient to say that there are 15 W generated, or 15 W absorbed?

The reason that I ask is that I was doing a homework assignment and I was doing a power check on a simple circuit. When it came down to it, I had "15 Watts generated with 15 Watts absorbed, so the power check is satisfied." My professor marked me wrong, because I supposedly needed "-15 Watts generated." Is he correct in doing this? (I specifically talked to him, all my work was correct, it was just the sign.)

If he is wrong, what source can I use to show him why it's not supposed to be that way? I don't think my opinion will persuade him, I need something more substantial.

• Could include the diagram or worked problem? Context is imortant Commented Oct 7, 2014 at 16:01