Hitachi HA-250 Amp no output

Normal dissipation will be very low indeed. At full output (sine wave, dummy load and all that) you might pull 50 to 100 milliamps through it which would be under 0.5 watt, plus it would be way less than that as an average...

So 0.5 watt covers every eventuality. Edit... 0.25 watt I'm sure will also be fine for any music program at any volume level you use.