Hi,
Oh ok, you can get away with more with only 5 watts.
But the 555 oscillator (as audioguru pointed out) will not be exactly symmetrical because of the internal transistor drop unless that drop is very small. It must be very small too, because the sensitivity analysis shows that for a 1 percent decrease in output voltage from the supply voltage will cause a 2.2 percent change in the ratio of the charge time to the discharge time, and that means an on time of about 51 percent and off time of about 49 percent of the total time, and that is for only a small drop in output voltage measured from the Vcc supply line. In other words, if the supply is +10v then 1 percent of that is 0.1v, and so if the transistor drops just 0.1v then the duty cycle is going to be 51 percent on and 49 percent off. This again is because the 2/3 point is only symmetrical with the 1/3 point when the supply itself is charging the capacitor. That might be acceptable for a small 5 watt unit however.
With a drop due to the transistor of VT volts the charge time is:
RC*log((3*VT-Vs)/(3*VT-2*Vs))
while the discharge time is:
log(2)*RC
Now if we make VT=Vs (Vs the supply voltage) then we do see the 50 percent duty cycle, but that does not include the transistor voltage drop then.
Im sure we could find a way to improve this though.