Whether it should be more or not depends on how much you are asking it to do. Every piece of hardware attached to your motherboard draws power as does the motherboard itself. Have you added things to the original system like extra RAM? A new graphics card? It all mounts up.
Do the calculation at that link. If the result is more than your 280W you need to replace your PSU urgently. An inadequate PSU will cause random reboots or shutdowns without warning, program slowdowns and blue screen of death PC crashes. It could even cause corruption of your hard-drive.

Expert in computer code including, but not limited to, BTW; IIRC; IMHO; LMAO; BRB; OIC; ROFL; TTYL. Black belt in Google-fu.