Wattage means shit when comparing PSUs.

A good quality (more efficient) 500 watt PSU will supply more power than a bad quality (read: cheap) 800 watt PSU. The cheaper PSU will also have more chance of dying and possibly taking out other components with it.

The PSU is the core of a system, and to be quite frank you'd have to be rather stupid (or careless) to buy a budget one for £10.