Load Average doesn't equate to a % of cpu usage.
It's a metric referring to (this is a gross simplification) how much of your processing power is being requested. And which is displayed in 1 minute, 5 minute, and 15 minute intervals.
In your case, you have 2 quad core CPUs without hyperthreading and therefore 8 cores total. Which means that a load average equivalent to 100% cpu use would be 8.
The fact your load average is at 15 means that you have need of more processing time than you have processor cores, so your applications end up having to wait. Ideally, you'd not want your load averages to go above 8 for best response times.